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I have an graphql/apollo-server/graphql-yoga endpoint. This endpoint exposes data returned from a database (or a REST endpoint or some other service).
I know my data source is returning the correct data -- if I log the result of the call to the data source inside my resolver, I can see the data being returned. However, my GraphQL field(s) always resolve to null.
If I make the field non-null, I see the following error inside the errors array in the response:
Cannot return null for non-nullable field
Why is GraphQL not returning the data?
There's two common reasons your field or fields are resolving to null: 1) returning data in the wrong shape inside your resolver; and 2) not using Promises correctly.
Note: if you're seeing the following error:
Cannot return null for non-nullable field
the underlying issue is that your field is returning null. You can still follow the steps outlined below to try to resolve this error.
The following examples will refer to this simple schema:
type Query {
post(id: ID): Post
posts: [Post]
}
type Post {
id: ID
title: String
body: String
}
Returning data in the wrong shape
Our schema, along with the requested query, defines the "shape" of the data object in the response returned by our endpoint. By shape, we mean what properties objects have, and whether those properties' values' are scalar values, other objects, or arrays of objects or scalars.
In the same way a schema defines the shape of the total response, the type of an individual field defines the shape of that field's value. The shape of the data we return in our resolver must likewise match this expected shape. When it doesn't, we frequently end up with unexpected nulls in our response.
Before we dive into specific examples, though, it's important to grasp how GraphQL resolves fields.
Understanding default resolver behavior
While you certainly can write a resolver for every field in your schema, it's often not necessary because GraphQL.js uses a default resolver when you don't provide one.
At a high level, what the default resolver does is simple: it looks at the value the parent field resolved to and if that value is a JavaScript object, it looks for a property on that Object with the same name as the field being resolved. If it finds that property, it resolves to the value of that property. Otherwise, it resolves to null.
Let's say in our resolver for the post field, we return the value { title: 'My First Post', bod: 'Hello World!' }. If we don't write resolvers for any of the fields on the Post type, we can still request the post:
query {
post {
id
title
body
}
}
and our response will be
{
"data": {
"post" {
"id": null,
"title": "My First Post",
"body": null,
}
}
}
The title field was resolved even though we didn't provide a resolver for it because the default resolver did the heavy lifting -- it saw there was a property named title on the Object the parent field (in this case post) resolved to and so it just resolved to that property's value. The id field resolved to null because the object we returned in our post resolver did not have an id property. The body field also resolved to null because of a typo -- we have a property called bod instead of body!
Pro tip: If bod is not a typo but what an API or database actually returns, we can always write a resolver for the body field to match our schema. For example: (parent) => parent.bod
One important thing to keep in mind is that in JavaScript, almost everything is an Object. So if the post field resolves to a String or a Number, the default resolver for each of the fields on the Post type will still try to find an appropriately named property on the parent object, inevitably fail and return null. If a field has an object type but you return something other than object in its resolver (like a String or an Array), you will not see any error about the type mismatch but the child fields for that field will inevitably resolve to null.
Common Scenario #1: Wrapped Responses
If we're writing the resolver for the post query, we might fetch our code from some other endpoint, like this:
function post (root, args) {
// axios
return axios.get(`http://SOME_URL/posts/${args.id}`)
.then(res => res.data);
// fetch
return fetch(`http://SOME_URL/posts/${args.id}`)
.then(res => res.json());
// request-promise-native
return request({
uri: `http://SOME_URL/posts/${args.id}`,
json: true
});
}
The post field has the type Post, so our resolver should return an object with properties like id, title and body. If this is what our API returns, we're all set. However, it's common for the response to actually be an object which contains additional metadata. So the object we actually get back from the endpoint might look something like this:
{
"status": 200,
"result": {
"id": 1,
"title": "My First Post",
"body": "Hello world!"
},
}
In this case, we can't just return the response as-is and expect the default resolver to work correctly, since the object we're returning doesn't have the id , title and body properties we need. Our resolver isn't needs to do something like:
function post (root, args) {
// axios
return axios.get(`http://SOME_URL/posts/${args.id}`)
.then(res => res.data.result);
// fetch
return fetch(`http://SOME_URL/posts/${args.id}`)
.then(res => res.json())
.then(data => data.result);
// request-promise-native
return request({
uri: `http://SOME_URL/posts/${args.id}`,
json: true
})
.then(res => res.result);
}
Note: The above example fetches data from another endpoint; however, this sort of wrapped response is also very common when using a database driver directly (as opposed to using an ORM)! For example, if you're using node-postgres, you'll get a Result object that includes properties like rows, fields, rowCount and command. You'll need to extract the appropriate data from this response before returning it inside your resolver.
Common Scenario #2: Array Instead of Object
What if we fetch a post from the database, our resolver might look something like this:
function post(root, args, context) {
return context.Post.find({ where: { id: args.id } })
}
where Post is some model we're injecting through the context. If we're using sequelize, we might call findAll. mongoose and typeorm have find. What these methods have in common is that while they allow us to specify a WHERE condition, the Promises they return still resolve to an array instead of a single object. While there's probably only one post in your database with a particular ID, it's still wrapped in an array when you call one of these methods. Because an Array is still an Object, GraphQL will not resolve the post field as null. But it will resolve all of the child fields as null because it won't be able to find the appropriately named properties on the array.
