Kotlin: How to insert a list of objects into Room?

Question

I am trying to define common CRUD methods in a base interface as shown here:

interface BaseDao<in I> {

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    fun create(obj: I)

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    fun createAll(objects: List<I>)

    @Delete
    fun delete(obj: I)

}

The following ProductDao interface for Room inherits from the base interface:

@Dao
interface ProductDao : BaseDao<Product> {

    // Specific methods

}

When I compile the definition for fun createAll(objects: List<I>) produces the following error:

Type of the parameter must be a class annotated with @Entity or a collection/array of it.

Answer 1

Try adding @JvmSuppressWildcards to your function.

@Insert
@JvmSuppressWildcards
fun createAll(objects: List<I>)

From the docs:

Instructs compiler to generate or omit wildcards for type arguments corresponding to parameters with declaration-site variance, for example such as Collection has.

It may be helpful only if declaration seems to be inconvenient to use from Java.

Answer 2

I've resolved issue by:

@Dao
interface BaseDao<T> {

    /**
     * Insert a list in the database. If the item already exists, replace it.
     *
     * @param list the items to be inserted.
     */
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    @JvmSuppressWildcards
    abstract fun insertAll(list: List<T>)

}

@JvmSuppressWildcards done the trick for me