跳到主要内容

Optional Fields

A common issue with Protobufs is that the way that nil values are represented: a zero-valued primitive field isn't encoded into the binary representation, this means that applications cannot distinguish between zero and not-set for primitive fields.

To support this, the Protobuf project supports some Well-Known types called "wrapper types". For example, the wrapper type for a bool, is called google.protobuf.BoolValue and is defined as:

// Wrapper message for `bool`.
//
// The JSON representation for `BoolValue` is JSON `true` and `false`.
message BoolValue {
// The bool value.
bool value = 1;
}

When entproto generates a Protobuf message definition, it uses these wrapper types to represent "Optional" ent fields.

Let's see this in action, modifying our ent schema to include an optional field:

// Fields of the User.
func (User) Fields() []ent.Field {
return []ent.Field{
field.String("name").
Unique().
Annotations(
entproto.Field(2),
),
field.String("email_address").
Unique().
Annotations(
entproto.Field(3),
),
field.String("alias").
Optional().
Annotations(entproto.Field(4)),
}
}

Re-running go generate ./..., observe that our Protobuf definition for User now looks like:

message User {
int32 id = 1;

string name = 2;

string email_address = 3;

google.protobuf.StringValue alias = 4; // <-- this is new
}

The generated service implementation also utilize this field. Observe in entpb_user_service.go:

// Create implements UserServiceServer.Create
func (svc *UserService) Create(ctx context.Context, req *CreateUserRequest) (*User, error) {
user := req.GetUser()
m := svc.client.User.Create()
if user.GetAlias() != nil {
m.SetAlias(user.GetAlias().GetValue())
}
m.SetEmailAddress(user.GetEmailAddress())
m.SetName(user.GetName())
res, err := m.Save(ctx)

switch {
case err == nil:
return toProtoUser(res), nil
case sqlgraph.IsUniqueConstraintError(err):
return nil, status.Errorf(codes.AlreadyExists, "already exists: %s", err)
case ent.IsConstraintError(err):
return nil, status.Errorf(codes.InvalidArgument, "invalid argument: %s", err)
default:
return nil, status.Errorf(codes.Internal, "internal: %s", err)
}
}

To use the wrapper types in our client code, we can use helper methods supplied by the wrapperspb package to easily build instances of these types. For example in cmd/client/main.go:

func randomUser() *entpb.User {
return &entpb.User{
Name: fmt.Sprintf("user_%d", rand.Int()),
EmailAddress: fmt.Sprintf("user_%d@example.com", rand.Int()),
Alias: wrapperspb.String("John Doe"),
}
}