Question

I have a method which is meant to return a number of items from the database based on a set of criteria:

scope :expired_not_marked, lambda { |client|
items = where('items.status > 0 AND items.expires_at < ? AND items.expired_at IS NULL AND (winning_bid_id IS NULL OR winner_id IS NULL)', Time.now)
unless client.nil?
    items.where('items.client_id = ?', client.id)
end
}

It’s being called as Item.expired_not_marked nil. When I run this from the IRB I get a lot of results but it shows the SQL query being executed as:

SELECT `items`.* FROM `items` 

It’s pretty obviously that was not the original author’s intent. As a result the same items are being processed over and over.

Why is this broken, and how do I fix it. The where clause seems correct. The above method is within the item.rb model.

Answer 1

Your problem is that your lambda sometimes returns nil and a scope that returns nil won’t do anything useful.

The lambda will return the value of its last expression. In your case, that expression will be the unless. So if client is not nil, it will return this:

items.where('items.client_id = ?', client.id)

and all will be good. But if client.nil? is true, the unless will evaluate to nil and your scope will return nil. I think you’d be better off with something like this:

scope :expired_not_marked, lambda { |client|
  items = where('items.status > 0 AND items.expires_at < ? AND items.expired_at IS NULL AND (winning_bid_id IS NULL OR winner_id IS NULL)', Time.now)
  unless client.nil?
    items = items.where('items.client_id = ?', client.id)
  end
  items
}

That way you always have a clear, explicit, and well defined return value.

---

The ActiveRecord Query Interface Guide recommends that you use class methods for scopes that take arguments:

Using a class method is the preferred way to accept arguments for scopes.

so you could also do this if the lambda approach is too noisy:

def self.expired_not_marked(client)
  items = where('items.status > 0')
  items = items.where('items.expires_at < ?', Time.now)
  items = items.where('items.expired_at IS NULL')
  items = items.where('winning_bid_id IS NULL OR winner_id IS NULL')
  unless client.nil?
    items = items.where('items.client_id = ?', client.id)
  end
  items
}

You don’t have to use a class method of course. And you don’t have to break the query into a bunch of little where calls for each component but it might be easier to read this way.