Add comments on foreign key constraint enforcement (#1675)

    /**

     * Used for testing the migration process from [toVersion]-1 to [toVersion].

     *

     * Note: SQLite's foreign key constraint enforcement is not enabled in tests. We need

     * to enable it ourselves using setting "PRAGMA foreign_keys=ON" directly after opening

     * a new database connection (works per connection). In tests it's usually more practical

     * not to do so, however. In production database connections room enables it for us.

     *

     * @param prepare      Callback to prepare the database. Will be run with database schema in version [toVersion] - 1.

     * @param validate     Callback to validate the migration result. Will be run with database schema in version [toVersion].

     */

        // Prepare the database with the initial version.

        val dbName = "test"

        helper.createDatabase(dbName, version = toVersion - 1).apply {

            // We could enable foreign key constraint enforcement here

            // by setting "PRAGMA foreign_keys=ON".

            prepare(this)

            close()

        }

import java.io.Writer

import javax.inject.Singleton

/**

 * The app database. Managed via android jetpack room. Room provides an abstraction

 * layer over SQLite.

 *

 * Note: In SQLite PRAGMA foreign_keys is off by default. Room activates it for

 * production (non-test) databases.

 */

@Database(entities = [

    Service::class,

    HomeSet::class,