From 0273856e4d9744c98aa42b98d485d726099e9020 Mon Sep 17 00:00:00 2001
From: Jinzhu <wosmvp@gmail.com>
Date: Mon, 31 Aug 2020 16:27:22 +0800
Subject: [PATCH] Don't alter column with full column data type, close #3339

---
 migrator/migrator.go | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/migrator/migrator.go b/migrator/migrator.go
index d93b8a6d..c736a3e0 100644
--- a/migrator/migrator.go
+++ b/migrator/migrator.go
@@ -297,10 +297,12 @@ func (m Migrator) DropColumn(value interface{}, name string) error {
 func (m Migrator) AlterColumn(value interface{}, field string) error {
 	return m.RunWithValue(value, func(stmt *gorm.Statement) error {
 		if field := stmt.Schema.LookUpField(field); field != nil {
+			fileType := clause.Expr{SQL: m.DataTypeOf(field)}
 			return m.DB.Exec(
 				"ALTER TABLE ? ALTER COLUMN ? TYPE ?",
-				clause.Table{Name: stmt.Table}, clause.Column{Name: field.DBName}, m.DB.Migrator().FullDataTypeOf(field),
+				clause.Table{Name: stmt.Table}, clause.Column{Name: field.DBName}, fileType,
 			).Error
+
 		}
 		return fmt.Errorf("failed to look up field with name: %s", field)
 	})