Named scopes and has_many :through relations in Rails

Named scopes in rails make for tidier code.

Instead of:

Response.find( :conditions => {:result => 'correct' } )

A named scope in the Response model looks like this:

named_scope :correct, :conditions => { :result => 'correct' }

and allows me to say:

Response.correct

Or, more usefully:

Question.last.responses.correct

This will even work with a has_many :through relationship:

class Course < ActiveRecord::Base
  has_many :questions
  has_many :responses, :through => :questions
  ...
end

Course.last.responses.correct

However, in Rails 2.2.2 there is a problem with named scopes that use inequalities. This usually requires a slightly different formulation of the :conditions like this:

named_scope :recent, :conditions => [ 'created_at > ?', Time.now - 3600 ]

Which allows me to say:

Question.last.responses.recent

But not:

Course.last.responses.recent

The first method produces:

SELECT * FROM `responses` WHERE (`responses`.question_id = 123) AND (created_at > '2009-06-13 22:21:38')

Whereas the second method produces:

SELECT `responses`.* FROM `responses` INNER JOIN questions ON responses.question_id = questions.id WHERE ((`questions`.course_id = 10)) AND (created_at > '2009-06-13 22:21:38')

Which causes the Rails log to complain:

Mysql::Error: Column 'created_at' in where clause is ambiguous

In other words, because of the way Rails translates the named_scope into SQL, it is not clear whether that inner join is selecting on questions.created_at or responses.created_at.

A simple workaround is to use ranges instead of inequalities:

named_scope :recent, :conditions => {:created_at => Time.now-3600..Time.now}

Which generates slightly different, and unambiguous, SQL:

SELECT `responses`.* FROM `responses` INNER JOIN questions ON responses.question_id = questions.id WHERE ((`questions`.course_id = 10)) AND (`responses`.`created_at` BETWEEN '2009-06-13 22:33:27' AND '2009-06-13 23:33:27')

This is unlikely to be the most elegant solution to the problem, but it works, which is good enough for me—at least for now.