Nested routesEdit

Rails allows you to set up your routes (in config/routes.rb) so that one or more resources are nested inside another resource.

Example

Consider a forum application with a forums resource routed as follows:

map.resources :forums

Different forums could be viewed using URLs like:

• http://example.com/forums/11
• http://example.com/forums/28

Now, imagine that forums can have multiple topics, and topics can have multiple posts:

map.resources :topics
map.resources :posts

Individual topics could be viewed as follows:

• http://example.com/topics/19
• http://example.com/topics/56

And individual posts like this:

• http://example.com/posts/20
• http://example.com/posts/34

You can define nested routes as follows, in recognition of the fact that posts belong to topics, and topics belong to forums:

map.resources :forums do |forum|
  forum.resources :topics do |topic|
    topic.resources :posts
  end
end

This in turn enables URLs like these:

• http://example.com/forums/2/topics/5
• http://example.com/forums/11/topics/5/posts/12

Note that the URL reflects the relationship that each entity has to its containing "parent". As far as I can tell, this is the only benefit that nested routes bring.

Verdict

Personally, I am not such a big fan of nested resources. I think they introduce too much additional complexity in your controller logic, which increases the scope for bugs, and in return offer little benefit. They redundantly encode information in the URL, making it unnecessarily longer. Nested routes are more complex and therefore harder to test, which increases the likelihood that they either won't be tested at all or they'll be tested inadequately or inaccurately.

Uniqueness

Note that the ids at each level must be unique: it is not possible for forum 1 and forum 2 to both have a topic with an id of 1. This leads to URLs like this one in which the very first post in a new topic can have an id like 3366:

• http://example.com/forums/2/topics/122/posts/3366

Given that the post id is clearly unique within the application I would rather just go directly to it with a URL like this one:

• http://example.com/posts/3366

In this way redundant information is eliminated from the URL and it is kept shorter.

The longer URL is also more brittle because if a moderator ever decides to move a topic to a different forum the old URL will no longer work.

Discoverability

Another problem with nested resources is that the URLs don't lend themselves to casual exploration. By way of example, consider a topic like this:

• http://example.com/forums/3/topics/135

It would be nice to look at that URL, wonder what the previous topic might be, and manually edit the URL to read:

• http://example.com/forums/3/topics/134

This will only work if topic number 134 belongs to forum number 3, something which may not necessarily be true.

Depending on your stand-point, this lack of discoverability may actually be a feature (security through obscurity of sorts), but for me it is a point against nested routes.

Testability

Because non-nested routing is simpler it is also easier to test.