Where is the documentation on url helpers in rails?

You can determine how many parameters a route helper requires by looking at the route definition.

For example, you might have this routes file:

resources :users

If you ran rake routes at the command line you would see something like this:

    users GET    /users(.:format)          users#index
          POST   /users(.:format)          users#create
 new_user GET    /users/new(.:format)      users#new
edit_user GET    /users/:id/edit(.:format) users#edit
     user GET    /users/:id(.:format)      users#show
          PUT    /users/:id(.:format)      users#update
          DELETE /users/:id(.:format)      users#destroy

The first column gives you the name of the route. You can append _path or _url to get the name of a route helper.

The third column shows the pattern. This is where you can figure out what the arguments are. Arguments are the parts prefixed with a colon, and optional arguments are shown in parentheses. For example the edit_user route has the pattern /users/:id/edit(.:format) which contains one required argument (id) and one optional argument (format), which tells me I need to pass at least one argument to the edit_user_path or edit_user_url helper:

edit_user_path(1) # => "/users/1/edit"
edit_user_path(2, :html) # => "/users/2/edit.html"

You can also use the argument names from the pattern as keys in a hash:

edit_user_path(id: 3, format: 'js') # => "/users/3/edit.js"

Finally, you can add extra arguments which will become part of the query string:

edit_user_path(id: 4, format: 'json', foo: 1) # => "/users/4/edit.json?foo=1"
edit_user_path(5, bar: 2) # => "/users/5/edit?bar=2"

See the Rails Routing Guide’s section on Listing Existing Routes for more information about rake routes.