If you are in development mode and if you have multi level STI then you might get unexpected result.

class CreateUser < ActiveRecord::Migration
  def self.up
    create_table :users do |t|
      t.string :name
      t.string :type
    end
  end
  def self.down
    drop_table :users
  end
end

class User < ActiveRecord::Base
end

class Agent < User
end

class SuperAgent < Agent
end

Now in console try this.

> Agent.find_by_name 'agent'
SELECT "users".* FROM "users" WHERE ("users"."type" = 'Agent') 
AND ("users"."name" = 'agent') LIMIT 1

> SuperAgent # just load this class
> Agent.find_by_name 'agent'
SELECT "users".* FROM "users" WHERE (("users"."type" = 'Agent' OR 
"users"."type" = 'SuperAgent')) AND ("users"."name" = 'agent') LIMIT 1

Notice that when SuperAgent is loaded then the generated sql query is different from the query generated when SuperAgent was not loaded. Since in development all the classes are loaded lazily you might get the first sql query. For the same code in production you will get second sql query.

Solution

There are many solutions. However I think it is best to let others know that if first level of STI is loaded then load all subclassess too. That can be done like this.

class Agent < User
end
require_dependecy 'super_agent'

Source: rails core discussion posted by me