如何设置核心数据实体之间的关系？

Question

So, I have a core data database with multiple relationship already populated with data. My problem is that I created the relationships between the tables but i don't know how to set them in code. I only need to set one and then I can figure it out for the rest of them. For example: Colleges has the "has Groups" relationship because one college has many groups. The Groupe table has the "bellongsToColegiu" relationship becouse one or more groups are in one college.

I know that, i have to set the ".bellongsToCollegiu = Colegiu?" or ".hasGrupe = Grupe?" but i don't know how to fetch the college in proper way.

For now I have only one college in database.

Can someone help me with this please? I've been knocking my head for a few days and got nothing.

Thank you and have a nice day!

Answer 1

You can fetch a existing 'Colegiu' from the data base or create a new one. for both fetching and creating new one you would need a managed object context. Managed object context is the last part of the data stack which has 3 components comprising of 1. Persistence store, 2. persistence store coordinator, and 3. managed object context.

Here is how to create a stack: https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/CoreData/InitializingtheCoreDataStack.html

Then you can create a new object:

let newItem = NSEntityDescription.insertNewObject(forEntityName: 'Colegiu',
                                                          into: 'reference to managed object context here')

or you can search the existing one by calling fetch on managed object context,

Keep in mind CoreData is a complicated topic and it takes time to put the head around. All the best.

Answer 2

When you create entities and relationship between them Xcode also generates code for handling those relationships. The name pattern for code that handles relationships are

1. The same as the name of the relation for to-one relationships
2. addToX and removeFromX (x is the relationship) for to-many relationship.

So you should have for instance addToHasGrupe(_ value: Grupe) on College and that you can add a group instance to and Grupe should have a belongsToColegiu property that you can set to a college instance. Autocompletion should help you.