How do I add a foreign key to my entity in Room 2.3.0?

Question

I have a project that I have worked with Room 2.2.5, I just updated to version 2.3.0 this is the code of an entity called photo:

    @Entity(tableName = Photo.TABLE_NAME, foreignKeys = @ForeignKey(entity = Event.class,
        parentColumns = "_id",
        childColumns = "id_event_ft",
        onDelete = ForeignKey.CASCADE))
public class Photo {

    public static final String TABLE_NAME = "Photo";
    public static final String COLUMN_ID = BaseColumns._ID;

    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(index = true, name = COLUMN_ID)
    public Long id_photo;

    @ColumnInfo(name = "path")
    private String path;

    @ForeignKey(entity = Event.class,
            parentColumns = "_id",
            childColumns = "id_event_ft",
            onDelete = ForeignKey.CASCADE)
    private Long id_event_ft;

    public Photo(Long id_photo, String path, Long id_event_ft) {
        this.id_photo = id_photo;
        this.path = path;
        this.id_event_ft = id_event_ft;
    }

    public Long getId_photo() {
        return id_photo;
    }

    public void setId_photo(Long id_photo) {
        this.id_photo = id_photo;
    }

    public String getPath() {
        return path;
    }

    public void setPath(String path) {
        this.path = path;
    }

    public Long getId_event_ft() {
        return id_event_ft;
    }

    public void setId_event_ft(Long id_event_ft) {
        this.id_event_ft = id_event_ft;
    }
}

Now I am getting the following error when trying to compile

error: annotation type not applicable to this kind of declaration @ForeignKey(entity = Event.class, parentColumns = "_id", childColumns = "id_event_ft", onDelete = ForeignKey.CASCADE) ^

The error is in the @ForeignKey that is above the variable private Long id_event_ft;

In the documentation I found this:

Added missing target to @ForeignKey annotation preventing its usage outside of the @Entity annotation. (Iced1e)

It is clear that using @ForeignKey outside of the @Entity annotation is no longer allowed, but then how do I bind the id_event_ft variable to the foreign key?, How do I assign a value to it now?

I hope someone can help me, thank you very much

Answer 1

Using ForeignKey does not automatically (magically) make a relationship. Rather it allows a relationship to be supported primarily by enforcing referential integrity.

• There is no need for a ForeignKey definition for a Foreign Key (relationship) to exist.

That is it is defining a rule that says that the value of the child column (id_event_ft) MUST be a value that is present in the parent column (_id). It also supports handling if there is a Foreign Key Conflict (e.g. onDelete as you have used).

Actually providing a suitable value is something that you have to do programmatically, that is id adding a photo you have to determine which Event the photo is to be linked/related to.

You can use @Relation to simplify extracting related data.

So consider the following:

An Event Entity (nice and simple for demonstration)

@Entity
public class Event {
    @PrimaryKey(autoGenerate = true)
    Long _id = null;
    String other_columns;

    public Event(){}
    @Ignore
    public Event(String other_columns) {
        this.other_columns = other_columns;
    }
}

• A Photo's parent column will be the _id column.
• Second Ignored (i.e. Ignored by Room) constructor otherwise Room issue a warning like *warning: There are multiple good constructors and Room will pick the no-arg constructor. *

A slightly changed Photo Entity :-

@Entity(tableName = Photo.TABLE_NAME,
        foreignKeys = @ForeignKey(
                entity = Event.class,
                parentColumns = "_id",
                childColumns = "id_event_ft",
                onDelete = ForeignKey.CASCADE),
        indices = @Index("id_event_ft") //<<<<<<<<<< ADDED as Room warns if omitted
)
public class Photo {

    public static final String TABLE_NAME = "Photo";
    public static final String COLUMN_ID = BaseColumns._ID;

    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(index = true, name = COLUMN_ID)
    public Long id_photo;
    @ColumnInfo(name = "path")
    private String path;
    /* <<<<<<<< COMMENTED OUT >>>>>>>>>>
    @ForeignKey(entity = Event.class,
            parentColumns = "_id",
            childColumns = "id_event_ft",
            onDelete = ForeignKey.CASCADE)

     */
    private Long id_event_ft;

    public Photo(Long id_photo, String path, Long id_event_ft) {
        this.id_photo = id_photo;
        this.path = path;
        this.id_event_ft = id_event_ft;
    }

    public Long getId_photo() {
        return id_photo;
    }

    public void setId_photo(Long id_photo) {
        this.id_photo = id_photo;
    }

    public String getPath() {
        return path;
    }

    public void setPath(String path) {
        this.path = path;
    }

    public Long getId_event_ft() {
        return id_event_ft;
    }

    public void setId_event_ft(Long id_event_ft) {
        this.id_event_ft = id_event_ft;
    }
}

For demonstration of retrieving via a relationship the POJO EventWithPhotos :-

public class EventWithPhotos {

    @Embedded
    Event event;
    @Relation(entity = Photo.class,parentColumn = "_id",entityColumn = "id_event_ft")
    List<Photo> photos;
}

Now a Dao AllDao:-

@Dao
interface AllDao {

    @Insert
    long insert(Event event);
    @Insert
    long insert(Photo photo);
    @Transaction
    @Query("SELECT * FROM event")
    List<EventWithPhotos> getAllEventsWithPhotos();
}

How do I assign a value to it now?

Now an example that puts it all together adding 2 Events the first with 2 photos, the second with 1 photo. Note the different techniques used:-

    dao = db.getAllDao();

    // Prepare to add an Event
    Event newEvent = new Event();
    newEvent.other_columns = "Event1";
    // Add the Event retrieving the id (_id column)
    long eventId = dao.insert(newEvent);
    // Prepare a photo to be added to Event1
    Photo newPhoto = new Photo(null,"photo1",eventId);
    // Add the Photo to Event1
    long photoid = dao.insert(newPhoto);
    // Add second photo to Event 1 using the 2nd constructor
    dao.insert(new Photo(null,"photo2",eventId));
    // Add Event2 with a photo all in a single line (again using the 2nd constrcutor)
    long event2Id;
    dao.insert(new Photo(null,"photo3",event2Id = dao.insert(new Event("Event2"))));

    // Get and output Each Event with the Photos for that Event
    List<EventWithPhotos> allEventsWithPhotosList = dao.getAllEventsWithPhotos();
    for (EventWithPhotos ewp: allEventsWithPhotosList) {
        Log.d("EVENTPHOTOINFO","Event is " + ewp.event.other_columns);
        for (Photo p: ewp.photos) {
            Log.d("EVENTWITHPHOTO","\tPhoto is " + p.getPath() + " ID is " + p.getId_photo());
        }
    }

Result

When run the log contains :-

D/EVENTPHOTOINFO: Event is Event1
D/EVENTWITHPHOTO:   Photo is photo1 ID is 1
D/EVENTWITHPHOTO:   Photo is photo2 ID is 2
D/EVENTPHOTOINFO: Event is Event2
D/EVENTWITHPHOTO:   Photo is photo3 ID is 3

The Database (view with Database Inspector) shows:-

The Event table :-

The Photo table :-

Answer 2

Relations

@Many To One

if the relation is many photo for one event, use ORM Hibernate, this is the easiest way to define foreign key with constraint on your Photo Entity

@ManyToOne
@JoinColumn(name = "id", nullable = false)
private Event id_event_ft;