ent/doc: add indexes doc and example

Summary: Pull Request resolved: https://github.com/facebookexternal/fbc/pull/1380

Reviewed By: alexsn

Differential Revision: D17075317

fbshipit-source-id: 021cb0bde3849af5a5d842b715a7b7dbd861c54f

examples/edgeindex/ent/schema/city.go

@@ -0,0 +1,30 @@
+// 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 schema
+
+import (
+	"github.com/facebookincubator/ent"
+	"github.com/facebookincubator/ent/schema/edge"
+	"github.com/facebookincubator/ent/schema/field"
+)
+
+// City holds the schema definition for the City entity.
+type City struct {
+	ent.Schema
+}
+
+// Fields of the City.
+func (City) Fields() []ent.Field {
+	return []ent.Field{
+		field.String("name"),
+	}
+}
+
+// Edges of the City.
+func (City) Edges() []ent.Edge {
+	return []ent.Edge{
+		edge.To("streets", Street.Type),
+	}
+}

examples/edgeindex/ent/schema/street.go

@@ -0,0 +1,42 @@
+// 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 schema
+
+import (
+	"github.com/facebookincubator/ent"
+	"github.com/facebookincubator/ent/schema/edge"
+	"github.com/facebookincubator/ent/schema/field"
+	"github.com/facebookincubator/ent/schema/index"
+)
+
+// Street holds the schema definition for the Street entity.
+type Street struct {
+	ent.Schema
+}
+
+// Fields of the Street.
+func (Street) Fields() []ent.Field {
+	return []ent.Field{
+		field.String("name"),
+	}
+}
+
+// Edges of the Street.
+func (Street) Edges() []ent.Edge {
+	return []ent.Edge{
+		edge.From("city", City.Type).
+			Ref("streets").
+			Unique(),
+	}
+}
+
+// Indexes of the Street.
+func (Street) Indexes() []ent.Index {
+	return []ent.Index{
+		index.Fields("name").
+			Edges("city").
+			Unique(),
+	}
+}