Relations in Room Persistence Library

Question

I wanted to use the @Relation annotation in an Android Project. Here I have found a description about that topic:

@Entity
public class User {
    @PrimaryKey
    public int id; // User id
}


@Entity
public class Pet {
    @PrimaryKey
    public int id;     // Pet id
    public int userId; // User id
    public String name;
}

public class UserWithPets {
   @Embedded
   public User user;

   @Relation(parentColumn = "id", entityColumn = "userId", entity = Pet.class)
   public List<Pet> pets;
}

My question would be: The parentColumn argument is id . But which id is it ? Is it the id of User or the id of Pet ? In the documentation it says that the parentColumn() is a reference column in the parent POJO. But which one is the parent POJO ?

Answer 1

In this scenario, a relation table is created under the hood where User column is the parent and Pet column is the child. The link between them is done by the id of User which is also present in Pet as userId.

What Room is saying here is :-- "give me all the pets whose userId is matching the current id from User entity" and then it repeats it for all the ids from the User entity.

I'd refer you to this link https://medium.com/androiddevelopers/database-relations-with-room-544ab95e4542 for a better understanding of the updated concept.