auto_increment

auto_increment automatically generates sequential values for Active Record attributes.

Common use cases:

• Invoice numbers (1, 2, 3)
• Customer numbers (1000, 1001, 1002)
• Per-account sequences
• Letter sequences (A, B, ..., Z, AA, AB)

Without the gem:

class Invoice < ApplicationRecord
  before_create :set_number

  private

  def set_number
    self.number = Invoice.maximum(:number).to_i + 1
  end
end

With auto_increment:

class Invoice < ApplicationRecord
  auto_increment :number
end

Invoice.create!.number #=> 1
Invoice.create!.number #=> 2
Invoice.create!.number #=> 3

Add the gem to your Gemfile:

gem "auto_increment"

Then run:

bundle install

The target column must exist in your database.

class Invoice < ApplicationRecord
  auto_increment :number
end

class Invoice < ApplicationRecord
  auto_increment :number
end

Generated values:

1
2
3
4
...

class Invoice < ApplicationRecord
  auto_increment :number, initial: 1000
end

Generated values:

1000
1001
1002
...

class User < ApplicationRecord
  auto_increment :code, initial: "A"
end

Generated values:

A
B
C
...
Z
AA
AB
...

String sequences follow the same pattern as Excel columns.

Generate independent sequences within a scope:

class Invoice < ApplicationRecord
  auto_increment :number, scope: :account_id
end

Result:

Account 1: 1, 2, 3
Account 2: 1, 2, 3

Multiple scopes are also supported:

class Invoice < ApplicationRecord
  auto_increment :number,
    scope: [:account_id, :year]
end

Result:

Account 1, 2026: 1, 2, 3
Account 1, 2027: 1, 2, 3
Account 2, 2026: 1, 2, 3

model_scope applies one or more Active Record scopes before calculating the maximum value.

This is useful when:

• Bypassing a default_scope
• Including archived records
• Restricting the sequence to a subset of records

class User < ApplicationRecord
  default_scope -> { where(active: true) }

  scope :unscoped_all, -> { unscoped }

  auto_increment :code,
    scope: :account_id,
    model_scope: :unscoped_all
end

In this example, the sequence is calculated using all records, including inactive ones.

By default, values are assigned during before_create.

auto_increment :code

You can change when the value is generated:

Example:

class Account < ApplicationRecord
  auto_increment :code, before: :validation

  validates :code, presence: true
end

By default, manually assigned values are preserved.

invoice.number = 500
invoice.save

To always generate a new value:

auto_increment :number, force: true

For applications that may create records concurrently, enable locking:

auto_increment :number, lock: true

This locks the record used to determine the next value before assigning it.

auto_increment :number,
  scope: [:account_id, :year],
  model_scope: :unscoped_all,
  initial: 1000,
  force: true,
  lock: true,
  before: :validation

When a record is created, auto_increment:

1. Builds a query for the target column.
2. Applies any configured scopes.
3. Applies any configured model scopes.
4. Finds the current maximum value.
5. Calculates the next value.
6. Assigns the value during the configured callback.

The generated value is stored in a normal database column and can be queried, indexed, and validated like any other attribute.

For older Ruby and Rails versions, use:

gem "auto_increment", "1.5.2"

Released under the MIT License. See LICENSE.txt.