等效核心数据联合查询

Question

I want to get the union of data from different entities. I have a number of entities (different kinds of tags eg location, events etc) and I want data for a table view that shows "All Tags" (ie the union of all tag entities). How do I make a fetch request with Core Data for this kind of a use case?

I know that Core Data is not an ORM but if my explanation above was not good enough, I will explain the corresponding database use case. I have different tables eg events, locations, people etc and I would like to UNION the results from these different tables. Remember that a UNION concatenates the rows and not columns.

One obvious solution is to get the data from the different entities separately and then just concatenate together the NSMutableArrays. I am wondering if there is a more efficient way.

Answer 1

There is no way to fetch more then one entity type in a fetch request ...

UNLESS, the entities you like to 'Union' have the same base class in your model (other than NSManagedObject ).

So if all your entities were to inherit from a base class named Tag for example, you would be able to fetch all of them together.

This however, will create a unified table for all these entities (a very sparse table if the intersection between the classes is small) in the actual database file.

In your case this might not be feasible as there is no real connection between 'Person' and 'Location' for instance, or you might decide that this will cause a performance issue.

The other solution will be (as you suggested) to create a fetch request for each entity.