领域java查询条件

Question

Assume the next situation:

//I don't put the getters and setters, but assume they are there
public class User extends RealmObject {
    private RealmList<Dog> dogs;
}

public class Dog extends RealmObject {
    //UPDATE: I've added the variable city, to make my question more exact
    private String city;
    private String color;
    private String name;
}

Assume: Person 1 has dogs: NY-white-Joe Person 2 has dogs: NY-brown-Mary, SF-white-Fluffy, LA-brown-Fluffy Person 3 has dogs: NY-brown-Fluffy, LA-white-Pepito

Question: How can I query all the persons that have a brown dog called Fluffy?

What I have tried using the implicit AND:

RealmQuery<User> userQuery = realm.where(User.class).equalTo("dogs.color", "brown").equalTo("dogs.name", "Fluffy");

Then I have read the documentation and the two equalTo() conditions are evaluated separately, that means that I will have:

All the users that have a brown dog AND a dog that is called Fluffy. (So the results are P2, P3).

How should I write this query to apply the conditions to the same dog?

Answer 1

It seems to me the best approach is to use query by primary key. I mean first adding primary key to Dog class:

class Dog extends RealmObject {
       @PrimaryKey
       private int id;
       private String color;
       private String name;
}

Then first step to find users that have a brown dog called Fluffy, is to find primary key of such a dog. So we making a query to find exact Dog :

Dog dog = realm.where(Dog.class).equalTo("color", "brown").equalTo("name", "Fluffy").findFirst();

After that we are searching for users that have a dog with defined primary key (id field):

RealmResults<User> users = realm.where(User.class).equalTo("dogs.id", dog.getId()).findAll();

Answer 2

The link queries in Realm are existential quantifiers ( https://en.wikipedia.org/wiki/Existential_quantification ). That implies that if a condition is fulfil for just one object in the child class, the object in the parent class is added to the RealmResults .

Link query example shows how the existential quantifiers will work. Currently, you will have to iterate User s and query the Dogs list individually.

There is an open issue on reverse lookups.