Ruby on Rails
PiggyBackQuery

The availability of EagerLoadingOfAssociations (as of Rails 0.12) removes the need for Piggy-back queries in nearly all cases.

Consider this model:

class Post < \ActiveRecord::Base
  has_one :author
end
 
class Author < \ActiveRecord::Base
  belongs_to :post
end

The easy way of displaying the author’s name alongside each post would be:

for post in Post.find_all
  puts post.title + " was written by " + post.author.name
end

Unfortunately, that’ll generate one additional SQL query per post looped over. Some times that doesn’t really matter, but if it’s a frequently used query and the number of posts you’re looping over is high, it can be a problem.

Piggy-backs to the rescue! Active Records are dynamic beasts that don’t mind carrying additional attributes with them. So what you do is create a specialized static finder method that grabs the author name along-side the rest of the post:

class Post < \ActiveRecord::Base
  def self.find_all_with_author_name
    find_by_sql(
      "SELECT posts.*, authors.name as author_name " +
      "FROM posts, authors " +
      "WHERE posts.author_id = authors.id"
    )
  end
end

Now you can do the same loop referencing the author_name attribute:

for post in Post.find_all_with_author_name
  puts post.title + " was written by " + post.author_name
end

Just a single SQL query!

Additional attributes are always strings. So if you wanted to do something like this:

class Order < \ActiveRecord::Base
  has_many :details
  def self.find_with_subtotal
    find_by_sql(
      "SELECT o.*, sum(d.quantity * d.price) AS subtotal" +
      "  FROM orders o INNER JOIN details d" +
      "    ON d.order_id = o.id"
    )
  end
end

and then be able to use order.subtotal to do calculations. In this scenario, you’ll have to use order.subtotal.to_f