该where子句永远不会返回nil，它返回一个ActiveRecord :: Relation对象，该对象可以为空！

尝试以下方法：

def create if @trained = Certificate.where(user_id: params[:certificate][:user_id]).first @trained.update_attributes(attend: "No") else @trained = Certificate.new(params[:certificate]) if @trained.save @trained.update_attributes(attend: "Yes") end end

redirect_to grandstreamers_resellers_path end 但我会将代码重构为如下形式：

def create @trained = Certificate.where(user_id: params[:certificate][:user_id]).first @trained ||= Certificate.new(params[:certificate]) if @trained.persisted? # tests if the records is persisted in the DB (has an ID) @trained.attend = "No" else @trained.attend = "Yes" end

@trained.save redirect_to grandstreamers_resellers_path end 非常简短的版本：

def create @trained = Certificate.where(user_id: params[:certificate][:user_id]).first || Certificate.new(params[:certificate]) @trained.attend = @trained.persisted? ? "No" : "Yes" @trained.save

redirect_to grandstreamers_resellers_path end 为了记录（@NitinJ，请看一下），您可以在if条件中分配变量：

if first_user = User.first

if User.first does not returns nil, executes the first block with local variable first_user available

first_user.id else # User.first returned nil end来源：stack overflow