Question

Normally if you have a 1 to many relationship in Core Data I understand that you should set that up as a relationship in the data model.
In this case, it is difficult to do because of the origin and management of the data.

I’m trying to essentially accomplish a join.

I’d like to fetch an entity A which meets some criteria on A but also meets a criteria on B.code and another attribute.

select statement would be

select attributeFromA from A, B where A.code = B.code and B.attrib="foo"

Is there a reasonable way to accomplish this without creating a relationship in core data?

Answer 1

I’ve only found two solutions, neither very good.
From what I’ve read, Core Data does not support a query against multiple entities unless they have a relationship between them.

1. Add a relationship anyway. This can be particularly bad since the data is coming from a server. No way to easily maintain relationships when individually updating each table from the server. Need to recreate relationships when data changes.

2. Manually perform the join outside of Core Data. In the above case, the intent is to get the set of object identifiers (‘code’) that match. One way to do that is to perform separate queries then get the intersection. Setup each query to only retrieve ‘code’, not managed objects.