You can easily fix this scenario by just grabbing the first item in the array and returning that in your resolver:
function post(root, args, context) {
return context.Post.find({ where: { id: args.id } })
.then(posts => posts[0])
}
If you're fetching data from another API, this is frequently the only option. On the other hand, if you're using an ORM, there's often a different method that you can use (like findOne) that will explicitly return only a single row from the DB (or null if it doesn't exist).
function post(root, args, context) {
return context.Post.findOne({ where: { id: args.id } })
}
A special note on INSERT and UPDATE calls: We often expect methods that insert or update a row or model instance to return the inserted or updated row. Often they do, but some methods don't. For example, sequelize's upsert method resolves to a boolean, or tuple of the the upserted record and a boolean (if the returning option is set to true). mongoose's findOneAndUpdate resolves to an object with a value property that contains the modified row. Consult your ORM's documentation and parse the result appropriately before returning it inside your resolver.
Common Scenario #3: Object Instead of Array
In our schema, the posts field's type is a List of Posts, which means its resolver needs to return an Array of objects (or a Promise that resolves to one). We might fetch the posts like this:
function posts (root, args) {
return fetch('http://SOME_URL/posts')
.then(res => res.json())
}
However, the actual response from our API might be an object that wraps the the array of posts:
{
"count": 10,
"next": "http://SOME_URL/posts/?page=2",
"previous": null,
"results": [
{
"id": 1,
"title": "My First Post",
"body" "Hello World!"
},
...
]
}
We can't return this object in our resolver because GraphQL is expecting an Array. If we do, the field will resolve to null and we'll see an error included in our response like:
Expected Iterable, but did not find one for field Query.posts.
Unlike the two scenarios above, in this case GraphQL is able to explicitly check the type of the value we return in our resolver and will throw if it's not an Iterable like an Array.
Like we discussed in the first scenario, in order to fix this error, we have to transform the response into the appropriate shape, for example:
function posts (root, args) {
return fetch('http://SOME_URL/posts')
.then(res => res.json())
.then(data => data.results)
}
Not Using Promises Correctly
GraphQL.js makes use of the Promise API under the hood. As such, a resolver can return some value (like { id: 1, title: 'Hello!' }) or it can return a Promise that will resolve to that value. For fields that have a List type, you may also return an array of Promises. If a Promise rejects, that field will return null and the appropriate error will be added to the errors array in the response. If a field has an Object type, the value the Promise resolves to is what will be passed down as the parent value to the resolvers of any child fields.
A Promise is an "object represents the eventual completion (or failure) of an asynchronous operation, and its resulting value." The next few scenarios outline some common pitfalls encountered when dealing with Promises inside resolvers. However, if you're not familiar with Promises and the newer async/await syntax, it's highly recommended you spend some time reading up on the fundamentals.
Note: the next few examples refer to a getPost function. The implementation details of this function are not important -- it's just a function that returns a Promise, which will resolve to a post object.
Common Scenario #4: Not Returning a Value
A working resolver for the post field might looks like this:
function post(root, args) {
return getPost(args.id)
}
getPosts returns a Promise and we're returning that Promise. Whatever that Promise resolves to will become the value our field resolves to. Looking good!
But what happens if we do this:
function post(root, args) {
getPost(args.id)
}
We're still creating a Promise that will resolve to a post. However, we're not returning the Promise, so GraphQL is not aware of it and it will not wait for it to resolve. In JavaScript functions without an explicit return statement implicitly return undefined. So our function creates a Promise and then immediately returns undefined, causing GraphQL to return null for the field.
If the Promise returned by getPost rejects, we won't see any error listed in our response either -- because we didn't return the Promise, the underlying code doesn't care about whether it resolves or rejects. In fact, if the Promise rejects, you'll see an
UnhandledPromiseRejectionWarning in your server console.
Fixing this issue is simple -- just add the return.
Common Scenario #5: Not chaining Promises correctly
You decide to log the result of your call to getPost, so you change your resolver to look something like this:
function post(root, args) {
return getPost(args.id)
.then(post => {
console.log(post)
})
}
When you run your query, you see the result logged in your console, but GraphQL resolves the field to null. Why?
When we call then on a Promise, we're effectively taking the value the Promise resolved to and returning a new Promise. You can think of it kind of like Array.map except for Promises. then can return a value, or another Promise. In either case, what's returned inside of then is "chained" onto the original Promise. Multiple Promises can be chained together like this by using multiple thens. Each Promise in the chain is resolved in sequence, and the final value is what's effectively resolved as the value of the original Promise.
In our example above, we returned nothing inside of the then, so the Promise resolved to undefined, which GraphQL converted to a null. To fix this, we have to return the posts:
function post(root, args) {
return getPost(args.id)
.then(post => {
console.log(post)
return post // <----
})
}
If you have multiple Promises you need to resolve inside your resolver, you have to chain them correctly by using then and returning the correct value. For example, if we need to call two other asynchronous functions (getFoo and getBar) before we can call getPost, we can do:
function post(root, args) {
return getFoo()
.then(foo => {
// Do something with foo
return getBar() // return next Promise in the chain
})
.then(bar => {
// Do something with bar
return getPost(args.id) // return next Promise in the chain
})
Pro tip: If you're struggling with correctly chaining Promises, you may find async/await syntax to be cleaner and easier to work with.
