Why Room entities don't work with immutable properties in Android

Question

I have been exploring Room database object mapping library and I figured something weird.

An entity data model cannot have immutable properties, as this answer sugge

Answer 1

It's weird because I can make my Entity class using val for all of my fields without an issue

@Entity(tableName = "repo")
data class RepoEntity(
        @PrimaryKey @ColumnInfo(name = "id") @SerializedName("id") val id: Int,
        @ColumnInfo(name = "name") @SerializedName("name") val name: String,
        @ColumnInfo(name = "full_name") @SerializedName("full_name") val fullName: String,
        @Embedded(prefix = "owner") @SerializedName("owner") val owner: RepoOwnerEntity,
        @ColumnInfo(name = "html_url") @SerializedName("html_url") val htmlUrl: String,
        @ColumnInfo(name = "description") @SerializedName("description") val description: String?
)

And the data still stored correctly inside the Database.

Answer 2

I believe that the issue stems from certain fields which can not be constructor parameters. From the Javadoc of the @Relation annotation:

Note that the @Relation annotated field cannot be a constructor parameter, it must be public or have a public setter.

As a workaround, I had a private constructor parameter _myRelationProperty and a public field:

val myRelationProperty: List<MyThings> get() = _myRelationProperty