Question

In a database I have a following situation, A has many B‘s and C also has many B‘s.
What is the effective way of using Core Data relationships to search for this type of query?

1. I need to search on both a.x and c.y attributes
2. then I need those B‘s which are common to both

For example:

records are separated by colon (";"), and attributes by comma (",")

A = {a;b}
C = {m;n}
B = {1,a,m;2,a,n;3,a,n;4,b,m;5,b,m;6,a,m;7,b,n;8,b,n}

Will the queries c.x = m and a.y = a result in following records from B = {1;6}?

Answer 1

Technical note: Core Data isn’t a relational database, so doesn’t really have ‘joins’. It’s more accurately described as an object graph.

It should be quite straightforward to implement what you want:

Set up a model with entities A, B, C.

B has a ‘to-many’ relationship to A (this property called a), and a ‘to-many’ relationship to C (this property called c).

Populate this model with data as appropriate.

Then to get your ‘join’, search using a predicate as follows:

NSFetchRequest* fetchRequest = [[NSFetchRequest alloc] init];

NSEntityDescription *entity = [NSEntityDescription entityForName:@"B" inManagedObjectContext:del.managedObjectContext];
[fetchRequest setEntity:entity];

NSString* predicateString = [NSString stringWithFormat:@"a.x = %@ AND b.y = %@", 
                        @"something1", @"something2"];

fetchRequest.predicate = [NSPredicate predicateWithFormat:predicateString];

NSError *error = nil;
NSArray* objects = [del.managedObjectContext executeFetchRequest:fetchRequest error:&error];

See also iPhone CoreData join.