I'm a nwebie in Core Data, i have designed a navigation based application and some of the data i use are created on run time(come from a URL via JSON). I took a few tutorials an searched for almost a day but haven't still realized how to save the incoming JSON data to the Entity (or event?) in my Core Data model. I fetch the data in the DetailViewController class and i need to save this data to Core Data(I have prepared an Entity with 7 properties). Can anyone please help?(If you know a good tutorial or sample code i will be pleased)
EDIT This may be a little specific but i really have trouble with and need just a little help.
My data comes to the app from a kind of restful server(i wrote it in PHP), firstly user enters his/her login informations(which i have saved to the database on server before) and when the response data comes i will use different elements of it in differen views(for example the user_id will be used on a view and the buttonData etc on other views). My question is, how will i save JSON data into my core data model(has tree Entities for the moment). Thanks in advance
Note: I lokked arround a lot but couldn't find any answer&tutorial about an app like mine
The best way to do that would be to create entities corresponding to JSON structure. Easiest was is when each JSON object becomes an entity, and arrays become arrays of entities. Be reasonable, however, and don't introduce too much overkill for JSON subobjects that are essentially part of its superobject.
When you have created entities, you can start off with the parsing and translation. Use some JSON framework (starting from iOS5 there's one from Apple) and parse JSON string into object tree, where root item is either an NSArray or NSDictionary, and subelements will be NSArray, NSDictionary, NSNumber, NSString or NSNull.
Go over them one by one in iterational loops and assign according values to your core data entity attributes. You can make use of NSKeyValueCoding here and avoid too much manual mapping of the attribute names. If your JSON attributes are of the same name as entity attributes, you'll be able to just go over all dictionary elements and parse them into attributes of the same name.
Example
My parsing code in the similar situation was as follows:
NSDictionary *parsedFeed = /* your way to get a dictionary */;
for (NSString *key in parsedFeed) {
id value = [parsedFeed objectForKey:key];
// Don't assign NSNull, it will break assignments to NSString, etc.
if (value && [value isKindOfClass:[NSNull class]])
value = nil;
#try {
[yourCreatedEntity setValue:value forKey:property];
} #catch (NSException *exception) {
// Exception means such attribute is not defined in the class or some other error.
}
}
This code will work in trivial situation, however, it may need to be expanded, depending on your needs:
With some kinds of custom mappings in case you want your JSON value be placed in differently named attribute.
If your JSON has sub-objects or arrays of sub-objects, you will need to detect those cases, for example in setters, and initiate new parsing one level deeper. Otherwise with my example you will face the situation that assigns NSDictionary object to an NSManagedObject.
I don't think it is reasonable to dive into these, more advanced matters in scope of this answer, as it will expand it too much.
I suggest you to use this library : https://github.com/TouchCode/TouchJSON
And then if you want to make a factory to parse json and feed your code data, you can use selectors to call methods to fill all your attributes.
Chances are your JSON data gets converted to an NSDictionary or NSArray (or some combination of the two). Simply extract the key/values from the JSON structure and add them to your entity class.
This lib helps me lot
Features
Attribute and relationship mapping to JSON key paths.
Value transformation using named NSValueTransformer objects.
Object graph preservation.
Support for entity inheritance
Works vice-versa
Related
I'm a nwebie in Core Data, i have designed a navigation based application and some of the data i use are created on run time(come from a URL via JSON). I took a few tutorials an searched for almost a day but haven't still realized how to save the incoming JSON data to the Entity (or event?) in my Core Data model. I fetch the data in the DetailViewController class and i need to save this data to Core Data(I have prepared an Entity with 7 properties). Can anyone please help?(If you know a good tutorial or sample code i will be pleased)
EDIT This may be a little specific but i really have trouble with and need just a little help.
My data comes to the app from a kind of restful server(i wrote it in PHP), firstly user enters his/her login informations(which i have saved to the database on server before) and when the response data comes i will use different elements of it in differen views(for example the user_id will be used on a view and the buttonData etc on other views). My question is, how will i save JSON data into my core data model(has tree Entities for the moment). Thanks in advance
Note: I lokked arround a lot but couldn't find any answer&tutorial about an app like mine
The best way to do that would be to create entities corresponding to JSON structure. Easiest was is when each JSON object becomes an entity, and arrays become arrays of entities. Be reasonable, however, and don't introduce too much overkill for JSON subobjects that are essentially part of its superobject.
