Pivots patch rails: named_scope with the :joins can cause table aliasing issues

September 29, 2008 David Stevenson

In order to accomplish some advanced search functionality, we’ve added a lot of named_scopes to our User model. This seems like a good idea, and well within the intended use for named_scopes. Unfortunately, we ran into issues with our :joins. We have a separate User and Profile model, but our advanced search scopes often needed both to make decisions. So we had some scopes that look like this:

class User
  named_scope :verified {
    :conditions => {:email_verified => true}
  }

  named_scope :answered_questions {
    :join => "INNER JOIN profiles ON profiles.user_id = users.id " +
                 "INNER JOIN answers ON answers.profile_id = profiles.id"
  }

  named_scope :with_name { lambda { |name|
    :join => "INNER JOIN profiles ON profiles.user_id = users.id",
    :conditions => ["profiles.name LIKE ?", "%#{name}%"]
  } }
end

Using these named_scopes, we wanted to dynamically construct a finder that would return the results the user was interested, such as: User.verified or User.answered_questions or even User.verified.answered_questions.with_name('Joseph'). The last scope caused issues, unfortunately, with table aliasing. The query ended up joining in the profiles table twice, in exactly the same way without renaming the table, so mysql rejects the query.

The easiest solution to this problem was to use only the hash form for :join clauses, such as :join => :profile. Rails correctly merges multiple consecutive join scopes that use hashes. If you need to use string joins (such as a LEFT JOIN rather than an INNER JOIN) or put a condition directly on your join, then merging goes out the window and the hashed form is immediately converted to a string and all consecutive joins are “merged” by appending them together.

We started by manually aliasing our scopes, but in some cases we were concerned about the amount of duplicate data this was causing in our queries.

We thought about creating a dependency framework for named_scopes, such that you could have a single :profile scope that other scopes were dependent on and it would only ever get added once. This seemed really difficult because of the way the with_scopes are constructed by named_scopes, there was no good place to keep track of these dependencies, and it would still cause problems if you had a manual with_scope, or :join in your find.

Finally we decided that rails fundamentally lacked the capability to deal with duplicate joins, and that we should solve this problem. It seemed a good solution was to allow :join options to take an array of strings as follows:

  named_scope :answered_questions {
    :join => ["INNER JOIN profiles ON profiles.user_id = users.id",
                 "INNER JOIN answers ON answers.profile_id = profiles.id"]
  }

Now calling User.answered_questions.with_name('Joseph') will create three values in a :join array, two of which are identical and will be uniq’d out. The downside to this approach is that each value in the :join array has to be string identical, or it will not be properly uniq’d.

So if you are mixing hash style :profile joins with string joins of the same table you need to be careful you match the rails generated syntax. We mostly use string style joins to avoid this issue.

Here’s the ticket the we filed and patched:
1077-chaining-scopes-with-duplicate-joins-causes-alias-problem

It has been commited and will roll out with rails 2.2. Since then we have filed two more issues related to :join and :include:

We hope to patch these two as well!

Joseph & David

About the Author

Biography

More Content by David Stevenson
Previous
Pivotal Labs Tech Talks now on iTunes!
Pivotal Labs Tech Talks now on iTunes!

At Pivotal, we host tech talks for our and guest developers. You can now subscribe to these video and audi...

Next
Solr demystified
Solr demystified

As I mentioned in this post, we've decided to set aside some of our weekly brown bags to spread around some...

Enter curious. Exit smarter.

Register Now