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Q&A

Cannot find definition of function that returns a path Question

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I want to find the place in the codebase where a path is defined, but searching for the function that creates it gives no results apart from the usage of that function.

For example, see thread.html.erb, line 142:

<%= link_to 'See the whole thread', comment_thread_path(@comment_thread) %>.

This calls a function comment_thread_path but searching for this function finds no definition of it.

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1 answer

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The function comment_thread_path is created automatically by Rails, based on the information provided in config/routes.rb.

See Rails Routing from the Outside In for further details.

Specifically, line 236 of config/routes.rb says (enclosing scope included for context):

scope 'comments' do
    ...
    get 'thread/:id', to: 'comments#thread', as: :comment_thread
    ...
end

This responds to a GET request on the route /comments/thread/:id by:

  • First calling the thread method of the CommentsController
  • Then rendering the app/views/comments/thread.html.erb view.

See Action Controller Overview for further details.

The comments#thread in the routes file determines both:

  • Which controller and method to call (being in the format controller#method)
  • Which view to render and where to find it (being in the format directory#viewfile)

The optional as: comment_thread overrides the default name that Rails would otherwise use in its automatically generated helper functions (otherwise it would use the default comments_thread - note the pluralisation). Now comment_thread_path and comment_thread_url are available, each a function taking a thread id as argument:

  • Calling comment_thread_path(123) returns /comments/thread/123
  • Calling comment_thread_url(123) returns the same thing but including the domain at the beginning, such as example.com/comments/thread/123.
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Thank you! I've gotten confused by routes too. (2 comments)

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