til/rails/set-default-as-sql-function-in-migration.md

Set Default As SQL Function In Migration

With static default values, like 0, true, or 'pending', we can set them directly as the value of default.

class CreateActionsTable < ActiveRecord::Migration[7.2]
  def change
    create_table :actions do |t|
      t.string :status, default: 'pending'
    end
  end
end

However, if we want our default value to be a SQL function like now(), we have to use a lambda.

Let's extend the above example to see what that looks like:

class CreateActionsTable < ActiveRecord::Migration[7.2]
  def change
    create_table :actions do |t|
      t.string :status, default: 'pending'

      t.column :created_at, :timestamptz, default: -> { 'now()' }, null: false
    end
  end
end

If we need to alter the default of an existing table's column, we can do something like this:

class AddDefaultTimestampsToActions < ActiveRecord::Migration[7.2]
  def up
    change_column_default :actions, :created_at, -> { "now()" }
    change_column_default :actions, :updated_at, -> { "now()" }
  end

  def down
    change_column_default :actions, :created_at, nil
    change_column_default :actions, :updated_at, nil
  end
end

I believe this functionality is available to Rails 5.0 and later.

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