Rails 3,如何确保Group has_one user.role = groupleader

class Group< ActiveRecord::Base
  has_one :user, -> { where(role: 'groupleader') }
end

class User< ActiveRecord::Base
  belongs_to :group
end

class AddRoleToUsers< ActiveRecord::Migration
  def change
    add_column :users, :role, :string
  end
end

class User< ActiveRecord::Base
  belongs_to :group
  validates :role, uniqueness: { scope: :group_id, conditions: -> { where(role: 'groupleader') } }
end

group = Group.find(1)
user = group.build_user(role: 'groupleader')
user.save!

group = Group.find(1)
groupleader = group.user