How to send custom array of objects with post body?

Question

There is data classes PlaylistInsertOperation, TrackId and function insertTracks via Retrofit Interface.

data class PlaylistInsertOperation(
    val tracks: List<TrackId>,
)

data class TrackId(
    val id: String,
    val albumId: String
)

@FormUrlEncoded
@POST("/patch")
suspend fun insertTracks(
    @Field("diff") diff: List<PlaylistInsertOperation>
): PlaylistResponse

When I send request, field diff equals the next string

diff=PlaylistInsertOperation(tracks=[TrackId(id=39117009, albumId=5034819), TrackId(id=89341636, albumId=18635889)])

But need to such string

diff=[{"tracks":[{"id":"61081889","albumId":9499401}]}]

How to tell gson or retrofit parse models without data class name?

Answer 1

I create additional data class PlaylistDiffRequest with list of InsertTracksOperation. And override toString function that call gson.

// Retrofit Interface
@FormUrlEncoded
@POST("/handlers/playlist-patch.jsx")
suspend fun patchPlaylist(
    @Field("diff") diff: PlaylistDiffRequest,
): PlaylistResponse

// Models
data class InsertTracksOperation(
    val tracks: List<TrackId>,
)

data class PlaylistDiffRequest(val diff: List<Any>) {
    override fun toString(): String = Gson().toJson(diff)
}

In this case output field will be as I need

diff=[{"tracks":[{"albumId":"6250697","id":"35629702"}]}]