Common Scenario #6
Before Promises, the standard way to handle asynchronous code was to use callbacks, or functions that would be called once the asynchronous work was completed. We might, for example, call mongoose's findOne method like this:
function post(root, args) {
return Post.findOne({ where: { id: args.id } }, function (err, post) {
return post
})
The problem here is two-fold. One, a value that's returned inside a callback isn't used for anything (i.e. it's not passed to the underlying code in any way). Two, when we use a callback, Post.findOne doesn't return a Promise; it just returns undefined. In this example, our callback will be called, and if we log the value of post we'll see whatever was returned from the database. However, because we didn't use a Promise, GraphQL doesn't wait for this callback to complete -- it takes the return value (undefined) and uses that.
Most more popular libraries, including mongoose support Promises out of the box. Those that don't frequently have complimentary "wrapper" libraries that add this functionality. When working with GraphQL resolvers, you should avoid using methods that utilize a callback, and instead use ones that return Promises.
Pro tip: Libraries that support both callbacks and Promises frequently overload their functions in such a way that if a callback is not provided, the function will return a Promise. Check the library's documentation for details.
If you absolutely have to use a callback, you can also wrap the callback in a Promise:
function post(root, args) {
return new Promise((resolve, reject) => {
Post.findOne({ where: { id: args.id } }, function (err, post) {
if (err) {
reject(err)
} else {
resolve(post)
}
})
})
I had the same issue on Nest.js.
If you like to solve the issue. You can add {nullable: true} option to your #Query decorator.
Here's an example.
#Resolver(of => Team)
export class TeamResolver {
constructor(
private readonly teamService: TeamService,
private readonly memberService: MemberService,
) {}
#Query(returns => Team, { name: 'team', nullable: true })
#UseGuards(GqlAuthGuard)
async get(#Args('id') id: string) {
return this.teamService.findOne(id);
}
}
Then, you can return null object for query.
Coming from Flutter here.
I couldn't find any flutter related solution to this so since my search always brought me here, lemme just add it here.
The exact error was:
Failure performing sync query to AppSync:
[GraphQLResponse.Error{message='Cannot return null for non-nullable
type: 'AWSTimestamp' within parent
So, in my schema (on the AppSync console) I had this:
type TypeName {
id: ID!
...
_version: Int!
_deleted: Boolean
_lastChangedAt: AWSTimestamp!
createdAt: AWSDateTime!
updatedAt: AWSDateTime!
}
I got the error from the field _lastChangedAt as AWSTimestamp couldn't be null.
All I had to do was remove the null-check (!) from the field and it was resolved.
Now, I don't know the implications of this in the long run but I'll update this answer if necessary.
EDIT: The implication of this as I have found out is anything I do, amplify.push that change is reversed. Just go back to your appsync console and change it again while you test. So this isn't a sustainable solution but chatter I've picked up online suggests improvements are coming to amplify flutter very soon.
#Thomas Hennes got it spot on for me
The title field was resolved even though we didn't provide a resolver for it because the default resolver did the heavy lifting -- it saw there was a property named title on the Object the parent field (in this case post) resolved to and so it just resolved to that property's value. The id field resolved to null because the object we returned in our post resolver did not have an id property. The body field also resolved to null because of a typo -- we have a property called bod instead of body!
Pro tip: If bod is not a typo but what an API or database actually returns, we can always write a resolver for the body field to match our schema. For example: (parent) => parent.bod
One important thing to keep in mind is that in JavaScript, almost everything is an Object. So if the post field resolves to a String or a Number, the default resolver for each of the fields on the Post type will still try to find an appropriately named property on the parent object, inevitably fail and return null. If a field has an object type but you return something other than object in its resolver (like a String or an Array), you will not see any error about the type mismatch but the child fields for that field will inevitably resolve to null.
In case anyone has used apollo-server-express and getting null value.
// This will return values, as you expect.
const typeDefs = require('./schema');
const resolvers = require('./resolver');
const server = new ApolloServer({typeDefs,resolvers});
// This will return null, since ApolloServer constructor is not using correct properties.
const withDifferentVarNameSchema = require('./schema');
const withDifferentVarNameResolver= require('./resolver');
const server = new ApolloServer({withDifferentVarNameSchema,withDifferentVarNameResolver});
Note: While creating an instance of Apolloserver pass the typeDefs and resolvers var name only.
If none of the above helped, and you have a global interceptor that envelopes all the responses for example inside a "data" field, you must disable this for graphql other wise graphql resolvers convert to null.
This is what I did to the interceptor on my case:
intercept(
context: ExecutionContext,
next: CallHandler,
): Observable<Response<T>> {
if (context['contextType'] === 'graphql') return next.handle();
return next
.handle()
.pipe(map(data => {
return {
data: isObject(data) ? this.transformResponse(data) : data
};
}));
}
I have an graphql/apollo-server/graphql-yoga endpoint. This endpoint exposes data returned from a database (or a REST endpoint or some other service).
I know my data source is returning the correct data -- if I log the result of the call to the data source inside my resolver, I can see the data being returned. However, my GraphQL field(s) always resolve to null.
If I make the field non-null, I see the following error inside the errors array in the response:
Cannot return null for non-nullable field
Why is GraphQL not returning the data?
