Sometimes I call a named route incorrectly: edit_user_project_path(project). And I get an illegible error message:

user_project_url failed to generate from {:action=>"show", :user_id=>#<Project id: 1, name: "Andy falls off a cliff", created_at: "2007-12-03 15:15:08", creator_id: 2, completed_at: nil, description: "", deleted_at: nil>, :controller=>"projects"}, expected: {:action=>"show", :controller=>"projects"}, diff: {:user_id=>#<Project id: 1, name: "Andy falls off a cliff", created_at: "2007-12-03 15:15:08", creator_id: 2, completed_at: nil, description: "", deleted_at: nil>}

I can’t read that. This, however, is much clearer:

user_project_url failed to generate from {:action=>"show", :user_id=>"1", :controller=>"projects"}, expected: {:action=>"show", :controller=>"projects"}, diff: {:user_id=>"1"}

The error message really ought to call #to_param on the path parts, don’t you think?

class ActionController::Routing::RouteSet
  # try to give a helpful error message when named route generation fails
  def raise_named_route_error(options, named_route, named_route_name)
    helpful_options = options.inject({}) {|hash, (key, value)| hash.merge(key => value.to_param) }
    diff = named_route.requirements.diff(options)
    unless diff.empty?
      raise RoutingError, "#{named_route_name}_url failed to generate  from #{helpful_options.inspect}, expected:  #{named_route.requirements.inspect}, diff:  #{named_route.requirements.diff(helpful_options).inspect}"
    else
      required_segments = named_route.segments.select {|seg| (!seg.optional?) && (!seg.is_a?(DividerSegment)) }
      required_keys_or_values = required_segments.map { |seg| seg.key rescue seg.value } # we want either the key or the value from the segment
      raise RoutingError, "#{named_route_name}_url failed to generate from #{helpful_options.inspect} - you may have ambiguous routes, or you may need to supply additional parameters for this route.  content_url has the following required parameters: #{required_keys_or_values.inspect} - are they all satisfied?"
    end
  end
end

Make it so!