Ruby Generated Code

This page describes the API of message objects that the protocol buffer compiler generates for any given protocol definition. You should read the proto3 language guide before reading this document.

For the moment only proto3 is supported. Support for proto2 is planned, but not available yet.

The protocol compiler for Ruby emits Ruby source files that use a DSL to define the message schema. However the DSL is still subject to change (especially as we add more features like proto2 support). In this guide we only describe the API of the generated messages, and not the DSL.

Compiler Invocation

The protocol buffer compiler produces Ruby output when invoked with the --ruby_out= command-line flag. The parameter to the --ruby_out= option is the directory where you want the compiler to write your Ruby output. The compiler creates a .rb file for each .proto file input. The names of the output files are computed by taking the name of the .proto file and making two changes:

  • The extension (.proto) is replaced with _pb.rb.
  • The proto path (specified with the --proto_path= or -I command-line flag) is replaced with the output path (specified with the --ruby_out= flag).

So, for example, let's say you invoke the compiler as follows:

protoc --proto_path=src --ruby_out=build/gen src/foo.proto src/bar/baz.proto

The compiler will read the files src/foo.proto and src/bar/baz.proto and produce two output files: build/gen/foo_pb.rb and build/gen/bar/baz_pb.rb. The compiler will automatically create the directory build/gen/bar if necessary, but it will not create build or build/gen; they must already exist.


The package name defined in the .proto file is used to generate a module structure for the generated messages. Given a file like:

package foo_bar.baz;

message MyMessage {}

The protocol compiler generates an output message with the name FooBar::Baz::MyMessage.


Given a simple message declaration:

message Foo {}

The protocol buffer compiler generates a class called Foo. The generated class derives from the Ruby Object class (protos have no common base class). Unlike C++ and Java, Ruby generated code is unaffected by the optimize_for option in the .proto file; in effect, all Ruby code is optimized for code size.

You should not create your own Foo subclasses. Generated classes are not designed for subclassing and may lead to "fragile base class" problems.

Ruby message classes define accessors for each field, and also provide the following standard methods:

  • Message#dup, Message#clone: Performs a shallow copy of this message and returns the new copy.
  • Message#==: Performs a deep equality comparison between two messages.
  • Message#hash: Computes a shallow hash of the message's value.
  • Message#to_hash, Message#to_h: Converts the object to a ruby Hash object. Only the top-level message is converted.
  • Message#inspect: Returns a human-readable string representing this message.
  • Message#[], Message#[]=: Gets or sets a field by string name. In the future this will probably also be used to get/set extensions.

The message classes also define the following methods as static. (In general we prefer static methods, since regular methods can conflict with field names you defined in your .proto file.)

  • Message.decode(str): Decodes a binary protobuf for this message and returns it in a new instance.
  • Message.encode(proto): Serializes a message object of this class to a binary string.
  • Message.decode_json(str): Decodes a JSON text string for this message and returns it in a new instance.
  • Message.encode_json(proto): Serializes a message object of this class to a JSON text string.
  • Message.descriptor: Returns the Google::Protobuf::Descriptor object for this message.

When you create a message, you can conveniently initialize fields in the constructor. Here is an example of constructing and using a message:

message = => 1,
                        :string_field => "String",
                        :repeated_int_field => [1, 2, 3, 4,
                        :submessage_field => => 42))
serialized = MyMessage.encode(message)

message2 = MyMessage.decode(serialized)
raise unless message2.int_field == 1

A message can be declared inside another message. For example: message Foo { message Bar { } }

In this case, the Bar class is declared as a class inside of Foo, so you can refer to it as Foo::Bar.


For each field in a message type, there are accessor methods to set and get the field. So given a field foo you can write: = get_value()

Whenever you set a field, the value is type-checked against the declared type of that field. If the value is of the wrong type (or out of range), an exception will be raised.

Singular Fields (proto3)

For singular primitive fields (numbers, strings, and boolean), the value you assign to the field should be of the correct type and must be in the appropriate range:

  • Number types: the value should be a Fixnum, Bignum, or Float. The value you assign must be exactly representable in the target type. So assigning 1.0 to an int32 field is ok, but assigning 1.2 is not.
  • Boolean fields: the value must be true or false. No other values will implicitly convert to true/false.
  • Bytes fields: the assigned value must be a String object. The protobuf library will duplicate the string, convert it to ASCII-8BIT encoding, and freeze it.
  • String fields: the assigned value must be a String object. The protobuf library will duplicate the string, convert it to UTF-8 encoding, and freeze it.

