examples/version: provide an optimistic locking example

examples/version/example_test.go

@@ -0,0 +1,46 @@
+// 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"
+
+	"entgo.io/ent/examples/version/ent"
+	_ "entgo.io/ent/examples/version/ent/runtime"
+	"entgo.io/ent/examples/version/ent/user"
+
+	_ "github.com/mattn/go-sqlite3"
+)
+
+func Example_OptimisticLock() {
+	client, err := ent.Open("sqlite3", "file:ent?mode=memory&cache=shared&_fk=1")
+	if err != nil {
+		log.Fatalf("failed opening connection to sqlite: %v", err)
+	}
+	defer client.Close()
+	ctx := context.Background()
+	// Run the auto migration tool.
+	if err := client.Schema.Create(ctx); err != nil {
+		log.Fatalf("failed creating schema resources: %v", err)
+	}
+
+	usr := client.User.Create().SetStatus(user.StatusOnline).SaveX(ctx)
+	fmt.Println(usr.ID, usr.Status)
+
+	usrCopy := client.User.Query().OnlyX(ctx)
+	usrCopy = usrCopy.Update().SetStatus(user.StatusOffline).SaveX(ctx)
+	fmt.Println(usrCopy.ID, usrCopy.Status)
+
+	// The operation fails because the user was updated by another process (usrCopy).
+	_, err = usr.Update().SetStatus(user.StatusOffline).Save(ctx)
+	fmt.Println(err)
+
+	// Output:
+	// 1 online
+	// 1 offline
+	// user 1 was changed by another process
+}