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/edgeindex.go

@@ -0,0 +1,68 @@
+// 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 main
+
+import (
+	"context"
+	"fmt"
+	"log"
+
+	"github.com/facebookincubator/ent/examples/edgeindex/ent"
+
+	"github.com/facebookincubator/ent/dialect/sql"
+	_ "github.com/mattn/go-sqlite3"
+)
+
+func main() {
+	db, err := sql.Open("sqlite3", "file:o2o2types?mode=memory&cache=shared&_fk=1")
+	if err != nil {
+		log.Fatalf("failed opening connection to sqlite: %v", err)
+	}
+	defer db.Close()
+	client := ent.NewClient(ent.Driver(db))
+	ctx := context.Background()
+	// run the auto migration tool.
+	if err := client.Schema.Create(ctx); err != nil {
+		log.Fatalf("failed creating schema resources: %v", err)
+	}
+	if err := Do(ctx, client); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func Do(ctx context.Context, client *ent.Client) error {
+	// Unlike `Save`, `SaveX` panics if an error occurs.
+	tlv := client.City.
+		Create().
+		SetName("TLV").
+		SaveX(ctx)
+	nyc := client.City.
+		Create().
+		SetName("NYC").
+		SaveX(ctx)
+	// Add a street "ST" to "TLV".
+	client.Street.
+		Create().
+		SetName("ST").
+		SetCity(tlv).
+		SaveX(ctx)
+	// This operation will fail because "ST"
+	// is already created under "TLV".
+	_, err := client.Street.
+		Create().
+		SetName("ST").
+		SetCity(tlv).
+		Save(ctx)
+	if err == nil {
+		return fmt.Errorf("expecting creation to fail")
+	}
+	// Add a street "ST" to "NYC".
+	client.Street.
+		Create().
+		SetName("ST").
+		SetCity(nyc).
+		SaveX(ctx)
+	return nil
+}