ActiveRecord::Base#find returns no records in Single Table Inheritance (STI)

Question

app/models

class Amodel < ActiveRecord::Base
end

class Bmodel < Amodel
end

class Cmodel < Bmodel  
end

db/migrate

create_table :amodels do |t|
  t.string :type
end

on script/console...

$ script/console
Loading development environment (Rails 2.3.4)
>> Cmodel.create
=> #<Cmodel id: 1, type: "Cmodel">
>> Bmodel.find(:all)
=> [#<Cmodel id: 1, type: "Cmodel">]

ok, but Bmodel returns no records after rebooting console like:

>> exit
$ script/console
Loading development environment (Rails 2.3.4)
>> Bmodel.find(:all)
=> []

however, it works after accessing Cmodel:

>> Cmodel
=> Cmodel(id: integer, type: string)
>> Bmodel.find(:all)
=> [#<Cmodel id: 1, type: "Cmodel">]

Amodel works like:

>> exit
$ script/console
Loading development environment (Rails 2.3.4)
>> Amodel.find(:all)
=> [#<Cmodel id: 1, type: "Cmodel">]

Does anyone know why it works like this?

Rails: 2.3.4
Ruby: 1.8.7
OS: Ubuntu 9.0.4

Answer 1

Because of the way ActiveRecord STI is built. When a class is loaded, it registers with it's parent (see the #inherited hook). Thus, when you call Amodel#find or Bmodel#find, if the subclass isn't known, it can't be found yet.

In production, this problem wouldn't be apparent, because Rails will load all models when it starts, preventing this kind of problem.