Question

I’m working with Core Data and a many-to-many relationship: a building can have multiple departments, and a department can be in multiple buildings. Having worked with databases before, I was unsure of how to implement this in Core Data, but I found this in the Core Data Programming Guide:

If you have a background in database management and this causes you
concern, don’t worry: if you use a SQLite store, Core Data
automatically creates the intermediate join table for you.

However, there’s not really any data integrity. I’ve tried inserting a few building objects, which for now only have one attribute (number), and every time I set the department object (relationship) it relates to. This results in the database containing multiple building objects with the same building number, all relating to a different department object. Ideally, there would be one object per building number, with in it all the different departments that are located in it.

So, my question is: can Core Data maintain data integrity somehow, or should I check to see if a building object with that number already exists before inserting it? It looks like I’ll have to manually check it, but it would be cool if Core Data could do this for me.

Answer 1

What melsam wrote is right. In addition to his answer I suggest you to use inverse relationships. About inverse, Apple says:

You should typically model relationships in both directions, and
specify the inverse relationships appropriately. Core Data uses this
information to ensure the consistency of the object graph if a change
is made (see “Manipulating Relationships and Object Graph Integrity”).
For a discussion of some of the reasons why you might want to not
model a relationship in both directions, and some of the problems that
might arise if you don’t, see “Unidirectional Relationships.”

A key point to understand is that when you work with Core Data, you work with objects. So, integrity criteria are resolved when you save the context or you explicity says to context to process also process pending changes (see processPendingChanges method).

About your question, I guess you have to create a fetch request and retrieve the object(s) you are looking for (e.g. you could provide to each object a specific id and set up a predicate with the id you want).
If the fetch request retrieve some objects, then you can update them. If not, create a new object with insertNewObjectForEntityForName:inManagedObjectContext:. Finally save the context.

I suggest you to read about Efficiently Importing Data.

Hope it helps.