No automatic #to_s, #to_i, etc. calls will happen to perform automatic conversion. You should convert values yourself first, if necessary.

Note that proto3 does not provide a way to check whether singular non-message fields have been explicitly set, so if a returned value is 0/false/"" there is no way to tell whether this value was set somewhere or is just the provided default.

Singular Message Fields

For submessages unset fields will return nil, so you can always tell if the message was explicitly set or not. You can also clear a submessage field by setting its value explicitly to nil.

if message.submessage_field.nil?
  puts "Submessage field is unset."
  message.submessage_field = nil
  puts "Cleared submessage field."

When you do assign a submessage it must be a generated message object of the correct type.

It is possible to create message cycles when you assign submessages. For example:

// foo.proto
message RecursiveMessage {
  RecursiveMessage submessage = 1;

# test.rb

require 'foo'

message =
message.submessage = message

If you try to serialize this, the library will detect the cycle and fail to serialize.

Repeated Fields

Repeated fields are represented using a custom class Google::Protobuf::RepeatedField. This class acts like a Ruby Array and mixes in Enumerable. Unlike a regular Ruby array, RepeatedField is constructed with a specific type and expects all of the array members to have the correct type. The types and ranges are checked just like message fields.

int_repeatedfield =, [1, 2, 3])

# Raises TypeError.
int_repeatedfield[2] = "not an int32"

# Raises RangeError
int_repeatedfield[2] = 2**33

message.int32_repeated_field = int_repeatedfield

# This isn't allowed; the regular Ruby array doesn't enforce types like we need.
message.int32_repeated_field = [1, 2, 3, 4]

# This is fine, since the elements are copied into the type-safe array.
message.int32_repeated_field += [1, 2, 3, 4]
The RepeatedField type supports all of the same methods as a regular Ruby Array. You can convert it to a regular Ruby Array with repeated_field.to_a.

Map Fields

Map fields are represented using a special class that acts like a Ruby Hash (Google::Protobuf::Map). Unlike a regular Ruby hash, Map is constructed with a specific type for the key and value and expects all of the map's keys and values to have the correct type. The types and ranges are checked just like message fields and RepeatedField elements.

int_string_map =, :string)

# Returns nil; items is not in the map.
print int_string_map[5]

# Raises TypeError, value should be a string
int_string_map[11] = 200

# Ok.
int_string_map[123] = "abc"

message.int32_string_map_field = int_string_map


Since Ruby does not have native enums, we create a module for each enum with constants to define the values. Given the .proto file:

message Foo {
  enum SomeEnum {
    VALUE_A = 0;
    VALUE_B = 5;
    VALUE_C = 1234;
  optional SomeEnum bar = 1;
You can refer to enum values like so:
print Foo::SomeEnum::VALUE_A  # => 0 = Foo::SomeEnum::VALUE_A

You may assign either a number or a symbol to an enum field. When reading the value back, it will be a symbol if the enum value is known, or a number if it is unknown. Since proto3 uses open enum semantics, any number may be assigned to an enum field, even if it was not defined in the enum. = 0
puts  # => :VALUE_A = :VALUE_B
puts  # => :VALUE_B = 999
puts  # => 999

# Raises: RangeError: Unknown symbol value for enum field. = :UNDEFINED_VALUE

# Switching on an enum value is convenient.
when :VALUE_A
  # ...
when :VALUE_B
  # ...
when :VALUE_C
  # ...
  # ...
An enum module also defines the following utility methods:
  • Enum#lookup(number): Looks up the given number and returns its name, or nil if none was found. If more than one name has this number, returns the first that was defined.
  • Enum#resolve(symbol): Returns the number for this enum name, or nil if none was found.
  • Enum#descriptor: Returns the descriptor for this enum.

We should do auto-stripping of enum prefixes like C# does.


Given a message with a oneof:

message Foo {
  oneof test_oneof {
     string name = 1;
     int32 serial_number = 2;

The Ruby class corresponding to Foo will have members called name and serial_number with accessor methods just like regular fields. However, unlike regular fields, at most one of the fields in a oneof can be set at a time, so setting one field will clear the others.

message =

# Fields have their defaults.
raise unless == ""
raise unless message.serial_number == 0
raise unless message.test_oneof == nil = "Bender"
raise unless == "Bender"
raise unless message.serial_number == 0
raise unless message.test_oneof == :name

# Setting serial_number clears name.
message.serial_number = 2716057
raise unless == ""
raise unless message.test_oneof == :serial_number

# Setting serial_number to nil clears the oneof.
message.serial_number = nil
raise unless message.test_oneof == nil