feat: limit Learn English parents to one LMS practice each

Reject creating a second practice on the same course, module, or lesson with 409 Conflict, and enforce the rule in the database via unique partial indexes.

Co-authored-by: Cursor <cursoragent@cursor.com>

db/migrations/000077_lms_one_practice_per_parent.down.sql

@@ -0,0 +1,3 @@
+DROP INDEX IF EXISTS idx_lms_practices_one_per_lesson;
+DROP INDEX IF EXISTS idx_lms_practices_one_per_module;
+DROP INDEX IF EXISTS idx_lms_practices_one_per_course;

db/migrations/000077_lms_one_practice_per_parent.up.sql

@@ -0,0 +1,31 @@
+-- Learn English: at most one lms_practice per course, module, or lesson parent.
+-- Remove duplicate rows (keep the earliest practice per parent) before adding constraints.
+DELETE FROM lms_practices a
+USING lms_practices b
+WHERE a.course_id IS NOT NULL
+  AND a.course_id = b.course_id
+  AND a.id > b.id;
+
+DELETE FROM lms_practices a
+USING lms_practices b
+WHERE a.module_id IS NOT NULL
+  AND a.module_id = b.module_id
+  AND a.id > b.id;
+
+DELETE FROM lms_practices a
+USING lms_practices b
+WHERE a.lesson_id IS NOT NULL
+  AND a.lesson_id = b.lesson_id
+  AND a.id > b.id;
+
+CREATE UNIQUE INDEX idx_lms_practices_one_per_course
+    ON lms_practices (course_id)
+    WHERE course_id IS NOT NULL;
+
+CREATE UNIQUE INDEX idx_lms_practices_one_per_module
+    ON lms_practices (module_id)
+    WHERE module_id IS NOT NULL;
+
+CREATE UNIQUE INDEX idx_lms_practices_one_per_lesson
+    ON lms_practices (lesson_id)
+    WHERE lesson_id IS NOT NULL;

db/query/lms_practices.sql

@@ -109,3 +109,18 @@ RETURNING *;
 -- name: DeleteLmsPractice :exec
 DELETE FROM lms_practices
 WHERE id = $1;
+
+-- name: CountLmsPracticesByCourseID :one
+SELECT COUNT(*)::bigint
+FROM lms_practices
+WHERE course_id = $1;
+
+-- name: CountLmsPracticesByModuleID :one
+SELECT COUNT(*)::bigint
+FROM lms_practices
+WHERE module_id = $1;
+
+-- name: CountLmsPracticesByLessonID :one
+SELECT COUNT(*)::bigint
+FROM lms_practices
+WHERE lesson_id = $1;

gen/db/lms_practices.sql.go

@@ -11,6 +11,45 @@ import (
 	"github.com/jackc/pgx/v5/pgtype"
 )
 
+const CountLmsPracticesByCourseID = `-- name: CountLmsPracticesByCourseID :one
+SELECT COUNT(*)::bigint
+FROM lms_practices
+WHERE course_id = $1
+`
+
+func (q *Queries) CountLmsPracticesByCourseID(ctx context.Context, courseID pgtype.Int8) (int64, error) {
+	row := q.db.QueryRow(ctx, CountLmsPracticesByCourseID, courseID)
+	var column_1 int64
+	err := row.Scan(&column_1)
+	return column_1, err
+}
+
+const CountLmsPracticesByLessonID = `-- name: CountLmsPracticesByLessonID :one
+SELECT COUNT(*)::bigint
+FROM lms_practices
+WHERE lesson_id = $1
+`
+
+func (q *Queries) CountLmsPracticesByLessonID(ctx context.Context, lessonID pgtype.Int8) (int64, error) {
+	row := q.db.QueryRow(ctx, CountLmsPracticesByLessonID, lessonID)
+	var column_1 int64
+	err := row.Scan(&column_1)
+	return column_1, err
+}
+
+const CountLmsPracticesByModuleID = `-- name: CountLmsPracticesByModuleID :one
+SELECT COUNT(*)::bigint
+FROM lms_practices
+WHERE module_id = $1
+`
+
+func (q *Queries) CountLmsPracticesByModuleID(ctx context.Context, moduleID pgtype.Int8) (int64, error) {
+	row := q.db.QueryRow(ctx, CountLmsPracticesByModuleID, moduleID)
+	var column_1 int64
+	err := row.Scan(&column_1)
+	return column_1, err
+}
+
 const CreateLmsPractice = `-- name: CreateLmsPractice :one
 INSERT INTO lms_practices (
     course_id, module_id, lesson_id,

internal/ports/lms_practice.go

@@ -30,4 +30,7 @@ type LmsPracticeStore interface {
 	ListLmsPracticesByLessonID(ctx context.Context, lessonID int64, publishedOnly bool, limit, offset int32) ([]domain.Practice, int64, error)
 	UpdateLmsPractice(ctx context.Context, id int64, input domain.UpdatePracticeInput) (domain.Practice, error)
 	DeleteLmsPractice(ctx context.Context, id int64) error
+	CountLmsPracticesByCourseID(ctx context.Context, courseID int64) (int64, error)
+	CountLmsPracticesByModuleID(ctx context.Context, moduleID int64) (int64, error)
+	CountLmsPracticesByLessonID(ctx context.Context, lessonID int64) (int64, error)
 }

