Add Category.BeforeSave func

db/category.go

@@ -1,6 +1,10 @@
 package db
 
-import "gorm.io/gorm"
+import (
+	"errors"
+
+	"gorm.io/gorm"
+)
 
 type Category struct {
 	gorm.Model
@@ -11,3 +15,26 @@ type Category struct {
 	UserID   uint
 	User     *User
 }
+
+var (
+	ERROR_CATEGORY_PARENT_NOT_FOUND = errors.New("ParentID is invalid for user")
+	ERROR_CATEGORY_NAME_NOT_UNIQUE  = errors.New("Name for Category have to be unique for user")
+)
+
+func (c *Category) BeforeSave(tx *gorm.DB) error {
+	var parent Category
+	if err := tx.Find(&parent, Category{ParentID: c.ParentID, UserID: c.UserID}).Error; err != nil {
+		return err
+	}
+	if parent.ID == 0 {
+		return ERROR_CATEGORY_PARENT_NOT_FOUND
+	}
+	var dup Category
+	if err := tx.Find(&dup, Category{Name: c.Name, UserID: c.UserID}).Error; err != nil {
+		return err
+	}
+	if dup.ID != 0 {
+		return ERROR_CATEGORY_NAME_NOT_UNIQUE
+	}
+	return nil
+}