"Unknown Error Unable to save background context" when fetching array of entries with injected primary keys.

[vkrikun]

Hello!

I need to fetch from server array of objects without unique identifiers. My API is:
GET /measurements
[{"bpm": 76,"timestamp" : "2014-01-02T19:20:30+01:00"}, {"bpm": 76,"timestamp" : "2014-01-02T19:20:35+02:00"}, ...]

I don't need to persist that data so I use temporary context (as you suggested in one of your previous answers). When server returns 1-5 objects - it works perfectly, but when response contain more then 5 records I get error:
Error Domain=com.mutualmobile.mmrecord Code=5 "Unknown Error Unable to save background context. Import operation unsuccessful." UserInfo=0x10d14d030 {NSLocalizedDescription=Unknown Error Unable to save background context. Import operation unsuccessful.}

I made few experiments:

1. I tried to add primary key to may data model and inject it with entityPrimaryKeyInjectionBlock. Key injection works, but the same error when more then 5 entries.

2. I took model scheme from 1), but instead of injecting keys I modified server to return id-s in response - everything works fine.

3. I thought it's something wrong with the rest of my app so I created very simple test project based on MMRecordFoursquare: AFNetworking 2.0, MMDataManager copied from example, one entity in data Model. The same experience.

Looks like I'm missing something very obvious here...

[cnstoll]

Hi there,

Looks like we need to update that error message, because that is definitely not describing the actual issue you're facing.

That error gets posted when there is a Core Data save operation error. That means that the save method on the managed object context actually failed.

Put a breakpoint on line 767 in MMRecord.m (inside the if statement that handles that error condition) and check what the error returned by core data is. It could easily be some sort of merge conflict, or a validation error. One time I saw this happen in another project where the value of a property was required, but wasn't being returned in the API so it was nil.

Let me know if that helps. For now, I've added this issue to the current milestone so that we can update that error message.

Thanks,

• Conrad

[cnstoll]

Also, good to know that the entity primary key injection works :)

[vkrikun]

Hello Conrad, thanks for your help! You were right and it was mistake in data model.

However I faced with another issue: MMRecord crashes when app injects non-unique primary key within one request. I would expect that library ignore entries with duplicated primary key or overwrite previous record with the same key. It looks like mistake in API design or server response, but when you compute primary key based on entry data it's difficult to "guaranty" it's uniqueness.

My app crashes in MMRecordResponse, - (NSArray *)recordsFromObjectGraph when nil record added to records array.

[cnstoll]

Interesting. Let me look into that. That would be an easy thing to fix the crash but you're right that it shouldn't really be happening in the first place.

[cnstoll]

I tried just very naively assigning every primary key to a specific string using that block and I was unable to reproduce the crash. Is there something else going on here that I'm missing?

[cnstoll]

Opened a pull request for improving the core data error handling flow.

#59

[cnstoll]

Closing this issue. Tracking error handling in a separate issue.

[cnstoll]

@vkrikun if you have a moment, please let me know what you think about this error handling process in this other PR: https://github.com/mutualmobile/MMRecord/pull/59/files