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 
