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Example

  • To adds a column adminb> to the users table, and gives that column the default value false.
class AddDetailsToUsers < ActiveRecord::Migration[5.0]
  def change
    add_column :users, :admin, :boolean, default: false
  end
end
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  • Migrations with defaults might take a long time in large tables with for example PostgreSQL.
  • This is because each row will have to be updated with the default value for the newly added column. To circumvent this (and reduce downtime during deployments).
  • We can split our migration into three steps
    • Add a add_column-migration similar to the one above, but set no default
    • Deploy and update the column in a rake task or on the console while your app is running. Make sure your application already writes data to that colum for new/updated rows.
    • Add another change_column migration, which then changes the default of that column to the desired default value

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