ActiveRecord callbacks are pretty popular in the Rails community, but they do have pitfalls. Probably the most significant one is that the responsibilities of a model can creep quite a lot once callbacks begin getting defined. Callbacks can also be hard to test, since they are triggered under a range of conditions, and have multiple phases per condition (before/around/after initialize, validate, create, etc).

There is an interesting, little-known ability to pass a block to the create method of an ActiveRecord model. This can be an elegant way of passing a callback method, from something like a service or form object to allow a model to set attributes on itself immediately before saving.

Here’s an example:

# app/services/ticket_creator.rb
class TicketCreator
  def create(summary:)
    Ticket.create!(summary: summary, &:assign_reference)

In this example, our ticket creator service passes the attributes to create as usual, but also passes a proc reference to the assign_reference method of the model - in other words, this is a shorthand, which could also be written as:

Ticket.create(attributes) do |ticket|

In this example, we would expect assign_reference to be implemented inside the Ticket model like so:

# app/models/ticket.rb
class Ticket
  def assign_reference!

Personally, I find this approach to be a lot cleaner than a normal callback, since the invocation of the callback method isn’t linked directly into the model. It is invoked on the model, but not by the model. In this case, it is called by a service object, but could just as easily be invoked from a controller, mailer, or any other class really. It is also relatively easy to test, since we can test the result of the service method being called, rather than the core create method.