How to implement inheritance for a Room entity (Android, Kotlin)

Question

I'm new to Room and haven't find any posts related to this. I have two classes: BaseModel and ChildModel. ChildModel is a Room entity class, but BaseModel - isn't. How do I extend BaseModel from ChildModel properly? I tried to do it as in the code bellow...

BaseModel.kt:

@Parcelize
open class BaseModel(
    open val id: String,
    open var name: String,
    open var creator: String,
    open var dateOfLastEdit: Timestamp,
    open var contributors: List<String>,
    open var color: Int,
    open var offline: Boolean
) : Parcelable {...} 

ChildModel.kt (empty constructor used for firebase):

@Entity
@Parcelize
data class ChildModel(
    @PrimaryKey(autoGenerate = false)
    override val id: String = "",
    override var name: String = "",
    override var creator: String = "",
    @TypeConverters(DateTypeConverter::class)
    override var dateOfLastEdit: Timestamp = currentTime(),
    @TypeConverters(ListStringConverter::class)
    override var contributors: List<String> = emptyList(),
    override var color: Int = NO_COLOR,
    override var offline: Boolean = false,
    var text: String = "",
    var lastEditTextPosition: Int = 0,
    var lastEditNamePosition: Int = 0,
    var lastUserEdited: String = ""
) : Parcelable, BaseModel(id, name, creator, dateOfLastEdit, contributors, color, offline) {
    constructor() : this("", "", "", currentTime(), emptyList(), NO_COLOR, false, "", 0, 0, "") 
...
}

... but it gives error: "Field has non-unique column name" for all the fields which are overridden from BaseModel. Btw, when create BaseModel as an interface it builds successfully.

My database class:

@Database(entities = [ChildModel::class], version = 1)
@TypeConverters(DateTypeConverter::class, ListStringConverter::class) 
    abstract class CustomDatabase : RoomDatabase() {
    abstract fun childModelDao(): ChildModelDao

    companion object {
        var INSTANCE: CustomDatabase? = null

        fun getDatabase(context: Context): CustomDatabase? {
            if (INSTANCE == null) {
                synchronized(CustomDatabase::class) {
                    INSTANCE = Room.databaseBuilder(
                        context.applicationContext,
                        CustomDatabse::class.java,
                        DATABASE
                    ).build()
                }
            }
            return INSTANCE
        }
        fun destroyDatabase() {
            INSTANCE = null
        }
    }
}

The error is not about upgrading database version, because I deleted the previous one.
Thanks in advance!

Answer 1

Ok, it seems that Room somehow creates two columns with same names (for parent class and for child class) I tried adding @Ignore annotation above every field inside BaseModel class. It fixed the issue, the project builds successfully, but I'm not sure that's the best approach.

Answer 2

In cases where an entity inherits fields from a parent entity, it's usually easier to use the ignoredColumns property of the @Entity attribute

Source: https://developer.android.com/training/data-storage/room/defining-data

The example in the article shows an @Entity data class inheriting from an open class.

Based on @Anastasia Dronina's answer, it sounds like the reason to use the ignoredColumns property of the @Entity attribute is because Room will duplicate columns corresponding to all the superclass fields.