Question

I’m having a bit of trouble with SQLite in Android. As my application supports Android 1.6, I am unable to use foreign keys, so have written triggers to enforce these constraints.

My two tables are vehicle and fuel_use. vehicle and fuel_use both have a registration_number column. My trigger ensures that I cannot update registration_number in vehicle when there are entries in fuel_use referencing registration_number in vehicle:

        //on update vehicle
    db.execSQL("CREATE TRIGGER fk_update_vehicle_trigger_fuel_references_vehicles_reg_no " +
            "BEFORE UPDATE ON " + VEHICLE_TABLE_NAME +
            " FOR EACH ROW BEGIN " +
            "SELECT RAISE(ROLLBACK, 'update on table " + VEHICLE_TABLE_NAME + " violates foreign key constraint fk_update_vehicle_trigger_fuel_references_vehicles_reg_no') " +
            "WHERE (SELECT " + REGISTRATION_NO_COLUMN + " FROM " + FUEL_USE_TABLE_NAME +
            " WHERE " + REGISTRATION_NO_COLUMN + " = OLD." + REGISTRATION_NO_COLUMN + ") IS NOT NULL; " +
            "END;");

This works fine in that it stops updates which would leave fuel_use rows referencing non-existent vehicle rows. However, I have just discovered a problem – when I try to update a differnt column in vehicle – initial_mileage – this trigger disallows it and forces a rollback. I understand why this is happening but don’t know how to rewrite the trigger so only updates on the registration_number column force a rollback. I have tried:

BEFORE UPDATE ON " + VEHICLE_TABLE_NAME + "." + REGISTRATION_NO_COLUMN

but this did not work. In fact an exception was thrown.

Any help would be gratefully received.

Answer 1

Try:

db.execSQL("CREATE TRIGGER fk_update_vehicle_trigger_fuel_references_vehicles_reg_no " +
        "BEFORE UPDATE OF " + REGISTRATION_NO_COLUMN + " ON " + VEHICLE_TABLE_NAME +
        " FOR EACH ROW BEGIN " +
        "SELECT RAISE(ROLLBACK, 'update on table " + VEHICLE_TABLE_NAME + " violates foreign key constraint fk_update_vehicle_trigger_fuel_references_vehicles_reg_no') " +
        "WHERE (SELECT " + REGISTRATION_NO_COLUMN + " FROM " + FUEL_USE_TABLE_NAME +
        " WHERE " + REGISTRATION_NO_COLUMN + " = OLD." + REGISTRATION_NO_COLUMN + ") IS NOT NULL; " +
        "END;");

See if that works.