If you use PostgreSQL to back your Active Record models, you should check the current names for your tables and their sequences. Prior to Active Record 3.2.7, renaming a table did not rename the associated sequence for the table’s primary key.

A demonstration may be in order.

robb@neoldian ~/code/pg_seq > rails generate model Product name:string description:text
      invoke  active_record
      create    db/migrate/20120727210734_create_products.rb
      create    app/models/product.rb
      invoke    test_unit
      create      test/unit/product_test.rb
      create      test/fixtures/products.yml

robb@neoldian ~/code/pg_seq > rake db:migrate
==  CreateProducts: migrating =================================================
-- create_table(:products)
NOTICE:  CREATE TABLE will create implicit sequence "products_id_seq" for serial column "products.id"
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "products_pkey" for table "products"
   -> 0.0194s
==  CreateProducts: migrated (0.0196s) ========================================

Note that on line 11 the CREATE TABLE created an implicit sequence products_id_seq for serial column products.id.

Let’s rename the table.

robb@neoldian ~/code/pg_seq > rails generate migration RenameProductsToWidgets
      invoke  active_record
      create    db/migrate/20120727211235_rename_products_to_widgets.rb

# db/migrate/20120727211235_rename_products_to_widgets.rb
class RenameProductsToWidgets < ActiveRecord::Migration
  def up
    rename_table :products, :widgets
  end

  def down
    rename_table :widgets, :products
  end
end

robb@neoldian ~/code/pg_seq > rake db:migrate
==  RenameProductsToWidgets: migrating ========================================
-- rename_table(:products, :widgets)
   -> 0.0019s
==  RenameProductsToWidgets: migrated (0.0020s) ===============================

What do you expect the sequence for the id column in the widgets table to be named at this point? widgets_id_seq?

                                      Table "public.widgets"
   Column    |            Type             |                       Modifiers
-------------+-----------------------------+-------------------------------------------------------
 id          | integer                     | not null default nextval('products_id_seq'::regclass)
 name        | character varying(255)      |
 description | text                        |
 created_at  | timestamp without time zone | not null
 updated_at  | timestamp without time zone | not null
Indexes:
    "products_pkey" PRIMARY KEY, btree (id)

The who the what?

On line 4, the next value for the widgets.id column is coming from a sequence named products_id_seq. This is not catastrophic for small code bases, but imagine coming across this database months or years from now. “Why’s the widget table have a sequence named ‘products’?”

The good news is that AR v3.2.7 includes a fix for this. When you rename a table, if the name of the sequence for the table’s primary key matches the AR default “tablename_columnname_seq”, then the sequence will be renamed as well. (I suppose the next order of business is to get that index renamed, too.)

So you might want to go take a look at your PostgreSQL tables. Their sequences probably have names that do not make any sense now.

You can use this method in your Rails console as a quick way to check.

def check_table_and_sequence_names
  connection = ActiveRecord::Base.connection
  if connection.adapter_name == "PostgreSQL"
    table_names = connection.tables.keep_if { |t| t !~ /schema_migrations/ }
    table_names.map { |t| [t, connection.pk_and_sequence_for(t)].flatten }
  end
end

check_table_and_sequence_names
# => [["tablename", "primarykeyname", "sequencename"]]