Help with primary_key_prefix_type

I put this config in environment.rb

  # Use table_id instead of id for primary key.
  config.active_record.primary_key_prefix_type =
:table_name_with_underscore

I have this migration:

class CreateEntities < ActiveRecord::Migration
  def self.up
    create_table :entities do |t|
      t.string :entity_name, :null => false,
                    :limit => 40
      t.string :entity_legal_name, :null => false,
                    :limit => 120
      t.string :entity_legal_form, :null => false,
                    :limit => 4

      t.timestamps
    end

    add_index :entities, :entity_name,
              :name => :idxU_entities_entity_name,
              :unique => true
  end

  def self.down
    remove_index :entities, :name => :idxU_entities_entity_name
    drop_table :entities
  end
end

When I run these rake tasks:

rake db:drop
rake migrate

Then I see this:

# sqlite3 db/development.sqlite3
SQLite version 3.3.6
Enter ".help" for instructions

.schema entities

CREATE TABLE entities ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
"entity_name" varchar(40) NOT NULL, "entity_legal_name" varchar(120) NOT
NULL, "entity_legal_form" varchar(4) NOT NULL, "created_at" datetime
DEFAULT NULL, "updated_at" datetime DEFAULT NULL);

But I expected to see this:

CREATE TABLE entities ("entity_id" INTEGER PRIMARY KEY AUTOINCREMENT NOT
NULL,...

It makes no difference if I pass the value to
config.active_record.primary_key_prefix_type as a string or a symbol as
neither works.

So, what am I doing wrong?