Kuidas luua ühendus kahe tabeli vahel andmebaasis, mis kasutab Room?

In Room, the relationship between two tables is established using foreign keys and annotations, as well as with special classes for data merging.

1. Foreign Key — in the entity, you specify a field with the @ForeignKey annotation, which references the primary key of another table.

2. Relations — to retrieve related data, POJO classes are created with @Relation annotations, which allow fetching nested objects from related tables.

Example of a "one-to-many" relationship:

@Entity
data class User(
    @PrimaryKey val userId: Int,
    val name: String
)

@Entity(
    foreignKeys = [ForeignKey(
        entity = User::class,
        parentColumns = ["userId"],
        childColumns = ["userOwnerId"],
        onDelete = ForeignKey.CASCADE
    )]
)
data class Book(
    @PrimaryKey val bookId: Int,
    val title: String,
    val userOwnerId: Int // foreign key
)

// Class to get user with their books
data class UserWithBooks(
    @Embedded val user: User,
    @Relation(
        parentColumn = "userId",
        entityColumn = "userOwnerId"
    )
    val books: List<Book>
)

This way, Room automatically links the tables and allows fetching related data through DAO.