There's two common reasons your field or fields are resolving to null: 1) returning data in the wrong shape inside your resolver; and 2) not using Promises correctly.
Note: if you're seeing the following error:
Cannot return null for non-nullable field
the underlying issue is that your field is returning null. You can still follow the steps outlined below to try to resolve this error.
The following examples will refer to this simple schema:
type Query {
post(id: ID): Post
posts: [Post]
}
type Post {
id: ID
title: String
body: String
}
Returning data in the wrong shape
Our schema, along with the requested query, defines the "shape" of the data object in the response returned by our endpoint. By shape, we mean what properties objects have, and whether those properties' values' are scalar values, other objects, or arrays of objects or scalars.
In the same way a schema defines the shape of the total response, the type of an individual field defines the shape of that field's value. The shape of the data we return in our resolver must likewise match this expected shape. When it doesn't, we frequently end up with unexpected nulls in our response.
Before we dive into specific examples, though, it's important to grasp how GraphQL resolves fields.
Understanding default resolver behavior
While you certainly can write a resolver for every field in your schema, it's often not necessary because GraphQL.js uses a default resolver when you don't provide one.
At a high level, what the default resolver does is simple: it looks at the value the parent field resolved to and if that value is a JavaScript object, it looks for a property on that Object with the same name as the field being resolved. If it finds that property, it resolves to the value of that property. Otherwise, it resolves to null.
Let's say in our resolver for the post field, we return the value { title: 'My First Post', bod: 'Hello World!' }. If we don't write resolvers for any of the fields on the Post type, we can still request the post:
query {
post {
id
title
body
}
}
and our response will be
{
"data": {
"post" {
"id": null,
"title": "My First Post",
"body": null,
}
}
}
The title field was resolved even though we didn't provide a resolver for it because the default resolver did the heavy lifting -- it saw there was a property named title on the Object the parent field (in this case post) resolved to and so it just resolved to that property's value. The id field resolved to null because the object we returned in our post resolver did not have an id property. The body field also resolved to null because of a typo -- we have a property called bod instead of body!
Pro tip: If bod is not a typo but what an API or database actually returns, we can always write a resolver for the body field to match our schema. For example: (parent) => parent.bod
One important thing to keep in mind is that in JavaScript, almost everything is an Object. So if the post field resolves to a String or a Number, the default resolver for each of the fields on the Post type will still try to find an appropriately named property on the parent object, inevitably fail and return null. If a field has an object type but you return something other than object in its resolver (like a String or an Array), you will not see any error about the type mismatch but the child fields for that field will inevitably resolve to null.
Common Scenario #1: Wrapped Responses
If we're writing the resolver for the post query, we might fetch our code from some other endpoint, like this:
function post (root, args) {
// axios
return axios.get(`http://SOME_URL/posts/${args.id}`)
.then(res => res.data);
// fetch
return fetch(`http://SOME_URL/posts/${args.id}`)
.then(res => res.json());
// request-promise-native
return request({
uri: `http://SOME_URL/posts/${args.id}`,
json: true
});
}
The post field has the type Post, so our resolver should return an object with properties like id, title and body. If this is what our API returns, we're all set. However, it's common for the response to actually be an object which contains additional metadata. So the object we actually get back from the endpoint might look something like this:
{
"status": 200,
"result": {
"id": 1,
"title": "My First Post",
"body": "Hello world!"
},
}
In this case, we can't just return the response as-is and expect the default resolver to work correctly, since the object we're returning doesn't have the id , title and body properties we need. Our resolver isn't needs to do something like:
function post (root, args) {
// axios
return axios.get(`http://SOME_URL/posts/${args.id}`)
.then(res => res.data.result);
// fetch
return fetch(`http://SOME_URL/posts/${args.id}`)
.then(res => res.json())
.then(data => data.result);
// request-promise-native
return request({
uri: `http://SOME_URL/posts/${args.id}`,
json: true
})
.then(res => res.result);
}
Note: The above example fetches data from another endpoint; however, this sort of wrapped response is also very common when using a database driver directly (as opposed to using an ORM)! For example, if you're using node-postgres, you'll get a Result object that includes properties like rows, fields, rowCount and command. You'll need to extract the appropriate data from this response before returning it inside your resolver.
Common Scenario #2: Array Instead of Object
What if we fetch a post from the database, our resolver might look something like this:
function post(root, args, context) {
return context.Post.find({ where: { id: args.id } })
}
where Post is some model we're injecting through the context. If we're using sequelize, we might call findAll. mongoose and typeorm have find. What these methods have in common is that while they allow us to specify a WHERE condition, the Promises they return still resolve to an array instead of a single object. While there's probably only one post in your database with a particular ID, it's still wrapped in an array when you call one of these methods. Because an Array is still an Object, GraphQL will not resolve the post field as null. But it will resolve all of the child fields as null because it won't be able to find the appropriately named properties on the array.
