How to add a default value to an existing column in a Rails migration

You probably know that you can easily set a default when adding a new column in an Active Record migration.

add_column(:events, :state, :string, default: 'draft', null: false)

ALTER TABLE "events" ADD "state" varchar(255) DEFAULT 'draft' NOT NULL

But what about adding a default to an existing column? Fortunately in Rails 4 and 5 there is an easy way to achieve that. Use change_column_default method.

Set default value for an existing column

The Api is:

change_column_default(
  table_name,
  column_name,
  default
)

or

change_column_default(
  table_name,
  column_name,
  from:,
  to:
)

Check out an example:

change_column_default(
  :events,
  :state,
  'draft'
)

Providing nil results in dropping the default.

change_column_default(
  :events,
  :state,
  nil,
)

However, it’s better to use :from and :to named arguments. It will make the migration reversible which is sometimes useful.

change_column_default(
  :events,
  :state,
  from: nil,
  to: "draft"
)

BTW. There is a similar method named change_column_null which (as you probably guessed right now) allows you to easily set or remove the NOT NULL constraint.

change_column_null(
  :events,
  :state,
  false
)

means that state cannot be NULL (null -> false).

change_column_null(
  :events,
  :state,
  true
)

means that state can be NULL (null -> true).

If you want, you can also set a new value for records who currently have NULL.

change_column_null(
  :events,
  :state,
  false,
  "draft"
)

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