How to insert data to many to many table - room(Sql) android

I created this table but i don't know how to add the data to the playListWithSongs table.

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@Entity
data class SongEntity(
  var songId : Long? = null,
  var name: String? = null,
 )

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@Entity
data class playlistEntity(
  var playListId : Long? = null,
  var playlistName: String? = null
)

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@Entity
data class playListWithSongs(
     var playlistId: Long,
     var songId: Long
)

Here How to insert the ids to the playListWithSongs table?

But in my case, the playlist may have the same songs or empty. Even songs will be present in one playlist , no playlist or many playlist so how can I insert the PlayListWithSongs Entity. Since both ids are primary key and I will have zero songs for a playlist there will be null and contains duplication (Here the list of song and list of course Size are not similar)

@Insert
fun insert(playListSong: playListWithSongs)

@Insert(onConflict = OnConflictStrategy.REPLACE)
fun insertPlayList(playList: PlayListEntity?)

@Insert(onConflict = OnConflictStrategy.REPLACE)
fun insertCourse(song: SongsEntity?)

@Transaction
@Query("SELECT * FROM playListEntity")
fun getAll(): LiveData<List<PlayListWithSong>>

If you define the return type of the insert functions as Long in all of your dao classes, you will be able to get the id of the inserted item, then use this id for inserting in PlayListWithSongs. For example:

@Dao
interface SongDao {

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insert(entity: Song): Long
}

Update:

Note that the Song and the Playlist are two distinct entities. So, while you want to assign a Song to a Playlist (which means adding a new row in playListWithSongs containing both their ids), the target song and playlist should exist in db.

I mean you would never try to add a song to a playlist which has not existed yet. So, At the first moment that a playlist gets created, there is no song assigned to it. It means that there is no row in the playListWithSongs table with the playlistId of the newly created one.

To sum up, it is obvious that when you want to assign a song to a playlist, you should have both ids. Also, it's better to not to use this couple as the primary key for the playListWithSongs. Let's see my written code for this subject:

@Entity(
    tableName = "playListWithSongs",
    foreignKeys = [
        ForeignKey(
            entity = Song::class,
            parentColumns = ["songId"],
            childColumns = ["songId"],
            onDelete = CASCADE
        ),
        ForeignKey(
            entity = Playlist::class,
            parentColumns = ["playlistId"],
            childColumns = ["playlistId"],
            onDelete = CASCADE
        )
    ],
    indices = [
        Index(
            value = ["songId", "playlistId"],
            unique = true
        )
    ]
)
data class PlayListWithSongs(
    @PrimaryKey(autoGenerate = true) val id: Int? = null,
    @ColumnInfo(name = "songId") val songId: Long,
    @ColumnInfo(name = "playlistId") val playlistId: Long
)

As you can see in the above code, if a song or playlist gets removed, all of its assignments on playListWithSongs get removed too (due to CASCADE). So there is no concern about unused rows and duplication (Note that the combination of songId and playlistId is defined as unique).

OTOH, You can do it without mapping. Create two entity one for songs another one for playlist with songs, instead of mapping keep the song id's as a list object in the playlist, and query the song table with the id's from the playlist accordingly. Here is how you can achieve that - https://medium.com/@toddcookevt/android-room-storing-lists-of-objects-766cca57e3f9.

Nevertheless, I do prefer the conventional mapping as @aminography eloborated in his answer so that u can make use of CASCADE action to propagate the delete or update operation on the parent key to each dependent child key.