You can easily fix this scenario by just grabbing the first item in the array and returning that in your resolver:
function post(root, args, context) {
return context.Post.find({ where: { id: args.id } })
.then(posts => posts[0])
}
If you're fetching data from another API, this is frequently the only option. On the other hand, if you're using an ORM, there's often a different method that you can use (like findOne) that will explicitly return only a single row from the DB (or null if it doesn't exist).
function post(root, args, context) {
return context.Post.findOne({ where: { id: args.id } })
}
A special note on INSERT and UPDATE calls: We often expect methods that insert or update a row or model instance to return the inserted or updated row. Often they do, but some methods don't. For example, sequelize's upsert method resolves to a boolean, or tuple of the the upserted record and a boolean (if the returning option is set to true). mongoose's findOneAndUpdate resolves to an object with a value property that contains the modified row. Consult your ORM's documentation and parse the result appropriately before returning it inside your resolver.
Common Scenario #3: Object Instead of Array
In our schema, the posts field's type is a List of Posts, which means its resolver needs to return an Array of objects (or a Promise that resolves to one). We might fetch the posts like this:
function posts (root, args) {
return fetch('http://SOME_URL/posts')
.then(res => res.json())
}
However, the actual response from our API might be an object that wraps the the array of posts:
{
"count": 10,
"next": "http://SOME_URL/posts/?page=2",
"previous": null,
"results": [
{
"id": 1,
"title": "My First Post",
"body" "Hello World!"
},
...
]
}
We can't return this object in our resolver because GraphQL is expecting an Array. If we do, the field will resolve to null and we'll see an error included in our response like:
Expected Iterable, but did not find one for field Query.posts.
Unlike the two scenarios above, in this case GraphQL is able to explicitly check the type of the value we return in our resolver and will throw if it's not an Iterable like an Array.
Like we discussed in the first scenario, in order to fix this error, we have to transform the response into the appropriate shape, for example:
function posts (root, args) {
return fetch('http://SOME_URL/posts')
.then(res => res.json())
.then(data => data.results)
}
Not Using Promises Correctly
GraphQL.js makes use of the Promise API under the hood. As such, a resolver can return some value (like { id: 1, title: 'Hello!' }) or it can return a Promise that will resolve to that value. For fields that have a List type, you may also return an array of Promises. If a Promise rejects, that field will return null and the appropriate error will be added to the errors array in the response. If a field has an Object type, the value the Promise resolves to is what will be passed down as the parent value to the resolvers of any child fields.
A Promise is an "object represents the eventual completion (or failure) of an asynchronous operation, and its resulting value." The next few scenarios outline some common pitfalls encountered when dealing with Promises inside resolvers. However, if you're not familiar with Promises and the newer async/await syntax, it's highly recommended you spend some time reading up on the fundamentals.
Note: the next few examples refer to a getPost function. The implementation details of this function are not important -- it's just a function that returns a Promise, which will resolve to a post object.
Common Scenario #4: Not Returning a Value
A working resolver for the post field might looks like this:
function post(root, args) {
return getPost(args.id)
}
getPosts returns a Promise and we're returning that Promise. Whatever that Promise resolves to will become the value our field resolves to. Looking good!
But what happens if we do this:
function post(root, args) {
getPost(args.id)
}
We're still creating a Promise that will resolve to a post. However, we're not returning the Promise, so GraphQL is not aware of it and it will not wait for it to resolve. In JavaScript functions without an explicit return statement implicitly return undefined. So our function creates a Promise and then immediately returns undefined, causing GraphQL to return null for the field.
If the Promise returned by getPost rejects, we won't see any error listed in our response either -- because we didn't return the Promise, the underlying code doesn't care about whether it resolves or rejects. In fact, if the Promise rejects, you'll see an
UnhandledPromiseRejectionWarning in your server console.
Fixing this issue is simple -- just add the return.
Common Scenario #5: Not chaining Promises correctly
You decide to log the result of your call to getPost, so you change your resolver to look something like this:
function post(root, args) {
return getPost(args.id)
.then(post => {
console.log(post)
})
}
When you run your query, you see the result logged in your console, but GraphQL resolves the field to null. Why?
When we call then on a Promise, we're effectively taking the value the Promise resolved to and returning a new Promise. You can think of it kind of like Array.map except for Promises. then can return a value, or another Promise. In either case, what's returned inside of then is "chained" onto the original Promise. Multiple Promises can be chained together like this by using multiple thens. Each Promise in the chain is resolved in sequence, and the final value is what's effectively resolved as the value of the original Promise.
In our example above, we returned nothing inside of the then, so the Promise resolved to undefined, which GraphQL converted to a null. To fix this, we have to return the posts:
function post(root, args) {
return getPost(args.id)
.then(post => {
console.log(post)
return post // <----
})
}
If you have multiple Promises you need to resolve inside your resolver, you have to chain them correctly by using then and returning the correct value. For example, if we need to call two other asynchronous functions (getFoo and getBar) before we can call getPost, we can do:
function post(root, args) {
return getFoo()
.then(foo => {
// Do something with foo
return getBar() // return next Promise in the chain
})
.then(bar => {
// Do something with bar
return getPost(args.id) // return next Promise in the chain
})
Pro tip: If you're struggling with correctly chaining Promises, you may find async/await syntax to be cleaner and easier to work with.
