has_manyな関連性のモデルを、元レコードの保存時に削除する

class Account < ActiveRecord::Base
  has_many :roles
end

class Role < ActiveRecord::Base
  belongs_to :account
end

# コントローラー側のメソッド内
def update 
  account = Account.find_by(params[:id])

  account.roles = [] #=> ここでrolesがdeleteされてしまう
  params[:roles].each do | role | 
    # ...
  end

  unless account.save
    render :action => "edit"
  end
end

def update 
  account = Account.find_by(params[:id])

  params[:roles].each do | role | 
    # ...
  end
  account.roles.each do | role | 
    role.mark_for_destruction unless roles.map{|r| r[:name]}.include?(role.name)
  end

  unless account.save
    render :action => "edit"
  end
end

role.marked_for_destruction?