Why the following error?

Question

I am building a nested resource like

 resources :blogs do
    resources :comments
  end

when I visit the following

blogs/1/comments/2, I received the error

undefined method `comment_url' for #<#<Class:0x4173108>:0x43b45d8>

Here is the code inside views\comments_comment.json.jbuilder

json.url comment_url(comment, format: :json)

How to fix?

Edit per suggestion

json.url blog_comments_url(comment.blog, comment, format: :json)

However the URl I get is always like "url":"http://localhost:3000/blogs/1/comments.json"

I would like to get something like "url":"http://localhost:3000/blogs/1/comments/2.json"

Answer 1

If you do a rake routes, you should see something like this:

    blog_comments GET    /blogs/:blog_id/comments(.:format)          comments#index                                     
                  POST   /blogs/:blog_id/comments(.:format)          comments#create                                    
 new_blog_comment GET    /blogs/:blog_id/comments/new(.:format)      comments#new                                       
edit_blog_comment GET    /blogs/:blog_id/comments/:id/edit(.:format) comments#edit                                      
     blog_comment GET    /blogs/:blog_id/comments/:id(.:format)      comments#show                                      
                  PATCH  /blogs/:blog_id/comments/:id(.:format)      comments#update                                    
                  PUT    /blogs/:blog_id/comments/:id(.:format)      comments#update                                    
                  DELETE /blogs/:blog_id/comments/:id(.:format)      comments#destroy                                   
            blogs GET    /blogs(.:format)                            blogs#index                                        
                  POST   /blogs(.:format)                            blogs#create                                       
         new_blog GET    /blogs/new(.:format)                        blogs#new                                          
        edit_blog GET    /blogs/:id/edit(.:format)                   blogs#edit                                         
             blog GET    /blogs/:id(.:format)                        blogs#show                                         
                  PATCH  /blogs/:id(.:format)                        blogs#update                                       
                  PUT    /blogs/:id(.:format)                        blogs#update                                       
                  DELETE /blogs/:id(.:format)                        blogs#destroy                                      

The first column has the path helper name without the _url or _path suffix.

Your :comments resources is nested within your :blogs resource so the helper names are also nested:

• blog_comments not comments
• new_blog_comment not new_comment
• ...

So the helper you're looking for is blog_comment_url. Also, since the resource is nested, the helper will want both the comment and blog as arguments:

json.url blog_comment_url(comment.blog, comment, format: :json)