accepts_nested_attributes

sample.rb

# http://api.rubyonrails.org/classes/ActiveRecord/NestedAttributes/ClassMethods.html

# need to specify id for an update:

class Member < ActiveRecord::Base
  has_one :avatar
  accepts_nested_attributes_for :avatar
end

params = { member: { name: 'Jack', avatar_attributes: { icon: 'smiling' } } }
member = Member.create(params[:member])
member.avatar.id # => 2
member.avatar.icon # => 'smiling'

# need to specify id of child record for an update
params = { member: { avatar_attributes: { id: '2', icon: 'sad' } } }
member.update params[:member]
member.avatar.icon # => 'sad'