Add field tag to ignore migration

.gitignore

@@ -2,3 +2,4 @@ TODO*
 documents
 coverage.txt
 _book
+.idea

migrator/migrator.go

@@ -396,7 +396,7 @@ func (m Migrator) MigrateColumn(value interface{}, field *schema.Field, columnTy
 		}
 	}
 
-	if alterColumn {
+	if alterColumn && !field.IgnoreMigration {
 		return m.DB.Migrator().AlterColumn(value, field.Name)
 	}
 

schema/field.go

@@ -70,6 +70,7 @@ type Field struct {
 	ReflectValueOf         func(reflect.Value) reflect.Value
 	ValueOf                func(reflect.Value) (value interface{}, zero bool)
 	Set                    func(reflect.Value, interface{}) error
+	IgnoreMigration        bool
 }
 
 func (schema *Schema) ParseField(fieldStruct reflect.StructField) *Field {
@@ -188,6 +189,10 @@ func (schema *Schema) ParseField(fieldStruct reflect.StructField) *Field {
 		field.Comment = val
 	}
 
+	if _, ok := field.TagSettings["IGNOREMIGRATION"]; ok {
+		field.IgnoreMigration = true
+	}
+
 	// default value is function or null or blank (primary keys)
 	skipParseDefaultValue := strings.Contains(field.DefaultValue, "(") &&
 		strings.Contains(field.DefaultValue, ")") || strings.ToLower(field.DefaultValue) == "null" || field.DefaultValue == ""