Add support to remove foreign key constraints (go-gorm#1686)

Author: ayadav

Diff for: main.go

@@ -611,6 +611,14 @@ func (s *DB) AddForeignKey(field string, dest string, onDelete string, onUpdate
 	return scope.db
 }
 
+// RemoveForeignKey Remove foreign key from the given scope, e.g:
+//     db.Model(&User{}).RemoveForeignKey("city_id", "cities(id)")
+func (s *DB) RemoveForeignKey(field string, dest string) *DB {
+	scope := s.clone().NewScope(s.Value)
+	scope.removeForeignKey(field, dest)
+	return scope.db
+}
+
 // Association start `Association Mode` to handler relations things easir in that mode, refer: https://jinzhu.github.io/gorm/associations.html#association-mode
 func (s *DB) Association(column string) *Association {
 	var err error

Diff for: scope.go

@@ -1175,6 +1175,16 @@ func (scope *Scope) addForeignKey(field string, dest string, onDelete string, on
 	scope.Raw(fmt.Sprintf(query, scope.QuotedTableName(), scope.quoteIfPossible(keyName), scope.quoteIfPossible(field), dest, onDelete, onUpdate)).Exec()
 }
 
+func (scope *Scope) removeForeignKey(field string, dest string) {
+	keyName := scope.Dialect().BuildForeignKeyName(scope.TableName(), field, dest)
+
+	if !scope.Dialect().HasForeignKey(scope.TableName(), keyName) {
+		return
+	}
+	var query = `ALTER TABLE %s DROP CONSTRAINT %s;`
+	scope.Raw(fmt.Sprintf(query, scope.QuotedTableName(), scope.quoteIfPossible(keyName))).Exec()
+}
+
 func (scope *Scope) removeIndex(indexName string) {
 	scope.Dialect().RemoveIndex(scope.TableName(), indexName)
 }