Add support for embed: :ids option for in associations

[guilleiguaran]

Hey!!

We would like to add an option to associations to allow the inclusion of only associated object ids instead of full objects, for this I'm proposing the embed: :ids option, example:

class AuthorSerializer < ActiveModel::Serializer
  attributes :id, :name

  has_many :posts, embed: :ids
end

in JSON API adapter this will not include the posts within the linked hash.

I'm not convinced by the option name, let me know if you have other suggestions.

[kurko]

This looks good to me. I'm thinking about something like the following would be helpful too:

class CompanySerializer < ActiveModel::Serializer
  attributes :id, :name

  has_many :employees, embed: :employees_ids
  has_many :addresses, embed: :ids

  private

  def employees_ids
    self.employees.pluck(:id) unless self.employees.count > 500
  end
end

[sogamoso]

+1 for this.

@kurko's suggestion would be really too, although that could be part of a later pull request.

[steveklabnik]

I like the name :)