html - .collect method get an error undefined method `collect' for nil:NilClass -


I am trying to list a drop down list, but give it to me when I try

Regarding 'Gathering' method for regulator: Null Class

Controller: 

  DRF Existing @ course = course. All-end DP duplicate course = course.fund_b_id (permitd_up [: ID ]) New_course = course.dup if new_course.save redirect_to: action = & gt; 'Show' and Redirect-to: Back End End   
 View:  
  & lt; H3 & gt; Choose a course & lt; / H3 & gt; & Lt;% = form_for: course, url: {: action = & gt; "Duplicate", Method: "Post"}. f |% & gt; & Lt;% = f.select: id, @ course. Collect {| c | | [Seed, CNN]}%> & Lt; Br> & Lt; Br> & lt;% = f.submit% & gt; & Lt;% end% & gt;    
 
 
 You will receive the following error  
  undefined blue Method to 'Collect': NilClass   
 at  
     
 Only when  @ course  example variable  was not set in the action that provided this particular view.   
 I think the  @ course  variable has been set in the  existing  method. If you are using  existing  as an action that presents this view then your view name should be  existing.html. ARB .  
 Or if you are rendering with a different action, in that case, you should set the  @ course  value in either that special action, either by directing the value within the direct action or by there Should be set by calling the  current  method.

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