Add migration for user group field (#1635)

modules/fbs-core/api/src/main/resources/migrations/21_user_group_field_added.sql

@@ -0,0 +1,51 @@
+BEGIN;
+
+CREATE TABLE IF NOT EXISTS `fbs`.`user_group` (
+    `user_id` INT NOT NULL,
+    `course_id` INT NOT NULL,
+    `group_id` INT NOT NULL,
+    PRIMARY KEY (`user_id`, `course_id`, `group_id`),
+    INDEX `user_has_groups_users_user_id_fk` (`user_id` ASC), 
+    CONSTRAINT `user_has_groups_users_user_id_fk` 
+        FOREIGN KEY (`user_id`)
+        REFERENCES `fbs`.`user` (`user_id`)
+        ON DELETE CASCADE
+        ON UPDATE CASCADE,
+    CONSTRAINT `user_group_course_course_id_fk`
+        FOREIGN KEY (`course_id`)
+        REFERENCES `fbs`.`course` (`course_id`)
+        ON DELETE CASCADE
+        ON UPDATE CASCADE,
+    CONSTRAINT `user_group_group_group_id_fk`
+        FOREIGN KEY (`group_id`)
+        REFERENCES `fbs`.`group` (`group_id`)
+        ON DELETE CASCADE
+        ON UPDATE CASCADE
+) ENGINE = InnoDB
+  DEFAULT CHARACTER SET = utf8mb4
+  COLLATE = utf8mb4_0900_ai_ci;
+
+-- -----------------------------------------------------
+-- trigger to check if user is participant of the course
+-- -----------------------------------------------------
+
+CREATE TRIGGER check_user_course_before_insert
+BEFORE INSERT ON `user_group`
+FOR EACH ROW
+BEGIN
+    DECLARE user_in_course INT;
+    SELECT COUNT(*)
+    INTO user_in_course
+    FROM user_course
+    WHERE user_id = NEW.user_id AND course_id = NEW.course_id;
+
+    IF user_in_course = 0 THEN
+        SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Der Nutzer gehört nicht zum angegebenen Kurs.';
+    END IF;
+END;
+
+
+INSERT INTO migration (number) VALUES (21);
+
+COMMIT;
+