When you have created entities, you can start off with the parsing and translation. Use some JSON framework (starting from iOS5 there's one from Apple) and parse JSON string into object tree, where root item is either an NSArray or NSDictionary, and subelements will be NSArray, NSDictionary, NSNumber, NSString or NSNull.
Go over them one by one in iterational loops and assign according values to your core data entity attributes. You can make use of NSKeyValueCoding here and avoid too much manual mapping of the attribute names. If your JSON attributes are of the same name as entity attributes, you'll be able to just go over all dictionary elements and parse them into attributes of the same name.
Example
My parsing code in the similar situation was as follows:
NSDictionary *parsedFeed = /* your way to get a dictionary */;
for (NSString *key in parsedFeed) {
id value = [parsedFeed objectForKey:key];
// Don't assign NSNull, it will break assignments to NSString, etc.
if (value && [value isKindOfClass:[NSNull class]])
value = nil;
#try {
[yourCreatedEntity setValue:value forKey:property];
} #catch (NSException *exception) {
// Exception means such attribute is not defined in the class or some other error.
}
}
This code will work in trivial situation, however, it may need to be expanded, depending on your needs:
With some kinds of custom mappings in case you want your JSON value be placed in differently named attribute.
If your JSON has sub-objects or arrays of sub-objects, you will need to detect those cases, for example in setters, and initiate new parsing one level deeper. Otherwise with my example you will face the situation that assigns NSDictionary object to an NSManagedObject.
I don't think it is reasonable to dive into these, more advanced matters in scope of this answer, as it will expand it too much.
I suggest you to use this library : https://github.com/TouchCode/TouchJSON
And then if you want to make a factory to parse json and feed your code data, you can use selectors to call methods to fill all your attributes.
Chances are your JSON data gets converted to an NSDictionary or NSArray (or some combination of the two). Simply extract the key/values from the JSON structure and add them to your entity class.
This lib helps me lot
Features
Attribute and relationship mapping to JSON key paths.
Value transformation using named NSValueTransformer objects.
Object graph preservation.
Support for entity inheritance
Works vice-versa
I want to read some data from Core Data for statistics. My datamodel is like in the tutorials found in the internet (a bit of clicking and voilà it is ready). But now I want to work with the data.
My function:
-(int)calcAve {
int ret=0;
ret = [[stats valueForKey:#"aveScore"] intValue];
NSLog(#"%d",ret);
return ret;
}
stats is the object connected from the .xib to the class, which. This object is bound to the entity Stats in the datamodel. [stats entity] returns the correct value.
aveScore is one object in the entity stats (no misspelling, checked it multiple times!). No the error shows me, that stats is not the correct datamodel:
this class is not key value coding-compliant for the key
What is wrong with this? Is there a simple way to read out the data from Core Data?
I am not sure what you managedObject in this case is. However usually you want to store more than one managedObject of the same class in a managedObjectContext. For example several recipes objects in a MOC. Every recipe has the properties.
Your call for value for key sounds reasonable. Check what that class of the stats object is. If it is not of the class NSManagedObject or your custom subclass of it you have a problem. You can only store NSManagedObjects within CoreData. You can check it like this:
NSString *className = NSStringFromClass([stats class]);
NSLog(#"class name: %#",className);
The way you are asking I suggest you try to read the CoreData docs again and look at the Recipes example in Apple's sample code library.
I have a CoreData model with 2 objects. The first object (Images) contains a list of image names. The second object (Book) has a relationship with a field from the Images object called imageId. The data type is set to int16 and is marked optional with a default value of 0, as not every book will have an image.
I request a JSON representation of the Book object from a WCF service. One of the book objects returned has "ImageId" : null as its value. So when converting the JSON object into a managed object in Objective-C I get the following error message:
Unacceptable type of value for attribute: property = "imageId"; desired type = NSNumber; given type = NSNull; value =
How do you handle this conversion? I thought of checking for null and then setting the value to 0 but this doesn't seem correct.
Many thanks.
