schema/field: add annotations for overriding field struct-tags

schema/field/annotation.go

@@ -0,0 +1,44 @@
+// Copyright 2019-present Facebook Inc. All rights reserved.
+// This source code is licensed under the Apache 2.0 license found
+// in the LICENSE file in the root directory of this source tree.
+
+package field
+
+import "github.com/facebook/ent/schema"
+
+// Annotation is a builtin schema annotation for
+// configuring the schema fields in codegen.
+type Annotation struct {
+	// The StructTag option allows overriding the struct-tag
+	// of the fields in the generated entity. For example:
+	//
+	//	field.Annotation{
+	//		StructTag: map[string]string{
+	//			"id": `json:"id,omitempty" yaml:"-"`
+	//		}
+	//	}
+	//
+	StructTag map[string]string
+}
+
+// Name describes the annotation name.
+func (Annotation) Name() string {
+	return "Fields"
+}
+
+// StructTag returns an annotation with the configured struct-tag mapping.
+//
+//	StructTag( "id", `json:"-" yaml:"-"`, "oid", `json:"oid,omitempty"`)
+//
+func StructTag(nametag ...string) Annotation {
+	if len(nametag)%2 == 1 {
+		panic("field.StructTag: odd argument count")
+	}
+	tags := make(map[string]string, len(nametag)/2)
+	for i := 0; i < len(nametag); i += 2 {
+		tags[nametag[i]] = nametag[i+1]
+	}
+	return Annotation{StructTag: tags}
+}
+
+var _ schema.Annotation = (*Annotation)(nil)