Common Scenario #6
Before Promises, the standard way to handle asynchronous code was to use callbacks, or functions that would be called once the asynchronous work was completed. We might, for example, call mongoose's findOne method like this:
function post(root, args) {
return Post.findOne({ where: { id: args.id } }, function (err, post) {
return post
})
The problem here is two-fold. One, a value that's returned inside a callback isn't used for anything (i.e. it's not passed to the underlying code in any way). Two, when we use a callback, Post.findOne doesn't return a Promise; it just returns undefined. In this example, our callback will be called, and if we log the value of post we'll see whatever was returned from the database. However, because we didn't use a Promise, GraphQL doesn't wait for this callback to complete -- it takes the return value (undefined) and uses that.
Most more popular libraries, including mongoose support Promises out of the box. Those that don't frequently have complimentary "wrapper" libraries that add this functionality. When working with GraphQL resolvers, you should avoid using methods that utilize a callback, and instead use ones that return Promises.
Pro tip: Libraries that support both callbacks and Promises frequently overload their functions in such a way that if a callback is not provided, the function will return a Promise. Check the library's documentation for details.
If you absolutely have to use a callback, you can also wrap the callback in a Promise:
function post(root, args) {
return new Promise((resolve, reject) => {
Post.findOne({ where: { id: args.id } }, function (err, post) {
if (err) {
reject(err)
} else {
resolve(post)
}
})
})
I had the same issue on Nest.js.
If you like to solve the issue. You can add {nullable: true} option to your #Query decorator.
Here's an example.
#Resolver(of => Team)
export class TeamResolver {
constructor(
private readonly teamService: TeamService,
private readonly memberService: MemberService,
) {}
#Query(returns => Team, { name: 'team', nullable: true })
#UseGuards(GqlAuthGuard)
async get(#Args('id') id: string) {
return this.teamService.findOne(id);
}
}
Then, you can return null object for query.
Coming from Flutter here.
I couldn't find any flutter related solution to this so since my search always brought me here, lemme just add it here.
The exact error was:
Failure performing sync query to AppSync:
[GraphQLResponse.Error{message='Cannot return null for non-nullable
type: 'AWSTimestamp' within parent
So, in my schema (on the AppSync console) I had this:
type TypeName {
id: ID!
...
_version: Int!
_deleted: Boolean
_lastChangedAt: AWSTimestamp!
createdAt: AWSDateTime!
updatedAt: AWSDateTime!
}
I got the error from the field _lastChangedAt as AWSTimestamp couldn't be null.
All I had to do was remove the null-check (!) from the field and it was resolved.
Now, I don't know the implications of this in the long run but I'll update this answer if necessary.
EDIT: The implication of this as I have found out is anything I do, amplify.push that change is reversed. Just go back to your appsync console and change it again while you test. So this isn't a sustainable solution but chatter I've picked up online suggests improvements are coming to amplify flutter very soon.
#Thomas Hennes got it spot on for me
The title field was resolved even though we didn't provide a resolver for it because the default resolver did the heavy lifting -- it saw there was a property named title on the Object the parent field (in this case post) resolved to and so it just resolved to that property's value. The id field resolved to null because the object we returned in our post resolver did not have an id property. The body field also resolved to null because of a typo -- we have a property called bod instead of body!
Pro tip: If bod is not a typo but what an API or database actually returns, we can always write a resolver for the body field to match our schema. For example: (parent) => parent.bod
One important thing to keep in mind is that in JavaScript, almost everything is an Object. So if the post field resolves to a String or a Number, the default resolver for each of the fields on the Post type will still try to find an appropriately named property on the parent object, inevitably fail and return null. If a field has an object type but you return something other than object in its resolver (like a String or an Array), you will not see any error about the type mismatch but the child fields for that field will inevitably resolve to null.
In case anyone has used apollo-server-express and getting null value.
// This will return values, as you expect.
const typeDefs = require('./schema');
const resolvers = require('./resolver');
const server = new ApolloServer({typeDefs,resolvers});
// This will return null, since ApolloServer constructor is not using correct properties.
const withDifferentVarNameSchema = require('./schema');
const withDifferentVarNameResolver= require('./resolver');
const server = new ApolloServer({withDifferentVarNameSchema,withDifferentVarNameResolver});
Note: While creating an instance of Apolloserver pass the typeDefs and resolvers var name only.
If none of the above helped, and you have a global interceptor that envelopes all the responses for example inside a "data" field, you must disable this for graphql other wise graphql resolvers convert to null.
This is what I did to the interceptor on my case:
intercept(
context: ExecutionContext,
next: CallHandler,
): Observable<Response<T>> {
if (context['contextType'] === 'graphql') return next.handle();
return next
.handle()
.pipe(map(data => {
return {
data: isObject(data) ? this.transformResponse(data) : data
};
}));
}
I am currently stuck with an issue in my MVC 4 application. I have private variable in controller, that holds instance of a simple class:
private InstallationStatus status = null;
When data get submitted on a view, it gets filled like this:
InstallationStatus installStatus = Install();
if (installStatus != null)
{
status = installStatus;
TempData["installPercent"] = 0;
return View("InstallationProgress", status);
}
This part works as intended, variable is set to the instance as it should be.
