Question

I’ve got two models in my application using STI: Entry and Sugar, they’re very simple.

Entries:

# == Schema Information
#
# Table name: entries
#
#  id         :integer         not null, primary key
#  created_at :datetime        not null
#  updated_at :datetime        not null
#  user_id    :integer
#  type       :string(255)
#

class Entry < ActiveRecord::Base
  # attr_accessible :title, :body
  belongs_to :user
end

Sugar (note the lack of amount in the schema info from the annotate gem):

# == Schema Information
#
# Table name: entries
#
#  id         :integer         not null, primary key
#  created_at :datetime        not null
#  updated_at :datetime        not null
#  user_id    :integer
#  type       :string(255)
#

class Sugar < Entry
  attr_accessible :amount
end

I created the Sugar model by running rails g model Sugar amount:integer and then edited it to be a subclass of the Entry model. The migration generated created an amount column:

class CreateSugars < ActiveRecord::Migration
  def change
    create_table :sugars do |t|
      t.integer :amount

      t.timestamps
    end
  end
end

And the column exists in my database:

bridges_development=# \d sugars
                                     Table "public.sugars"
   Column   |            Type             |                      Modifiers                      
------------+-----------------------------+-----------------------------------------------------
 id         | integer                     | not null default nextval('sugars_id_seq'::regclass)
 amount     | integer                     | 
 created_at | timestamp without time zone | not null
 updated_at | timestamp without time zone | not null
Indexes:
    "sugars_pkey" PRIMARY KEY, btree (id)

However, the “amount” attribute and/or methods don’t seem to exist. Here’s an example:

1.9.2-p290 :002 > s.amount = 2
NoMethodError: undefined method `amount=' for #<Sugar:0xb84041c> (...)

1.9.2-p290 :003 > s = Sugar.new(:amount => 2)
ActiveRecord::UnknownAttributeError: unknown attribute: amount (...)

Why wouldn’t the amount attribute and associated methods be made available?

Answer 1

The idea of STI is N models – 1 table. Sugar is in fact accessing table entries, where there is no attribute amount.