Question

In a view I can do this and the link works fine:

<%= link_to "Most popular comment", comment_path( Comment.find(5) ) %>

So I know that my routes are set up to show an individual comment record via comment_path(). However when I try this:

 <%= link_to "Most popular comment", comment_path( @post.comments.order("vote_cnt DESC").first )

I get a “No route matches {:action=>"show", :controller=>"comments"}” error. But I know this not an accurate error description because the first link_to() statement listed above works. I’ve confirmed the route exists – from rake routes I get this:

 comment GET    /comments/:id(.:format)  {:action=>"show", :controller=>"comments"}

In IRB I can see that the two statements I’m passing to comment_path() both generate the same class, namely “Comment“:

irb(main):022:0> top_comment = post.comments.order("vote_cnt DESC").first
  Comment Load (0.6ms)  SELECT "comments".* FROM "comments" WHERE "post_id" = 2 ORDER BY vote_cnt DESC LIMIT 1
=> #<Comment id: 5, heading: nil, body: nil, user_id: 5, created_at: "2012-02-03 01:23:30", updated_at: "2012-02-03 01:23:30",vote_cnt: 0>

irb(main):023:0> top_comment.class
=> Comment(id: integer, heading: string, body: text, user_id: integer, created_at: datetime, updated_at: datetime, vote_cnt: integer)

irb(main):024:0> comment_5 = Comment.find(5)
  Comment Load (3.2ms)  SELECT "comments".* FROM "comments" WHERE "comments"."id" = $1 LIMIT 1  [["id", 5]]
=> #<Comment id: 5, heading: nil, body: nil, user_id: 5, created_at: "2012-02-03 01:23:30", updated_at: "2012-02-03 01:23:30", vote_cnt: 0>

irb(main):025:0> comment_5.class
=> Comment(id: integer, heading: string, body: text, user_id: integer, created_at: datetime, updated_at: datetime, vote_cnt: integer)

If both statements generate the same class of object, how can link_to() work with one and cause a route error on the other, especially when both resolve to the same exact record? I tried:

<% top_comment = post.comments.order("vote_cnt DESC").first %>
<%= link_to "Most popular comment", comment_path( top_comment )

and that generates the same “No path matches…” error.

Any ideas what is going on here? It seems that link_to() works well with a record that comes from a straight up query against a table, but errors out on that same record if was retrieved via an ActiveRecord::Relation action. Why? How can that be?

Answer 1

[converting my comment above into an answer]

Sorry guys – programmer logic error.

I tried “rkb”s suggestion of …first.id, and got an error saying I can’t get the ID of a nil object. This link_to call was in a loop:

@top_ten_posts each do |post|
...
end

Since they are recent posts, one of them didn’t have any comments. Duh! My code should check for that first.

My bad. However, it is very frustrating that link_to won’t give you a clear indication that you have passed it a nil object. “No route matches…”?!?!?! Pretty poor error message for the situation!