Ajax call for JSON using EF Core is failing/aborting - asp.net-core

Question

I'm at a loss with why this isn't working...
I have a .NET Core app using EF Core, and I'm making an Ajax call via jQuery to my controller to retrieve some data from the database via EF Core. Debugging the call via my developer tools in the browser (IE/Chrome) results in a status of failed/aborted. Yet when I step through my method in my controller, the method seems to be able to retrieve the data from the database via EF Core just fine.
Here's my controller:
public ActionResult GetInfo(string term)
{
using (var dbContext = new DatabaseContext())
{
// use DbContext to get data from the database
var retrievedData = dbContext.TableName.Where(...);
return Json(retrievedData.Select(data => new {
id = data.id,
text = data.text
}));
}
}
And here's the jQuery:
$(#element).select2({
...
ajax: {
url: $(#element).attr("data-getinfo"),
dataType: 'json', // tried this with jsonp and application/JSON with no luck
contentType: 'application/json; charset=utf-8',
delay: 250,
data: function (params) {
return: { term: params.term};
},
processResults: function (data) {
return {
results: $.map(data, function (item) {
return {
id: item.id, text: item.text
}
})
}
},
},
....
});
The Ajax calls work with previous apps I've worked on, but they used MVC 5 and EF 6. This also works if I retrieve dummy data, IE instead of using EF Core to get the data, I return fake data built into the controller. What gives?

Answer 1

To clarify the root of your problem: You are querying your database and returning an IEnumerable as a JsonResult. But first, you need to understand one step before. Calling .Where returns an IQueryable. You can think of an IQueryable as if it is a TSQL command that was not yet execute on the database. Only calls that will enumerate the results will trigger the materialization of the query.
So you did this:
// .Where returns an IQueryable. You can "chain" more wheres later.
// the query will still not be executed
var retrievedData = dbContext.TableName.Where(...);
// This then returns an IEnumerable to the client.
// The Select will materialize (execute) the query
return Json(retrievedData.Select(data => new {
id = data.id,
text = data.text
}));
The problem with your code is: .Select returns an IEnumerable which enumerates the results. But, by the time the browser or whatever client you are dealing with starts to enumerate the results, your database connection is already closed, because you used using block around your dbContext (which is kind of correct.. see comments in the end).
So, to fix it, you need basically to enumerate the results yourself or not close the connection (let the framework close for you when the request is finished..). This minor change fix the problem:
// ToList() will enumerate all the results in memory
var retrievedData = dbContext.TableName.Where(...).ToList();
Other comments:
You don't need (also shouldn't) manage the creation of the dbContext by yourself. You can register it in the DI container and the framework will do the rest for you. You can take a look in the EF Core docs to have an idea on how it is done.

Answer 2

Not an ideal solution, but I got it working. I suspect it might have to do with how .NET Core or EF Core was returning data to the browser, but I can't say for sure yet.
I ended up using Json.NET for a workaround. Performance isn't bad (I tried a query with hundreds of records and it only took a couple of seconds at most), and I was already using it for an external API call.
public ActionResult GetInfo(string term)
{
using (var dbContext = new DatabaseContext())
{
// use DbContext to get data from the database
var retrievedData = dbContext.TableName.Where(...);
var initJson = Json(retrievedData.Select(data => new {
id = data.id,
text = data.text
}));
var serializedJson = Newtonsoft.Json.JsonConvert.SerializeObject(initJson);
var deserializedJson = Newtonsoft.Json.JsonConvert.DeserializeObject(serializedJson);
return Json(deserializedJson);
}
}