I am using Room in my Android App (Java) and there I have two entities with a 1:many relation.

Lens entity One lens can have multiple wears.

@Entity(tableName = "lens_table")
public class Lens {

    @PrimaryKey(autoGenerate = true)
    private int lensId;
    private String name;
}

Wear entity One wear can only relate to one lens.

@Entity(tableName = "wear_table",
        foreignKeys = {@ForeignKey(
                entity = Lens.class,
                parentColumns = "lensId",
                childColumns = "fk_lensId",
                onDelete = ForeignKey.CASCADE)},
        indices = {@Index("fk_lensId")})
public class Wear {

    @PrimaryKey(autoGenerate = true)
    private int wearId;
    private String name;
    private int fk_lensId;
}

So far so good. I am fine with the "standard" queries (create, get all, update, delete,...) so far, where a lot of documentation is around. I also was successful implementing the query to get all lenses with their wears based on below relation.

public class LensWithWears {

    @Embedded
    public Lens lens;

    @Relation(
            parentColumn = "lensId",
            entityColumn = "fk_lensId"
    )
    public List<Wear> wears;
}

But now I need to query the following information:

Get a single wear with the associated lens by looking up the wearId

The relationship class I am currently using looks like following:

public class WearWithLens {
    @Embedded
    public Wear wear;

    @Relation(
            parentColumn = "wearId",
            entityColumn = "lensId"
    )
    public Lens lens;
}

And the Dao Query looks like that:

@Query("SELECT * FROM wear_table WHERE wearId = :wearId LIMIT 1")
LiveData<WearWithLens> getWearWithLensByWearId(int wearId);

My code obviously does not work, otherwise I would not ask... The problem is, that an object WearWithLens is returned, but the lens object in it is always null.

In other words, I would like to query a Wear, which has a 1:1 relation to a lens and get both objects together in the class WearWithLens.

Can somebody tell me how the query should look like?

Thanks!

CodePudding user response：

The parent column needs to be the column that forms the relationship between the two. That is it should be the fk_lensId column.

So :-

public class WearWithLens {
    @Embedded
    public Wear wear;

    @Relation(
            parentColumn = "fk_lensId",
            entityColumn = "lensId"
    )
    public Lens lens;
}

As an example

• not using LiveData<WearWithLens> getWearWithLensByWearId(int wearId); but instead WearWithLens getWearWithLensByWearId(int wearId); for convenience and brevity.
• using your classes/entities with getters and setters added and also additional constructors to reduce the coding
• and obviously the @Relationship as above.

With the following :-

    db = TheDatabase.getInstance(this);
    dao = db.getAllDao();

    int l1id = (int) dao.insert(new Lens("Lens1"));
    int l2id = (int) dao.insert(new Lens("Lens2"));
    int l3id = (int) dao.insert(new Lens("Lens3"));

    dao.insert(new Wear("Wear1 child of Lens1",l1id));
    dao.insert(new Wear("Wear2 child of Lens1",l1id));
    dao.insert(new Wear("Wear3 child of Lens1",l1id));
    dao.insert(new Wear("Wear4 child of Lens2",l2id));
    dao.insert(new Wear("Wear5 child of Lens2",l2id));
    dao.insert(new Wear("Wear6 child of Lens2",l2id));
    dao.insert(new Wear("Wear7 child of Lens3",l3id));

    for (Wear wear: dao.getAllWears()) {
        WearWithLens currentWearWithLens = dao.getWearWithLensByWearId(wear.getWearId());
        Log.d("DBINFO","Current Wear is "   currentWearWithLens.wear.getName()   " parent Lens is "   currentWearWithLens.lens.getName());
    }

The Result is:-

2021-12-09 06:34:58.105 D/DBINFO: Current Wear is Wear1 child of Lens1 parent Lens is Lens1
2021-12-09 06:34:58.110 D/DBINFO: Current Wear is Wear2 child of Lens1 parent Lens is Lens1
2021-12-09 06:34:58.112 D/DBINFO: Current Wear is Wear3 child of Lens1 parent Lens is Lens1
2021-12-09 06:34:58.114 D/DBINFO: Current Wear is Wear4 child of Lens2 parent Lens is Lens2
2021-12-09 06:34:58.115 D/DBINFO: Current Wear is Wear5 child of Lens2 parent Lens is Lens2
2021-12-09 06:34:58.116 D/DBINFO: Current Wear is Wear6 child of Lens2 parent Lens is Lens2
2021-12-09 06:34:58.120 D/DBINFO: Current Wear is Wear7 child of Lens3 parent Lens is Lens3