Return type for Android Room joins

Question

Let\'s say I want to do an INNER JOIN between two entities Foo and Bar:

@Query(\"SELECT * FROM Foo INNER JOIN Bar ON F         


        

Answer 1

Try this way. For example, I have M2M (many-to-many) relations between Product and Attribute. Many Products have many Attributes and I need to get all Attributes by Product.id with sorted records by PRODUCTS_ATTRIBUTES.DISPLAY_ORDERING.

|--------------|  |--------------|  |-----------------------|
| PRODUCT      |  | ATTRIBUTE    |  | PRODUCTS_ATTRIBUTES   |
|--------------|  |--------------|  |-----------------------|
| _ID:  Long   |  | _ID: Long    |  | _ID: Long             |
| NAME: Text   |  | NAME: Text   |  | _PRODUCT_ID: Long     |
|______________|  |______________|  | _ATTRIBUTE_ID: Long   |
                                    | DISPLAY_ORDERING: Int |
                                    |_______________________|

So, models will be like below:

@Entity(
    tableName = "PRODUCTS",
    indices = [
        Index(value = arrayOf("NAME"), unique = true)
    ]
)
class Product {

    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(name = "_ID")
    var _id: Long = 0

    @ColumnInfo(name = "NAME")
    @SerializedName(value = "NAME")
    var name: String = String()

}


@Entity(
    tableName = "ATTRIBUTES",
    indices = [
        Index(value = arrayOf("NAME"), unique = true)
    ]
)
class Attribute {

    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(name = "_ID")
    var _id: Long = 0

    @ColumnInfo(name = "NAME")
    @SerializedName(value = "NAME")
    var name: String = String()

}

And the "join" table will be:

@Entity(
    tableName = "PRODUCTS_ATTRIBUTES",
    indices = [
        Index(value = ["_PRODUCT_ID", "_ATTRIBUTE_ID"])
    ],
    foreignKeys = [
        ForeignKey(entity = Product::class, parentColumns = ["_ID"], childColumns = ["_PRODUCT_ID"]),
        ForeignKey(entity = Attribute::class, parentColumns = ["_ID"], childColumns = ["_ATTRIBUTE_ID"])
    ]
)
class ProductAttribute {

    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(name = "_ID")
    var _id: Long = 0

    @ColumnInfo(name = "_PRODUCT_ID")
    var _productId: Long = 0

    @ColumnInfo(name = "_ATTRIBUTE_ID")
    var _attributeId: Long = 0

    @ColumnInfo(name = "DISPLAY_ORDERING")
    var displayOrdering: Int = 0

}

In, AttributeDAO, to get all attributes based on Product._ID, you can do something like below:

@Dao
interface AttributeDAO {

    @Query("SELECT ATTRIBUTES.* FROM ATTRIBUTES INNER JOIN PRODUCTS_ATTRIBUTES ON PRODUCTS_ATTRIBUTES._ATTRIBUTE_ID = ATTRIBUTES._ID INNER JOIN PRODUCTS ON PRODUCTS._ID = PRODUCTS_ATTRIBUTES._PRODUCT_ID WHERE PRODUCTS._ID = :productId ORDER BY PRODUCTS_ATTRIBUTES.DISPLAY_ORDERING ASC")
    fun getAttributesByProductId(productId: Long): LiveData>

}

If you have any questions, please tell me.