How can I use NSPredicate to filter on core data r

问题:

Say I have core data objects of type "obj" that has a property "propertyA" and a one-to-many relationship with an object of type "sub" that has two properties, "propertyB" and "propertyC".

I want to fetch all the objs that have propertyA equal to a value and a sub obj with propertyB and propertyC set.

If it was just propertyA and propertyB, I would do

[NSPredicate predicateWithFormat:@"ANY sub.propertyB = %@ AND propertyA == %@", ...];

The problem is that I can't figure out how to add in the second property. I want only the objs that have at least one sub that has the two properties true. I've tried the following, but it doesn't work:

[NSPredicate predicateWithFormat:@"ANY (sub.propertyB = %@ AND sub.propertyC) AND propertyA == %@", ...];

I've tried it without the ANY but that doesn't work either. How can I do this?

回答1:

Since you have a to-many relationship with the sub object, the subs property of obj returns a set instead of a single object. To query the set, you need to use a SUBQUERY.

Subqueries have the form:

SUBQUERY(collection, $individualCollectionItem, expression-with-collection-item)

in this case you would want something like

SUBQUERY(subs,$s,$s.propertyB==%@) AND SUBQUERY(subs,$s,$s.propertyC!=NULL)

回答2:

The solution seems to be:

[NSPredicate predicateWithFormat:@"propertyA == %@ AND (SUBQUERY(sub, $s, $s.propertyB == %@ AND $s.propertyC == %@).@count != 0)",  propertyAvalue, propertyBvalue, propertyCvalue];

where the values at the end are the values you want the various properties to be equal to.