CoreData获取嵌套对象

Question

I have a json which contain objects where each object have child object, for example

[
    {
        "id": 1,
        "name": "obj1"
    },
    {
        "id": 2,
        "name": "obj2"
    },
    {
        "id": 3,
        "name": "obj3",
        "child": {
            "id": 2,
            "name": "obj2"
        }
    },
    {
        "id": 4,
        "name": "obj4",
        "child": {
            "id": 5,
            "name": "obj5"
        }
    }
]

So in core data i have entity MyObject which has relation to child (to itself, to one), and relation to parent (to many)

So when i try fetch object by predicate like

[NSPredicate predicateWithFormat:@"parent.@count = 0"]

I got only object with id in [1,3,4], but no with id = 2 (here error) and id = 5 (here all correct), because it's was sets parent when it's mapped when object with id = 3 save to data base.

If no use predicate i got 5 object, include obj5.

But i need fetch from coredata exactly count of object what i got from json.

I need fetch only object with id in [1,2,3,4].

How do i need write predicate, if it's possible?

Answer 1

The data you got from JSON is a subset of all your objects, but the logic according to which it is selected is not persisted in your object model.

You persist the relationship between child and parent (hint: for readability, rename it as parents if it is to-many), but the selection in your JSON example is not based on any of those criteria.

One easy way to get all the objects is to extract the top level ids delivered by the JSON feed and use that in the predicate:

NSArray *ids = [jsonArray valueForKey:@"id"];
[NSPredicate predicateWithFormat:@"idNumber in %@", ids];

(Note that I changed your attribute name from id to idNumber in order to avoid possible language quirks, as id is a reserved word.)