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