How to correctly implement the many to many relationship in Room?

Question

Android

How to correctly implement the many to many relationship in Room?
such tables

Answer 1

In short, in the link table, the keys should be described something like this:

@Entity(tableName = "invoiceDetail",
        primaryKeys = { "p_id", "inv_id" },
        foreignKeys = {
                @ForeignKey(entity = Product.class,
                            parentColumns = "p_id",
                            childColumns = "p_id"),
                @ForeignKey(entity = Invoice.class,
                            parentColumns = "inv_id",
                            childColumns = "inv_id")
                })

Well, requests are quite normal - through inner join.
Here is an article on the subject

Answer 2

I tried to get cards with a list of categories in this way, but it does not work. All data is null

public class CardWithCategoryEntity {
    @Embedded
    public CardEntity cardEntity;

    @Ignore
    @Relation(parentColumn = "id", entityColumn = "cards_id", entity = CardCategoryEntity.class, projection = {"cards_id, category_id, record_type"})
    public List<CategoryEntity> categoryList;
}