领域：如何仅查询一对多关系中父亲的对象

Question

I have the following problem:

Father containing a RealmList<Kid> (with Kid being another RealmObject)

I would like to query a list of all Kid s belonging to Father , without querying Father itself, via:

Realm.getDefaultInstance().where(Kid.class).equalTo("fatherId", id);

But I'm pretty certain that this is not possible, as Kid has no reference to Father .

Do I have to query the Father object and then call the getKids() method?

If you're wondering why I'm not simply querying Father : I have a generic method for querying, querying any class given to it:

public <T extends RealmObject> List<T> getForFather(Class<T> clazz, String fatherId)

Most of the classes queried belong to Father and I want to avoid if-else statements. I also would like to avoid multiple methods doing the same thing, just with another class. Above generic method would work, but only, if there's a way to query Kid for a specific Father .

Answer 1

For your case, you have to use the conventional method of design the ER for the database.

That is, Instead of having RealmList<Kid> in your father object create a primaryKey in the father and have the reference of that primary key in child fatherPrimaryKey

then you can simply query

RealmResults<Kid> result = realm.where(Kid.class).equals("fatherPrimaryKey", "some_key").findAll();

And while storing the data in the kid object, make sure you pass the correct value of father's primary key to the child as reference.