add gorm field tag "update"

This attempts to address a concern in issues #202 and #192.

It adds a gorm field tag that allows one to specify that the tagged field should always be updated. (essentially it forces isBlank to always be false)

scope.go

@@ -287,7 +287,6 @@ func (scope *Scope) fieldFromStruct(fieldStruct reflect.StructField, withRelatio
 	value := scope.IndirectValue().FieldByName(fieldStruct.Name)
 	indirectValue := reflect.Indirect(value)
 	field.Field = value
-	field.IsBlank = isBlank(value)
 
 	// Search for primary key tag identifier
 	settings := parseTagSetting(fieldStruct.Tag.Get("gorm"))
@@ -304,7 +303,11 @@ func (scope *Scope) fieldFromStruct(fieldStruct reflect.StructField, withRelatio
 	if fieldStruct.Tag.Get(tagIdentifier) == "-" {
 		field.IsIgnored = true
 	}
-
+	if _, ok := settings["UPDATE"]; ok {
+		field.IsBlank = false
+	} else {
+		field.IsBlank = isBlank(value)
+	}
 	if !field.IsIgnored {
 		// parse association
 		if !indirectValue.IsValid() {

update_test.go

@@ -158,3 +158,41 @@ func TestUpdateColumn(t *testing.T) {
 		t.Errorf("updatedAt should not be updated with update column")
 	}
 }
+
+type Always struct {
+	Id    int64
+	Name  string
+	Code  string
+	Price int64 `gorm:"update"`
+}
+
+func TestAlwaysUpdate(t *testing.T) {
+	DB.DropTable(&Always{})
+	DB.CreateTable(&Always{})
+
+	obj1 := Always{Name: "obj1", Code: "code_1", Price: 10}
+	obj2 := Always{Name: "obj2", Code: "code_2", Price: 20}
+
+	// save initial
+	DB.Save(&obj1).Save(&obj2).UpdateColumn(map[string]interface{}{"code": "columnUpdate2"})
+	// fetch and verify
+	var obj3, obj4 Always
+	DB.First(&obj3, obj1.Id)
+	if obj3.Code != "code_1" || obj3.Price != 10 {
+		t.Errorf("obj1 was not saved correctly: expected: %#v got: %#v", obj1, obj3)
+	}
+	DB.First(&obj4, obj2.Id)
+	if obj4.Code != "columnUpdate2" || obj4.Price != 20 {
+		t.Errorf("obj2 was not saved correctly: expected: %#v got: %#v", obj2, obj4)
+	}
+
+	// now update via struct price should change to zero
+	obj5 := Always{Name: "obj2update", Code: "code_2"}
+	DB.Model(obj5).Updates(obj5)
+
+	var obj6 Always
+	DB.First(&obj6, obj2.Id)
+	if obj6.Code != "code_2" || obj6.Name != "obj2update" || obj6.Price != 0 {
+		t.Errorf("obj2 was not saved correctly: got: %#v", obj6)
+	}
+}