*UPDATE *
In the process of implementing Daniels solution I discovered that other fields could also be returned from the service as null. Is it therefore possible to modify the NSDictionary category to enable it to traverse itself and replace any instance of with nil? I am using jsonKit as my JSON parser and it seems to return the parsed result as a NSMutableDictionary with NSDictionary objects representing my JSON objects. I have spent much of the day trying to modify my objects but as they are returned as NSDictionaries they are immutable. I could simply use Daniels solution on every field that needed checking for however if the service changed and another field was returned as then this would break my app. Alternatively I could implement Daniels solution on every field just incase it was ever but not sure if thats a good solution especially for performance.
Some JSON libraries let you set an option so that JSON nulls are simply omitted from the dictionary as opposed to using NSNull. If you do that, then it will work because [theDict objectForKey:#"imageID"] will return nil.
But other than that, the right thing to do is look out for NSNull explicitly. Making an NSDictionary category is a handy way to do it:
#implementation NSDictionary (DDNullGetter)
- (id)nonNullObjectForKey:(id)key
{
id val = [self objectForKey:key];
return (val == [NSNull null] ? nil : val);
}
#end
In my quest to update a Core Data model within my iOS project, I'm querying a server for JSON objects that correspond - to some extent - with the managed entities of my model. The end result I'm striving for is a reliable update solution from JSON output.
For the examples in this question, I'll name the core data managed object existingObj and the incoming JSON deserialized dictionary updateDict. The tricky part is dealing with these facts:
Not all properties of the existingObj are present in the updateDict
Not all properties of the updateDict are available in the extistingObj.
Not all types of existingObj's properties match the JSON deserialized properties. (some strings may need a custom Objective-C wrapper).
updateDict may contain values for keys that are uninitialized (nil) in existingObj.
This means that while iterating through the updated dictionaries, there has to be some testing of properties back and forth. First I have to test whether the properties of the updateDict exist in existingObj, then I set the value using KVC, like so:
// key is an NSString, e.g. #"displayName"
if ([existingObj respondsToSelector:NSSelectorFromString(key)) {
[existingObj setValue:[updateDict objectForKey:key] forKey:key];
}
Although this part works, I don't like the fact that I'm actually testing for displayName as a getter, while I'm about to call the setDisplayName: setter (indirectly via KVC). What I'd rather to is something like [existingObj hasWritablePropertyWithName:key], but something that does this I can't find.
This makes for subquestion A: How does one test for a property setter, if you only have the property's name?
The next part is where I'd like to automate the property identification based on their types. If both the updateDict and the existingObj have an NSString for key #"displayName", setting the new value is easy. However, if the updateDict contains an NSString for key #"color" that is #"niceShadeOfGreen", I'd like to transform this into the right UIColor instance. But how do I test the type of the receiving property in existingObj so I know when to convert values and when to simply assign? I was hoping for something along the lines of typeOfSelector:
if ([existingObj typeOfSelector:sel] == [[updateDict objectForKey:key] class]) {
// regular assignment
} else {
// perform custom assignment
}
Of course this is boguscode. I can't rely on testing the type of the existingObj-property's value, for it may be unitialized or nil.
Subquestion B: How does one test for the type of a property, if you only have the property's name?
I guess that's it. I figured this must be a dupe of something that's already on here, but I couldn't find it. Maybe you guys can?
Cheers, EP.
P.S. If you'd have a better way to synchronize custom Objective-C objects to deserialized JSON objects, please do share! In the end, the result is what counts.
If you want to query whether an object has a setter for a given KVC key called key which corresponds to a declared property, you need to check whether it responds to a selector method called setKey: (starts with set, capitalise the first character in key, add a trailing colon). For instance,
NSString *key = #"displayName";
NSString *setterStr = [NSString stringWithFormat:#"set%#%#:",
[[key substringToIndex:1] capitalizedString],
[key substringFromIndex:1]];
if ([obj respondsToSelector:NSSelectorFromString(setterStr)]) {
NSLog(#"found the setter!");
[obj setValue:someValue forKey:key];
}
Two remarks:
Even though properties can have setters with names that do not follow the pattern described above, they wouldn’t be KVC compliant, so it is safe to check for set<Key>: since you’re using KVC to set the corresponding value.
KVC doesn’t use the setter method only. If it doesn’t find a setter method, it checks whether the class allows direct access to instance variables and, if so, use the instance variable to set the value. Also, if no setter method or instance variable is found, it sends -setValue:forUndefinedKey: to the receiver, whose class might have overridden the standard implementation that throws an exception. This is described in the Key-Value Coding Programming Guide.That said, if you’re always using properties, checking for the setter method should be safe.