internal/repository/lms_practices.go

@@ -259,3 +259,15 @@ func (s *Store) UpdateLmsPractice(ctx context.Context, id int64, input domain.Up
 func (s *Store) DeleteLmsPractice(ctx context.Context, id int64) error {
 	return s.queries.DeleteLmsPractice(ctx, id)
 }
+
+func (s *Store) CountLmsPracticesByCourseID(ctx context.Context, courseID int64) (int64, error) {
+	return s.queries.CountLmsPracticesByCourseID(ctx, int64PtrToPg8(&courseID))
+}
+
+func (s *Store) CountLmsPracticesByModuleID(ctx context.Context, moduleID int64) (int64, error) {
+	return s.queries.CountLmsPracticesByModuleID(ctx, int64PtrToPg8(&moduleID))
+}
+
+func (s *Store) CountLmsPracticesByLessonID(ctx context.Context, lessonID int64) (int64, error) {
+	return s.queries.CountLmsPracticesByLessonID(ctx, int64PtrToPg8(&lessonID))
+}

internal/services/practices/service.go

@@ -3,6 +3,7 @@ package practices
 import (
 	"Yimaru-Backend/internal/domain"
 	"Yimaru-Backend/internal/ports"
+	"Yimaru-Backend/internal/repository"
 	"Yimaru-Backend/internal/services/courses"
 	"Yimaru-Backend/internal/services/lessons"
 	"Yimaru-Backend/internal/services/modules"
@@ -16,6 +17,7 @@ var (
 	ErrPracticeNotFound            = errors.New("practice not found")
 	ErrQuestionSetNotFound         = errors.New("question set not found")
 	ErrInvalidPracticeParent       = errors.New("parent_kind and parent_id do not match an allowed parent")
+	ErrPracticeParentLimitExceeded = errors.New("this course, module, or lesson already has a practice")
 )
 
 type Service struct {
@@ -101,6 +103,28 @@ func (s *Service) resolveParent(ctx context.Context, in domain.CreatePracticeInp
 	}
 }
 
+func (s *Service) ensureParentPracticeLimit(ctx context.Context, courseID, moduleID, lessonID *int64) error {
+	var count int64
+	var err error
+	switch {
+	case courseID != nil:
+		count, err = s.practices.CountLmsPracticesByCourseID(ctx, *courseID)
+	case moduleID != nil:
+		count, err = s.practices.CountLmsPracticesByModuleID(ctx, *moduleID)
+	case lessonID != nil:
+		count, err = s.practices.CountLmsPracticesByLessonID(ctx, *lessonID)
+	default:
+		return ErrInvalidPracticeParent
+	}
+	if err != nil {
+		return err
+	}
+	if count >= 1 {
+		return ErrPracticeParentLimitExceeded
+	}
+	return nil
+}
+
 func (s *Service) Create(ctx context.Context, in domain.CreatePracticeInput) (domain.Practice, error) {
 	if err := s.validateQuestionSet(ctx, in.QuestionSetID); err != nil {
 		return domain.Practice{}, err
@@ -114,7 +138,14 @@ func (s *Service) Create(ctx context.Context, in domain.CreatePracticeInput) (do
 	if err != nil {
 		return domain.Practice{}, err
 	}
-	return s.practices.CreateLmsPractice(ctx, in, courseID, moduleID, lessonID)
+	if err := s.ensureParentPracticeLimit(ctx, courseID, moduleID, lessonID); err != nil {
+		return domain.Practice{}, err
+	}
+	p, err := s.practices.CreateLmsPractice(ctx, in, courseID, moduleID, lessonID)
+	if err != nil && repository.IsUniqueViolation(err) {
+		return domain.Practice{}, ErrPracticeParentLimitExceeded
+	}
+	return p, err
 }
 
 func (s *Service) TryGetByQuestionSetID(ctx context.Context, questionSetID int64) (domain.Practice, bool, error) {

internal/web_server/handlers/practice_handler.go

@@ -48,6 +48,11 @@ func (h *Handler) CreatePractice(c *fiber.Ctx) error {
 			return c.Status(fiber.StatusNotFound).JSON(domain.ErrorResponse{Message: "Persona not found", Error: err.Error()})
 		case errors.Is(err, practices.ErrInvalidPracticeParent):
 			return c.Status(fiber.StatusBadRequest).JSON(domain.ErrorResponse{Message: "Invalid parent", Error: err.Error()})
+		case errors.Is(err, practices.ErrPracticeParentLimitExceeded):
+			return c.Status(fiber.StatusConflict).JSON(domain.ErrorResponse{
+				Message: "This course, module, or lesson already has a practice",
+				Error:   err.Error(),
+			})
 		}
 		return c.Status(fiber.StatusInternalServerError).JSON(domain.ErrorResponse{Message: "Failed to create practice", Error: err.Error()})
 	}