Android with Room - How to set a foreign key nullable

问题

I'm using Room in my Android app. One table (stock_move) contains several Foreign keys. In some case, I need to insert a stock_move without a FK (locationDestId). In this case, SQLite raise an error:

io.reactivex.exceptions.OnErrorNotImplementedException: foreign key constraint failed (code 19)

There is my entity:

@Entity(tableName = "stock_move",
    foreignKeys = {
            @ForeignKey(
                    entity = LocationEntity.class,
                    parentColumns = "id",
                    childColumns = "location_id",
                    onDelete = CASCADE

            ),
            @ForeignKey(
                    entity = LocationEntity.class,
                    parentColumns = "id",
                    childColumns = "location_dest_id",
                    onDelete = CASCADE
            ),
            @ForeignKey(
                    entity = ProductEntity.class,
                    parentColumns = "id",
                    childColumns = "product_id",
                    onDelete = CASCADE
            ),
            @ForeignKey(
                    entity = UomEntity.class,
                    parentColumns = "id",
                    childColumns = "uom_id",
                    onDelete = CASCADE
            )
    }
)
public class StockMoveEntity {
    @PrimaryKey(autoGenerate = true)
    private int id;

    private String name;

    @ColumnInfo(name="product_id")
    private int mProductId;

    @ColumnInfo(name="uom_id")
    private int mUomId;

    @ColumnInfo(name="location_id")
    private int mLocationId;

    @ColumnInfo(name="location_dest_id")
    private int mLocationDestId;

    private int mProductQty;

    public StockMoveEntity(int id, String name, int productId, int uomId, int locationId, int locationDestId, int productQty) {
        this.id = id;
        this.name = name;
        mProductId = productId;
        mUomId = uomId;
        mLocationId = locationId;
        mLocationDestId = locationDestId;
        mProductQty = productQty;
    }
}

回答1:

If you change the data type for the mLocationDestId variable from int to Integer the schema should be generated without the NOT NULL constraint for your location_dest_id column since Integer is nullable.

I've only tested this with version 1.1.0-alpha3 of room so I don't know if it works with 1.0.0 as well.

来源：https://stackoverflow.com/questions/48933887/android-with-room-how-to-set-a-foreign-key-nullable