Setting a default timestamp to "now" in postgresql

bad_idea.rb

class CreateMessages < ActiveRecord::Migration
  def change
    create_table :messages do |t|
      t.references :chat
      t.text :content
      t.timestamp :received_at, :default => "now"
      t.timestamps
    end
  end
end

good_idea.rb

class CreateMessages < ActiveRecord::Migration
  def change
    create_table :messages do |t|
      t.references :chat
      t.text :content
      t.timestamp :received_at
      t.timestamps
    end

    ActiveRecord::Base.connection.execute "ALTER TABLE messages ALTER COLUMN received_at SET DEFAULT now()"
  end
end