The best way to save a dictionary containing a bunch of basic data objects?


I wish to save a dictionary containing some Core Data objects (bunch of different entities). The objects also have quite a few relationships (and inverse relationships) defined. What would be the best way to go about it?

I tried using NSKeyedArchiver and writing to a file. While that works great, when trying to read from the file using NSKeyedUnarchiver, it fails on one of the classes with the error

-[SomeEntity initWithCoder:]: unrecognized selector sent to instance

EDIT - More details

I have a bunch of objects, each having properties based on which they can be filtered. The properties are in themselves Core Data entity objects since they have a complex structure.

I wish to save the filters the user has selected so that the next time they view the objects, the objects can be filtered as per their previous selection.

Say there are 3 filters, Filter A, B and C and each can have 5 different values. Now the user might select Filter A1, A2, B1 and C3 (or a different combination). My question, how do I save these selected filters (A1, A2, B1 and C3 in this case) using Core Data?

Let me see if I understand your question: You have a collection of managedObjects that are already saved in a context. They may already be persisted in the SQL database. You want to save that collection ALSO to another file for other purposes. You have already considered saving the information of this collection inside core-data in some way and have already rejected it. You have also considered simply saving the query generation tokens to save the state of the database as it currently is, but that also is not what you want. The point is to have a file that contains a copy of some of the managedObjects organized in a way that you can get the data back without using the SQL database that was already designed exactly for that purpose.

Solution 1: Turn each managed object in a dictionary.

You can get every attribute and every property of every object by getting a managed object's entity and then accessing the attributesByName and relationshipsByName property of the entity. From there you make a simple loop to put each property into a dictionary. I also suggest you store the objectID and point to the objectID when encoding the relationships. Then replace the managedObject in your dictionary with dictionary that contains all the attributes and relationship. This new dictionary should be easy to archive and unarchive.

This make sure that the data when you unarchive is exactly how you left it. When you unarchive you will get a COPY of data and if the managed objects have changed in your database since then, you will get the OLD values. Also these copies are not core-data object because they are unconnected to a managed Object Context.

Solution 2: Just save the Managed Object's ObjectId.

Replace every managed object in your collection with the object's objectId. This dictionary can be easily archived. When you unarchive it replace every objectId with a core data object (if found) using existingObjectWithID: on the context. If entities have been deleted then you won't get them back. If entities have changed then you will get the NEW values.

Solution 3: Don't do any of this

It seems to me that you may not be aware core-data are already saved in a database. If you have some collection of managedObjects, you should be able to recreated it from your database. If you aren't able to, then you should add properties and/or relationships that will allow you to so.