After that view periodically checks another variable (using ajax):
<script type="text/javascript">
$(document).ready(function () {
var progress = 0;
$("div.status-message").text("Progress: " + progress + "%");
var statusUpdate = setInterval(function () {
$.ajax({
type: 'GET',
url: "/AppStart/GetInstallProgress",
datatype: "application/html; charset=utf-8",
success: function (data) {
progress = parseInt(data);
if (progress >= 100) {
clearInterval(statusUpdate);
var data = $(this).serialize();
$.ajax({
type: 'POST',
url: "#Url.Action("CompletedStatus", "AppStart")",
success: function () {
window.location = "/Login/Login"
}
});
}
$("div.status-message").text("Progress: " + progress + "%");
}
});
}, 2000);
});
</script>
When it calls "CompletedStatus" action on the controller, variable "status" on the controller is null (the instance set previously is not there?
How do I ensure that its value will persist? It seems to me like whole instance of controller gets lost, but that doesnt really matter to me - the source for "status" is webservice and once I get the instance of InstallationStatus, I cant get it again - I need to keep it.
I tried using TempData to store it but since there can be more than one step between storing it and retrieving it TempData proved unreliable.
The final process is:
Request installation status and navigate to view for installation progress (status will be received when progress will finish).
navigate to view where I will by updating installation progress
using javascript whenever I get callback from server with info about
progress
when installation finishes (status is returned) pass that status to
another view
In the example above I have some dummy code-behind, so the status is returned immediately, but that has no impact on the problem I have.
Currently I can do 1 and 2 and I can call the final view, but I cant pass the status in there because I dont have it on controller anymore and TempData are not reliable for this (sometimes it is still there, sometimes it is not).
Any help will be greatly appreciated.
When it calls "CompletedStatus" action on the controller, variable
"status" on the controller is null (the instance set previously is not
there?
How do I ensure that its value will persist?
private InstallationStatus status = null;
It won't unless it's a static value and that would be a very bad thing to do. Remember that variable values (private members' values) are only scoped within the http request. If you do another request then that's a totally whole new scope for your private variables.
I tried using TempData to store it but since there can be more than
one step between storing it and retrieving it TempData proved
unreliable.
That's because TempData will not have the value you expect it to have once you do another request. One good example of using this TempData is when you want to pass/move some values between a POST and GET, that is when you do a POST and do a redirect. TempData does not fit your case.
Now for a possible solution to your scenario, a good question is: is the installation process called once? Is it unique per user? If it is, which I highly suspect it is, then you need to uniquely identify each request. You can simply use a GUID to identify each request. Save that into your database (better than saving in a session) along with some other information like the status of the installation. Pass that guid back to your client and let them pass it back to the controller and retrieve an update on the status of the installation.
My adapter uses findHasMany to load child records for a hasMany relationship.
My findHasMany adapter method is directly based on the test case for findHasMany. It retrieves the contents of the hasMany on demand, and eventually does the following two operations:
store.loadMany(type, hashes);
// ...
store.loadHasMany(record, relationship.key, ids);
(The full code for the findHasMany is below, in case the issue is there, but I don't think so.)
The really strange behavior is: it seems that somewhere within loadHasMany (or in some subsequent async process) only the first and last child records get their inverse belongsTo property set, even though all the child records are added to the hasMany side. I.e., if posts/1 has 10 comments, this is what I get, after everything has loaded:
var post = App.Posts.find('1');
post.get('comments').objectAt(0).get('post'); // <App.Post:ember123:1>
post.get('comments').objectAt(1).get('post'); // null
post.get('comments').objectAt(2).get('post'); // null
// ...
post.get('comments').objectAt(8).get('post'); // null
post.get('comments').objectAt(9).get('post'); // <App.Post:ember123:1>
My adapter is a subclass of DS.RESTAdapter, and I don't think I'm overloading anything in my adapter or serializer that would cause this behavior.
Has anybody seen something like this before? It's weird enough I though someone might know why it's happening.
Extra
Using findHasMany lets me load the contents of the hasMany only when the property is accessed (valuable in my case because calculating the array of IDs would be expensive). So say I have the classic posts/comments example models, the server returns for posts/1:
{
post: {
id: 1,
text: "Linkbait!"
comments: "/posts/1/comments"
}
}
Then my adapter can retrieve /posts/1/comments on demand, which looks like this:
{
comments: [
{
id: 201,
text: "Nuh uh"
},
{
id: 202,
text: "Yeah huh"
},
{
id: 203,
text: "Nazi Germany"
}
]
}
Here is the code for the findHasMany method in my adapter:
findHasMany: function(store, record, relationship, details) {
var type = relationship.type;
var root = this.rootForType(type);
var url = (typeof(details) == 'string' || details instanceof String) ? details : this.buildURL(root);
var query = relationship.options.query ? relationship.options.query(record) : {};
this.ajax(url, "GET", {
data: query,
success: function(json) {
var serializer = this.get('serializer');
var pluralRoot = serializer.pluralize(root);
var hashes = json[pluralRoot]; //FIXME: Should call some serializer method to get this?
store.loadMany(type, hashes);
// add ids to record...
var ids = [];
var len = hashes.length;
for(var i = 0; i < len; i++){
ids.push(serializer.extractId(type, hashes[i]));
}
store.loadHasMany(record, relationship.key, ids);
}
});
}
Solution
Override the DS.RelationshipChange.getByReference method by inserting the following code into your app:
DS.RelationshipChange.prototype.getByReference = function(reference) {
var store = this.store;
// return null or undefined if the original reference was null or undefined
if (!reference) { return reference; }
if (reference.record) {
return reference.record;
}
return store.materializeRecord(reference);
};
Yes, this is overriding a private, internal method in Ember Data. Yes, it may break at any time with any update. I'm pretty sure this is a bug in Ember Data, but I'm not 100% certain this is the right solution. But it does solve this problem, and possibly other relationship-related problems.
