Robot Has No Heart

Xavier Shay blogs here

A robot that does not have a heart

Class Table Inheritance and Eager Loading

Consider a typical class table inheritance table structure with items as the base class and dvds and cars as two subclasses. In addition to what is strictly required, items also has an item_type parameter. This denormalization is usually a good idea, I will save the justification for another post so please take it for granted for now.

The easiest way to map this relationship with Rails and ActiveRecord is to use composition, rather than trying to hook into the class loading code. Something akin to:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31

class Item < ActiveRecord::Base
  SUBCLASSES = [:dvd, :car]
  SUBCLASSES.each do |class_name|
    has_one class_name
  end

  def description
    send(item_type).description
  end
end

class Dvd < ActiveRecord::Base
  belongs_to :item

  validates_presence_of :title, :running_time
  validates_numericality_of :running_time

  def description
    title
  end
end

class Car < ActiveRecord::Base
  belongs_to :item

  validates_presence_of :make, :registration

  def description
    make
  end
end

A naive way to fetch all the items might look like this:

1

Item.all(:include => Item::SUBCLASSES)

This will issue one initial query, then one for each subclass. (Since Rails 2.1, eager loading is done like this rather than joining.) This is inefficient, since at the point we preload the associations we already know which subclass tables we should be querying. There is no need to query all of them. A better way is to hook into the Rails eager loading ourselves to ensure that only the tables required are loaded:

1
2
3

Item.all(opts).tap do |items|
  preload_associations(items, items.map(&:item_type).uniq)
end

Wrapping that up in a class method on items is neat because we can then use it as a kicker at the end of named scopes or associations – person.items.preloaded, for instance.

Here are some tests demonstrating this:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35

require 'test/test_helper'

class PersonTest < ActiveRecord::TestCase
  setup do
    item = Item.create!(:item_type => 'dvd')
    dvd  = Dvd.create!(:item => item, :title => 'Food Inc.')
  end

  test 'naive eager load' do
    items = []
    assert_queries(3) { items = Item.all(:include => Item::SUBCLASSES) }
    assert_equal 1, items.size
    assert_queries(0) { items.map(&:description) }
  end

  test 'smart eager load' do
    items = []
    assert_queries(2) { items = Item.preloaded }
    assert_equal 1, items.size
    assert_queries(0) { items.map(&:description) }
  end
end

# Monkey patch stolen from activerecord/test/cases/helper.rb
ActiveRecord::Base.connection.class.class_eval do
  IGNORED_SQL = [/^PRAGMA/, /^SELECT currval/, /^SELECT CAST/, /^SELECT @@IDENTITY/, /^SELECT @@ROWCOUNT/, /^SAVEPOINT/, /^ROLLBACK TO SAVEPOINT/, /^RELEASE SAVEPOINT/, /SHOW FIELDS/]

  def execute_with_query_record(sql, name = nil, &block)
    $queries_executed ||= []
    $queries_executed << sql unless IGNORED_SQL.any? { |r| sql =~ r }
    execute_without_query_record(sql, name, &block)
  end

  alias_method_chain :execute, :query_record
end

I talk about this sort of thing in my “Your Database Is Your Friend” training sessions. They are happening throughout the US and UK in the coming months. One is likely coming to a city near you. Head on over to www.dbisyourfriend.com for more information and free screencasts

A pretty flower Another pretty flower