ent/doc: transaction example and docs

Summary: {F205899335}

Reviewed By: dlvhdr

Differential Revision: D17149531

fbshipit-source-id: cb8595d41ede6f813370564ca688f33d0dfe6905

examples/traversal/traversal.go

@@ -10,6 +10,7 @@ import (
 	"log"
 
 	"github.com/facebookincubator/ent/examples/traversal/ent/user"
+	"github.com/pkg/errors"
 
 	"github.com/facebookincubator/ent/examples/traversal/ent/pet"
 
@@ -41,6 +42,20 @@ func main() {
 	if err := Traverse2(ctx, client); err != nil {
 		log.Fatal(err)
 	}
+	// Generate a group of entities in a transaction.
+	if err := GenTx(ctx, client); err != nil {
+		log.Fatal(err)
+	}
+	// Wrap an existing function ("Gen") in a transaction.
+	if err := WrapGen(ctx, client); err != nil {
+		log.Fatal(err)
+	}
+	// WithTx helper.
+	if err := WithTx(ctx, client, func(tx *ent.Tx) error {
+		return Gen(ctx, tx.Client())
+	}); err != nil {
+		log.Fatal(err)
+	}
 }
 
 func Gen(ctx context.Context, client *ent.Client) error {
@@ -117,6 +132,7 @@ func Traverse(ctx context.Context, client *ent.Client) error {
 	return nil
 }
 
+// Traverse2 example from the doc.
 func Traverse2(ctx context.Context, client *ent.Client) error {
 	pets, err := client.Pet.
 		Query().
@@ -136,3 +152,89 @@ func Traverse2(ctx context.Context, client *ent.Client) error {
 	// [Pet(id=1, name=Pedro) Pet(id=2, name=Xabi)]
 	return nil
 }
+
+// GenTx example from the doc.
+func GenTx(ctx context.Context, client *ent.Client) error {
+	tx, err := client.Tx(ctx)
+	if err != nil {
+		return fmt.Errorf("starting a transaction: %v", err)
+	}
+	hub, err := tx.Group.
+		Create().
+		SetName("Github").
+		Save(ctx)
+	if err != nil {
+		return rollback(tx, fmt.Errorf("failed creating the group: %v", err))
+	}
+	// Create the admin of the group.
+	dan, err := tx.User.
+		Create().
+		SetAge(29).
+		SetName("Dan").
+		AddManage(hub).
+		Save(ctx)
+	if err != nil {
+		return rollback(tx, err)
+	}
+	// Create user "Ariel".
+	a8m, err := tx.User.
+		Create().
+		SetAge(30).
+		SetName("Ariel").
+		AddGroups(hub).
+		AddFriends(dan).
+		Save(ctx)
+	if err != nil {
+		return rollback(tx, err)
+	}
+	fmt.Println(a8m)
+	// Output:
+	// User(id=1, age=30, name=Ariel)
+	return tx.Commit()
+}
+
+// WrapGen wraps the existing "Gen" function in a transaction.
+func WrapGen(ctx context.Context, client *ent.Client) error {
+	tx, err := client.Tx(ctx)
+	if err != nil {
+		return err
+	}
+	txClient := tx.Client()
+	// Use the same "Gen" as above, but give it the transactional client; no code changes to "Gen".
+	if err := Gen(ctx, txClient); err != nil {
+		return rollback(tx, err)
+	}
+	return tx.Commit()
+}
+
+// WithTx example from the doc.
+func WithTx(ctx context.Context, client *ent.Client, fn func(tx *ent.Tx) error) error {
+	tx, err := client.Tx(ctx)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if v := recover(); v != nil {
+			tx.Rollback()
+			panic(v)
+		}
+	}()
+	if err := fn(tx); err != nil {
+		if rerr := tx.Rollback(); rerr != nil {
+			err = errors.Wrapf(err, "rolling back transaction: %v", rerr)
+		}
+		return err
+	}
+	if err := tx.Commit(); err != nil {
+		return errors.Wrapf(err, "committing transaction: %v", err)
+	}
+	return nil
+}
+
+// rollback calls to tx.Rollback and wraps the given error with the rollback error if occurred.
+func rollback(tx *ent.Tx, err error) error {
+	if rerr := tx.Rollback(); rerr != nil {
+		err = fmt.Errorf("%v: %v", err, rerr)
+	}
+	return err
+}