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pg_sequencer ↑

pg_sequencer adds methods to your migrations to allow you to create, drop and change sequence objects in PostgreSQL. It also dumps sequences to schema.rb.

This is especially useful if you are connecting to a legacy database where the primary key field is declared as an INTEGER and a sequence is queried for the value of the next record.

The design of pg_sequencer is heavily influenced on Matthew Higgins’ Foreigner gem:

Installation ↑

Add this to your Gemfile:

gem 'pg_sequencer'

API ↑

pg_sequencer adds the following methods to migrations:

  • create_sequence(sequence_name, options)

  • change_sequence(sequence_name, options)

  • drop_sequence(sequence_name)

The methods closely mimic the syntax of the PostgreSQL SQL for CREATE SEQUENCE, DROP SEQUENCE and ALTER SEQUENCE. See the REFERENCES section below for more information.

Options ↑

For create_sequence and change_sequence, all options are the same, except create_sequence will look for :start or :start_with, and change_sequence will look for :restart or :restart_with.

  • :increment/:increment_by (integer) - The value to increment the sequence by.

  • :min (integer/false) - The minimum value of the sequence. If specified as false (e.g. :min => false), “NO MINVALUE” is sent to Postgres.

  • :max (integer/false) - The maximum value of the sequence. May be specified as “:max => false” to generate “NO MAXVALUE”

  • :start/:start_with (integer) - The starting value of the sequence (create_sequence only)

  • :restart/:restart_with (integer) The value to restart the sequence with (change_sequence only)

  • :cache (integer) - The number of values the sequence should cache.

  • :cycle (boolean) - Whether the sequence should cycle. Generated at “CYCLE” or “NO CYCLE”

Examples ↑

Creating a sequence ↑

Create a sequence called “seq_user”, incrementing by 1, min of 1, max of 2000000, starts at 1, caches 10 values, and disallows cycles:

create_sequence("seq_user", {
  :increment => 1,
  :min => 1,
  :max => 2000000,
  :start => 1,
  :cache => 10,
  :cycle => false
})

This is equivalent of the following query:

CREATE SEQUENCE seq_user INCREMENT BY 1 MIN 1 MAX 2000000 START 1 CACHE 10 NO CYCLE

If sequence already exists, creation is skipped. If You pass option ‘drop_if_exists: true`, than existing one is dropped and new is created.

Reset a sequence’s value: ↑

change_sequence "seq_accounts", :restart_with => 50

This is equivalent to:

ALTER SEQUENCE seq_accounts RESTART WITH 50

Removing a sequence: ↑

drop_sequence "seq_products"

Caveats / Bugs ↑

  • Tested with postgres 11.7, should work down to 8.4.

  • Listing all the sequences in a database creates n+1 queries (1 to get the names and n to describe each sequence). Is there a way to fully describe all sequences in a database in one query?

  • The “SET SCHEMA” fragment of the ALTER command is not implemented.

  • Oracle/other databases not supported

  • Other unknown bugs :)

References ↑