```ruby
require 'croudia/base'

module Croudia
  class Identity < Croudia::Base
    def initialize(*)
      super
      raise ArgumentError, 'argument must have an :id key' unless id
      @attrs[:id_str] = id.to_s
    end

    # @param other [Croudia::Identity]
    # @return [Boolean]
    def ==(other)
      super || self.class == other.class && id == other.id
    end

    # @return [Integer]
    def id
      @attrs[:id]
    end
  end
end
```

Set :id_str only if not set
user
```ruby
require 'croudia/base'

module Croudia
  class Identity < Croudia::Base
    def initialize(*)
      super
      raise ArgumentError, 'argument must have an :id key' unless id
      @attrs[:id_str] ||= id.to_s
    end

    # @param other [Croudia::Identity]
    # @return [Boolean]
    def ==(other)
      super || self.class == other.class && id == other.id
    end

    # @return [Integer]
    def id
      @attrs[:id]
    end
  end
end
```
assistant