This fix is designed to be applied to Ember Data master as of 29 Apr 2013.
Reason
DS.Store.loadHasMany calls DS.Model.hasManyDidChange, which retrieves references for all the child records and then sets the hasMany's content to the array of references. This kicks off a chain of observers., eventually calling DS.ManyArray.arrayContentDidChange, in which the first line is this._super.apply(this, arguments);, calling the superclass method Ember.Array.arrayContentDidChange. That Ember.Array method includes an optimization that caches the first and last object in the array and calls objectAt on only those two array members. So there's the part that singles out the first and last record.
Next, since DS.RecordArray implements an objectAtContent method (from Ember.ArrayProxy), the objectAtContent implementation calls DS.Store.recordForReference, which in turn calls DS.Store.materializeRecord. This last function adds a record property to the reference that is passed in as a side effect.
Now we get to what I think is a bug. In DS.ManyArray.arrayContentDidChange, after calling the superclass method, it loops through all the new references and creates a DS.RelationshipChangeAdd instance that encapsulates the owner and child record references. But the first line inside the loop is:
var reference = get(this, 'content').objectAt(i);
Unlike what happens above to the first and last record, this calls objectAt directly on the Ember.NativeArray and bypasses the ArrayProxy methods including the objectAtContent hook, which means that DS.Store.materializeRecord--which adds the record property on the reference object--may have never been called on some references.
Next, the relationship changes created in the loop are immediately afterward (in the same run loop) applied with this call tree: DS.RelationshipChangeAdd.sync -> DS.RelationshipChange.getFirstRecord -> DS.RelationshipChange.getByReference. This last method expects the reference object to have a record property. However, the record property is only set on the first and last reference objects, for reasons explained above. Therefore, for all but the first and last records, the relationship fails to be established because it doesn't have access to the child record object!
The above fix calls DS.Store.materializeRecord whenever the record property doesn't exist on the reference. The last line in the function is the only thing added. On the one hand, it looks like this was the original intention: that var store = this.store; line in the original declares a variable that isn't otherwise used in the function, so what's it there for? Also, without the added line, the function doesn't always return a value, which is a little unusual for a function which is expected to do so. On the other hand, this could lead to mass materialization in some cases where that would be undesirable (but, the relationships just won't work without it in some cases, it seems).
Possibly related
The "chain of observers" I mentioned takes a bit of an odd path. The initiating event was setting the content property on a DS.ManyArray, which extends Ember.ArrayProxy--therefore the content property has a dependent property arrangedContent. Importantly, the observers on arrangedContent are executed before observers on content are executed (see Ember.propertyDidChange). However, the default implementation of Ember.ArrayProxy.arrangedContentArrayDidChange simply calls Ember.Array.arrayContentDidChange, which DS.ManyArray implements! The point being, this looks like a recipe for some code to execute in an unintended order. That is, I think Ember.ManyArray.arrayContentDidChange may getting executed earlier than expected. If this is the case, the above mentioned code that expects the record property to already exist on all references may have been expecting this reasonably, as one of the observers directly on the content property may call DS.Store.materializeRecord on each reference. But I haven't dug deep enough to find out if this is true.
I'm trying to replace an object with a new one and am getting the mentioned exception. I've tried several combination and can't get around it.
I have a Playlist that has Items (the Items has another nested object, but I'm leaving it out to help make my question clearer. The user can change which items are in the playlist.
if (playlistChanged)
{
// remove selectedForRemoval
IEnumerable<PlaylistItemViewModel> nonSelectedItems = selectedDisplayTemplates.Where(pivm => pivm.IsSelectedForRemoval);
foreach (temViewModel ivm in nonSelectedItems)
{
context.DeleteObject(ivm.Model);
}
// clear out and remove old items
foreach (Item item in playlist.PlaylistItems)
{
context.DeleteObject(item);
}
playlist.PlaylistItems.Clear();
// add the selectedItem(s) to the playlist
// these items can be from the Media, or other tables
// so are newly created in code on the client
foreach (ItemViewModel ivm in selectedItems)
{
playlist.Items.Add(ivm.PlaylistItemModel);
context.AddToItems(ivm.PlaylistItemModel);
}
context.BeginSaveChanges(SaveChangesOptions.Batch, new AsyncCallback((iar) =>
{
try
{
// Callback method for the async request, retrieves the status of the requested action
DataServiceResponse response = context.EndSaveChanges(iar);
}
catch (DataServiceRequestException)
{
throw;
}
}), context);
}
Any help is appreciated.
EDIT: I was overriding the Equals and ToString in Playlist partial class. After I removed those, it started working.
I was overriding the Equals and ToString in Playlist partial classes in Silverlight. After I removed those, it started working. I'm going to avoid that from now on with WCF Data Services.
If you fetched the data using a different context from the one you are trying to add/delete with, you will get the exception you posted. Either dispose of the original context you fetched the data with or explicitly call Detach on the item you are calling AddItem/DeleteObject on.