As for your second question, it is not possible to query the runtime to know the actual Objective-C class of a property. From the runtime perspective, there’s an implementation specific type encoding for properties and general types (such as method parameters/return types). This type encoding uses a single encoding (namely #) for any Objective-C object, so the type encoding of an NSString property is the same as the type encoding of a UIColor property since they’re both Objective-C classes.
If you do need this functionality, one alternative is to process your classes and add a class method that returns a dictionary with keys and corresponding types for every property (or the ones you’re interested in) declared in that class and superclasses, or maybe some sort of description language. You’d have to do this on your own and rely on information not available during runtime.
Short:
I need to find core data objects by a key, which holds a unique immutable array (fixed length, but chosen at runtime) of arbitrary objects (for which not only element membership, but also element order determines uniqueness). NSManagedObject however forbids overriding [isEqual:]. Now what?
Long:
I have an entity (see diagram image for entity "…Link") in my Core Data model for which I have to guarantee uniqueness based on an attribute key ("tuple"). So far so good.
The entity's unique attribute however has to be an NSArray.
And to make things a bit more difficult I neither know the class type of the tuple's elements.
Nor do I know the tuple's element count. Well, actually the count is the same for every tuple (per core data context at least), but not known before the app runs.
There must only ever be one instance of my link entity with a given tuple.
And for obvious reason only ever one tuple instance with a given array of arbitrary objects.
Whereas two tuples are to be considered equal if [tuple_1 isEqual:tuple_n] returns YES. NSManagedObject forbids the overriding of [isEqual:] and [hash] though, otherwise things would be pretty much a piece of cake.
"…Tuple" objects are created together with their array of tokens (via a convenience method) and are immutable (and so is each "…Token" and its data attribute). (think of "…Tuple" as a "…Link"'s dictionary key.)
"…Tuple" implements "- (NSArray *)tokens;", which returnes a neatly ordered array of tokens, based on the "order" keys of "…TokenOrder". (Tuples are expected to contain at most 5 elements.)
I however expect to have tens of thousands (potentially even more in some edge cases) of "…Link" objects, which I have to (frequently) find based on their "tuple" attribute.
Sadly I couldn't find any article (let alone solution) for such a scenario in any literature or the web.
Any ideas?
A possible solution I've come up with so far would be:
Narrow amount of elements to compare
by tuple by adding another attribute
to "…Tuple" called "tupleHash",
which is pre-calculated on
object creation via: Snippet 1
Query with NSPredicate for objects of matching tupleHash (narrowing down the list of candidates quite a bit).
Find "…Link" featuring given tuple in narrowed candidate list by: Snippet 1
Snippet 1:
NSUInteger tupleHash = [[self class] hash];
for (id token in self.tokens) {
tupleHash ^= [token.data hash];
}
Snippet 2:
__block NSArray *tupleTokens = someTokens;
NSArray *filteredEntries = [narrowedCandidates filteredArrayUsingPredicate:
[NSPredicate predicateWithBlock: ^(id evaluatedObject, NSDictionary *bindings) {
return [evaluatedObject.tuple.tokens isEqualToArray:tupleTokens];
}]];
(Sorry, markdown appears to oppose mixing of lists with code snippets.)
Good idea of or just insane?
Thanks in advance!
I strongly suggest that you calculate a hash for your objects and store it in your database.
Your second snippet will seriously hurt performance, that's for sure.
Update:
You don't need to use the hash method of NSArray.
To calculate the hash, you can perform a SHA1 or MD5 on the array values, concatenated. There are many algorithms for hashing, these are just two.
You can create a category for NSArray, say myHash to make the code reusable.
As recommended in a comment by Joe Blow I'm just gonna go with SQLite. Core Data simply appears to be the wrong tool here.
Benefits:
Fast thanks to SQL's column indexing
No object allocation/initialization on SELECT, prior to returning the results. (which Core Data would require for attribute checks)
Easily query link tuples using JOINs.
Easy use of SQLite's JOIN, GROUP BY, ORDER BY, etc
Little to no wrapper code thanks to EGODatabase (FMDB-inspired SQLite Objective-C wrapper)