Add RemoveForeignKey functionality.

Works for mssql, mysql, postgresql. Droping foreign keys for sqlite isn't supported without a work-around.

dialect.go

@@ -29,6 +29,8 @@ type Dialect interface {
 	HasForeignKey(tableName string, foreignKeyName string) bool
 	// RemoveIndex remove index
 	RemoveIndex(tableName string, indexName string) error
+	// RemoveForeignKey remove foreign key
+	RemoveForeignKey(tableName string, foreignKeyName string) error
 	// HasTable check has table or not
 	HasTable(tableName string) bool
 	// HasColumn check has column or not

dialect_common.go

@@ -105,6 +105,11 @@ func (s commonDialect) HasForeignKey(tableName string, foreignKeyName string) bo
 	return false
 }
 
+func (s commonDialect) RemoveForeignKey(tableName string, foreignKeyName string) error {
+	_, err := s.db.Exec(fmt.Sprintf("ALTER TABLE %v DROP CONSTRAINT %v", tableName, foreignKeyName))
+	return err
+}
+
 func (s commonDialect) HasTable(tableName string) bool {
 	var count int
 	s.db.QueryRow("SELECT count(*) FROM INFORMATION_SCHEMA.TABLES WHERE table_schema = ? AND table_name = ?", s.CurrentDatabase(), tableName).Scan(&count)

dialect_mysql.go

@@ -132,6 +132,11 @@ func (s mysql) HasForeignKey(tableName string, foreignKeyName string) bool {
 	return count > 0
 }
 
+func (s mysql) RemoveForeignKey(tableName string, foreignKeyName string) error {
+	_, err := s.db.Exec(fmt.Sprintf("ALTER TABLE %v DROP FOREIGN KEY %v", s.Quote(tableName), foreignKeyName))
+	return err
+}
+
 func (s mysql) CurrentDatabase() (name string) {
 	s.db.QueryRow("SELECT DATABASE()").Scan(&name)
 	return

main.go

@@ -602,6 +602,14 @@ func (s *DB) AddForeignKey(field string, dest string, onDelete string, onUpdate
 	return scope.db
 }
 
+// RemoveForeignKey Removes foreign key from the given scope, e.g:
+//     db.Model(&User{}).RemoveForeignKey("user_city_id_city_id_foreign")
+func (s *DB) RemoveForeignKey(keyName string) *DB {
+	scope := s.NewScope(s.Value)
+	scope.removeForeignKey(keyName)
+	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

migration_test.go

@@ -191,6 +191,16 @@ type Comment struct {
 	Post    Post
 }
 
+type Class struct {
+	Id   int64
+	Year string
+}
+
+type Student struct {
+	Id      int64
+	ClassID int64
+}
+
 // Scanner
 type NullValue struct {
 	Id      int64
@@ -254,7 +264,7 @@ func runMigration() {
 		DB.Exec(fmt.Sprintf("drop table %v;", table))
 	}
 
-	values := []interface{}{&Short{}, &ReallyLongThingThatReferencesShort{}, &ReallyLongTableNameToTestMySQLNameLengthLimit{}, &NotSoLongTableName{}, &Product{}, &Email{}, &Address{}, &CreditCard{}, &Company{}, &Role{}, &Language{}, &HNPost{}, &EngadgetPost{}, &Animal{}, &User{}, &JoinTable{}, &Post{}, &Category{}, &Comment{}, &Cat{}, &Dog{}, &Hamster{}, &Toy{}, &ElementWithIgnoredField{}}
+	values := []interface{}{&Short{}, &ReallyLongThingThatReferencesShort{}, &ReallyLongTableNameToTestMySQLNameLengthLimit{}, &NotSoLongTableName{}, &Product{}, &Email{}, &Address{}, &CreditCard{}, &Company{}, &Role{}, &Language{}, &HNPost{}, &EngadgetPost{}, &Animal{}, &User{}, &JoinTable{}, &Post{}, &Category{}, &Comment{}, &Cat{}, &Dog{}, &Hamster{}, &Toy{}, &ElementWithIgnoredField{}, &Class{}, &Student{}}
 	for _, value := range values {
 		DB.DropTable(value)
 	}
@@ -332,6 +342,25 @@ func TestIndexes(t *testing.T) {
 	}
 }
 
+func TestForeignKeys(t *testing.T) {
+	if err := DB.Model(&Student{}).AddForeignKey("class_id", "classes (id)", "RESTRICT", "RESTRICT").Error; err != nil {
+		t.Errorf("Got error while trying to create foreign key: %+v", err)
+	}
+
+	scope := DB.NewScope(&Student{})
+	if !scope.Dialect().HasForeignKey(scope.TableName(), "student_class_id_class_id_foreign") {
+		t.Errorf("Student should have foreign key students_class_id_classes_id_foreign")
+	}
+
+	if err := DB.Model(&Student{}).RemoveForeignKey("students_class_id_classes_id_foreign").Error; err != nil {
+		t.Errorf("Got error while trying to remove foreign key: %+v", err)
+	}
+
+	if scope.Dialect().HasForeignKey(scope.TableName(), "student_class_id_class_id_foreign") {
+		t.Errorf("Student should no longer have foreign key students_class_id_classes_id_foreign")
+	}
+}
+
 type EmailWithIdx struct {
 	Id           int64
 	UserId       int64

scope.go

@@ -1168,6 +1168,14 @@ func (scope *Scope) removeIndex(indexName string) {
 	scope.Dialect().RemoveIndex(scope.TableName(), indexName)
 }
 
+func (scope *Scope) removeForeignKey(keyName string) {
+	if !scope.Dialect().HasForeignKey(scope.TableName(), keyName) {
+		return
+	}
+
+	scope.Dialect().RemoveForeignKey(scope.TableName(), keyName)
+}
+
 func (scope *Scope) autoMigrate() *Scope {
 	tableName := scope.TableName()
 	quotedTableName := scope.QuotedTableName()