feat: add test for M2M relations with composite keys

internal/dbtest/orm_test.go

@@ -32,6 +32,7 @@ func TestORM(t *testing.T) {
 		{testM2MRelationExcludeColumn},
 		{testRelationBelongsToSelf},
 		{testCompositeHasMany},
+		{testCompositeM2M},
 	}
 
 	testEachDB(t, func(t *testing.T, dbName string, db *bun.DB) {
@@ -441,6 +442,78 @@ func testCompositeHasMany(t *testing.T, db *bun.DB) {
 	require.Equal(t, 2, len(department.Employees))
 }
 
+func testCompositeM2M(t *testing.T, db *bun.DB) {
+	type Item struct {
+		ID     int64 `bun:",pk"`
+		ShopID int64 `bun:",pk"`
+	}
+
+	type Order struct {
+		ID     int    `bun:",pk"`
+		ShopID int    `bun:",pk"`
+		Items  []Item `bun:"m2m:orders_to_items,join:Order=Item"`
+	}
+
+	type OrderToItem struct {
+		bun.BaseModel `bun:"table:orders_to_items"`
+
+		ShopID int `bun:""`
+
+		OrderID int    `bun:""`
+		Order   *Order `bun:"rel:belongs-to,join:shop_id=shop_id,join:order_id=id"`
+		ItemID  int    `bun:""`
+		Item    *Item  `bun:"rel:belongs-to,join:shop_id=shop_id,join:item_id=id"`
+	}
+
+	db.RegisterModel((*OrderToItem)(nil))
+	mustResetModel(t, ctx, db, (*Order)(nil), (*Item)(nil), (*OrderToItem)(nil))
+
+	items := []Item{
+		{ID: 1, ShopID: 22},
+		{ID: 2, ShopID: 22},
+		{ID: 3, ShopID: 22},
+	}
+	_, err := db.NewInsert().Model(&items).Exec(ctx)
+	require.NoError(t, err)
+
+	orders := []Order{
+		{ID: 12, ShopID: 22},
+		{ID: 13, ShopID: 22},
+	}
+	_, err = db.NewInsert().Model(&orders).Exec(ctx)
+	require.NoError(t, err)
+
+	orderItems := []OrderToItem{
+		{OrderID: 12, ItemID: 1, ShopID: 22},
+		{OrderID: 12, ItemID: 2, ShopID: 22},
+		{OrderID: 13, ItemID: 3, ShopID: 22},
+	}
+	_, err = db.NewInsert().Model(&orderItems).Exec(ctx)
+	require.NoError(t, err)
+
+	var ordersOut []Order
+
+	err = db.NewSelect().
+		Model(&ordersOut).
+		Where("id = ?", 12).
+		Relation("Items").
+		Scan(ctx)
+	require.NoError(t, err)
+	require.Equal(t, 1, len(ordersOut))
+	require.Equal(t, 2, len(ordersOut[0].Items))
+
+	var ordersOut2 []Order
+
+	err = db.NewSelect().
+		Model(&ordersOut2).
+		Where("id = ?", 13).
+		Relation("Items").
+		Scan(ctx)
+	require.NoError(t, err)
+	require.Equal(t, 1, len(ordersOut2))
+	require.Equal(t, 1, len(ordersOut2[0].Items))
+}
+
 type Genre struct {
 	ID     int `bun:",pk"`
 	Name   string