What's better way to build NSPredicate with to-many deep relationships?
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18-09-2019 - |
Question
I have three entities: EntityA, EntityB and EntityC connected with to-many relationships.
See schema for details:
alt text http://img706.imageshack.us/img706/9974/screenshot20091220at124.png
For getting all instance of EntityA which depend from EntityB.name I use the predicate like this:
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"ANY EntityB.name like 'SomeName'"];
What should be predicate for getting all instance of EntityA which depend from EntityC.name?
I tried query like @"ANY EntityB.entitiesC.name like 'SomeName'"
but get exception "multiple to-many keys not allowed here"
.
Best regards,
Victor
Solution
My final solution is to use SUBQUERY.
NSPredicate *p = [NSpredicate predicateWithFormat:@"(name like %@) AND (0 != SUBQUERY(entitiesB, $x, (0 != SUBQUERY($x.entitiesC, $y, $y.name like %@).@count)).@count)", nameA, nameC];
Unfortunately I was unable to expand this query on nsExpression objects.
OTHER TIPS
While I was stopped at the following decision:
First, I get all the EntityC that satisfy the condition EntityC.name equal to 'SomeName'
NSPredicate *p = [NSPredicate predicateWithFormat:@"name like %@", @"SomeName];
...
NSArray *res = [managedObjectContext executeFetchRequest:fetchRequest error:&error];
Then I get an array of EntityB from above query
NSArray *parentBs = [res valueForKeyPath:@"@distinctUnionOfObjects.parent"];
Than get array of EntityB that satisfy the condition EntityB.EntitiesC.name equal to 'SomeName':
NSExpression *leftExpression = [NSExpression expressionForEvaluatedObject];
NSExpression *rightExpression = [NSExpression expressionForConstantValue:parentBs];
NSPredicate *p = [NSComparisonPredicate predicateWithLeftExpression:leftExpression rightExpression: rightExpression modifier:NSDirectPredicateModifier type:NSInPredicateOperatorType options:0];
I repeat the same for EntityA.
The effectiveness of this solution in doubt and I still expect a better solution for this problem.