NoMethodError

Question

エラー：

You have a nil object when you didn't expect it! 
You might have expected an instance of Array. 
The error occurred while evaluating nil.[] 


app/controllers/users_controller.rb:45:in `update' 

コントローラusers_controller

def update 
    @user.groups = Group.find(@params[:group_ids]) if @params[:group_ids] 
    if @user.update_attributes(params[:user]) 
     flash[:success] = "User updated." 
     #Redirect back to current users, i've change this as after the admin edit a user it would redirect them 
     #Too that users account which conflicts. 
     #if current_user.admin? # If the current user is an admin move them back to the user list page. 
     #redirect_to users_path 
     #else # if its a normal user just redirect them back to there dashboard 
     redirect_to @user 
     #end 
    else 
     @title = "Edit user" 
     render 'edit' 
    end 
    end 

図（edit.html.erb）：

<% for group in @groups %> 
    <%= check_box_tag "user[group_ids][]", group.id %> 
    <%= group.description %> 
    <% end %> 

Answer 1

@user.groups = Group.find(@params[:group_ids]) if @params[:group_ids] 

@user.groups = Group.find(params[:group_ids]) if params[:group_ids]