feature: model Joins & Preload methods support multiple RelationField (…

…go-gorm#424)

* feat: modify preload function on template to support Preloading multiple relations

* feat: modify Joins function on template to support Joins multiple relations

* fix(tempale-method): use plural for Joins & Preload args

internal/template/method.go

@@ -179,12 +179,18 @@ func ({{.S}} {{.NewStructName}}Do) Assign(attrs ...field.AssignExpr) *{{.NewStru
 	return {{.S}}.withDO({{.S}}.DO.Assign(attrs...))
 }
 
-func ({{.S}} {{.NewStructName}}Do) Joins(field field.RelationField) *{{.NewStructName}}Do {
-	return {{.S}}.withDO({{.S}}.DO.Joins(field))
-}
-
-func ({{.S}} {{.NewStructName}}Do) Preload(field field.RelationField) *{{.NewStructName}}Do {
-	return {{.S}}.withDO({{.S}}.DO.Preload(field))
+func ({{.S}} {{.NewStructName}}Do) Joins(fields ...field.RelationField) *{{.NewStructName}}Do {
+	for _, _f := range fields {
+        {{.S}} = *{{.S}}.withDO({{.S}}.DO.Joins(_f))
+    }
+	return &{{.S}}
+}
+
+func ({{.S}} {{.NewStructName}}Do) Preload(fields ...field.RelationField) *{{.NewStructName}}Do {
+    for _, _f := range fields {
+        {{.S}} = *{{.S}}.withDO({{.S}}.DO.Preload(_f))
+    }
+	return &{{.S}}
 }
 
 func ({{.S}} {{.NewStructName}}Do) FirstOrInit() (*{{.StructInfo.Package}}.{{.StructInfo.Type}}, error) {