type column reader not defined for STI until you access the column using [:type], read_attribute(:type), or create a new instance.

20111109172903_create_users.rb

class CreateUsers < ActiveRecord::Migration
  def change
    create_table :users do |t|
      t.string :type
      t.string :name
      t.timestamps
    end
  end
end

overview.txt

Using a vanilla app via rails new with rails 3.1.1 with ruby 1.8.7-p352.  This issue does not occur with ruby 1.9.2-p290.

1) Create a SubUser in dev mode:

joerafaniello@:~/Code/playground/rails311_type_method$ bundle exec rails c
Loading development environment (Rails 3.1.1)
ruby-1.8.7-p352 :001 > SubUser.create(:name => "Joe")
   (0.1ms)  BEGIN
  SQL (52.9ms)  INSERT INTO "users" ("created_at", "name", "type", "updated_at") VALUES ($1, $2, $3, $4) RETURNING "id"  [["created_at", Fri, 11 Nov 2011 23:04:59 UTC +00:00], ["name", "Joe"], ["type", "SubUser"], ["updated_at", Fri, 11 Nov 2011 23:04:59 UTC +00:00]]
   (0.4ms)  COMMIT
 => #<SubUser id: 1, type: "SubUser", name: "Joe", created_at: "2011-11-11 23:04:59", updated_at: "2011-11-11 23:04:59"> 
ruby-1.8.7-p352 :002 > User.first.type
  User Load (0.5ms)  SELECT "users".* FROM "users" LIMIT 1
 => "SubUser" 
ruby-1.8.7-p352 :003 > exit

2) type method is defined on the Subuser instance until you exit

3) Start a console and find the SubUser and call type, which is not defined, so you get the ruby type method.

joerafaniello@:~/Code/playground/rails311_type_method$ bundle exec rails c
Loading development environment (Rails 3.1.1)
ruby-1.8.7-p352 :001 > User.first.type
  User Load (0.4ms)  SELECT "users".* FROM "users" LIMIT 1
(irb):1: warning: Object#type is deprecated; use Object#class
 => SubUser(id: integer, type: string, name: string, created_at: datetime, updated_at: datetime)

4) Accessing the type column directly through read_attribute or hash access loads the type method.

ruby-1.8.7-p352 :002 > u = User.first
  User Load (0.4ms)  SELECT "users".* FROM "users" LIMIT 1
 => #<SubUser id: 1, type: "SubUser", name: "Joe", created_at: "2011-11-11 23:04:59", updated_at: "2011-11-11 23:04:59"> 
ruby-1.8.7-p352 :003 > u[:type]
 => "SubUser" 
ruby-1.8.7-p352 :004 > u.type
 => "SubUser" 
ruby-1.8.7-p352 :005 > exit

5) Same test with production mode.

joerafaniello@:~/Code/playground/rails311_type_method$ bundle exec rails c production
Loading production environment (Rails 3.1.1)
ruby-1.8.7-p352 :001 > SubUser.create(:name => "Joe")
   (0.1ms)  BEGIN
  SQL (3.0ms)  INSERT INTO "users" ("created_at", "name", "type", "updated_at") VALUES ($1, $2, $3, $4) RETURNING "id"  [["created_at", Fri, 11 Nov 2011 23:07:00 UTC +00:00], ["name", "Joe"], ["type", "SubUser"], ["updated_at", Fri, 11 Nov 2011 23:07:00 UTC +00:00]]
   (0.7ms)  COMMIT
 => #<SubUser id: 2, type: "SubUser", name: "Joe", created_at: "2011-11-11 23:07:00", updated_at: "2011-11-11 23:07:00"> 
ruby-1.8.7-p352 :002 > User.first.type
  User Load (0.4ms)  SELECT "users".* FROM "users" LIMIT 1
 => "SubUser" 
ruby-1.8.7-p352 :003 > exit
joerafaniello@:~/Code/playground/rails311_type_method$ bundle exec rails c production
Loading production environment (Rails 3.1.1)
ruby-1.8.7-p352 :001 > User.first.type
  User Load (0.3ms)  SELECT "users".* FROM "users" LIMIT 1
(irb):1: warning: Object#type is deprecated; use Object#class
 => SubUser(id: integer, type: string, name: string, created_at: datetime, updated_at: datetime) 
ruby-1.8.7-p352 :002 > u = User.first
  User Load (0.3ms)  SELECT "users".* FROM "users" LIMIT 1
 => #<SubUser id: 1, type: "SubUser", name: "Joe", created_at: "2011-11-11 23:04:59", updated_at: "2011-11-11 23:04:59"> 
ruby-1.8.7-p352 :003 > u[:type]
 => "SubUser" 
ruby-1.8.7-p352 :004 > u.type
 => "SubUser" 
ruby-1.8.7-p352 :005 > exit

6) Creating a SubUser also will define the type method correctly so subsequent found instances have type.

joerafaniello@:~/Code/playground/rails311_type_method$ bundle exec rails c production
Loading production environment (Rails 3.1.1)
ruby-1.8.7-p352 :001 > SubUser.create(:name => "Joe2")
   (0.1ms)  BEGIN
  SQL (3.1ms)  INSERT INTO "users" ("created_at", "name", "type", "updated_at") VALUES ($1, $2, $3, $4) RETURNING "id"  [["created_at", Fri, 11 Nov 2011 23:09:50 UTC +00:00], ["name", "Joe2"], ["type", "SubUser"], ["updated_at", Fri, 11 Nov 2011 23:09:50 UTC +00:00]]
   (0.5ms)  COMMIT
 => #<SubUser id: 3, type: "SubUser", name: "Joe2", created_at: "2011-11-11 23:09:50", updated_at: "2011-11-11 23:09:50"> 
ruby-1.8.7-p352 :002 > u = User.find(1)
  User Load (0.6ms)  SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1  [["id", 1]]
 => #<SubUser id: 1, type: "SubUser", name: "Joe", created_at: "2011-11-11 23:04:59", updated_at: "2011-11-11 23:04:59"> 
ruby-1.8.7-p352 :003 > u.type
 => "SubUser" 
ruby-1.8.7-p352 :004 > u = User.find(2)
  User Load (0.3ms)  SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1  [["id", 2]]
 => #<SubUser id: 2, type: "SubUser", name: "Joe", created_at: "2011-11-11 23:07:00", updated_at: "2011-11-11 23:07:00"> 
ruby-1.8.7-p352 :005 > u = User.find(3)
  User Load (0.3ms)  SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1  [["id", 3]]
 => #<SubUser id: 3, type: "SubUser", name: "Joe2", created_at: "2011-11-11 23:09:50", updated_at: "2011-11-11 23:09:50"> 
ruby-1.8.7-p352 :006 > u.type
 => "SubUser" 

sub_user.rb

class SubUser < User
end

user.rb

class User < ActiveRecord::Base
end