Start method just once for addition / removal of association elements

Question

I have a Composition model which has a has_and_belongs_to_many :authors.

I need to fire a method after a composition changed its authors, although, since it involves the creation of a PDF file (with the name of the authors), I want to call this method only once, regardless of the number of authors added / removed.

Of course I can add / remove existing authors from the composition, so a before_save / after_save won't work here (somehow it recognizes new authors added to the composition, but not existing ones).

So I tried using after_add / after_remove, but the callbacks specified here will be invoked for every author item added to / removed from the composition.

Is there a way to have a method called only once for every "batch action" of adding / removing items from this kind of relationship?

Sounds like a good use case for a `service` rather than a `callback`. — jvillian, Apr 18 '18 at 23:23
@jvillian can you suggest an example of how to write such a service? — Simon, Apr 19 '18 at 00:56

score 0 · Answer 1 · answered Apr 19 '18 at 01:47

Here's what a service might look like:

class UpdateCompositionAuthorsService

  attr_accessor *%w(
    args
  ).freeze

  class << self 

    def call(args={})
      new(args).call
    end

  end # Class Methods

  #======================================================================================
  # Instance Methods
  #======================================================================================

    def initialize(args={})
      @args = args
      assign_args
    end

    def call
      do_stuff_to_update_authors
      generate_the_pdf
    end

  private

    def do_stuff_to_update_authors
      # do your 'batch' stuff here
    end

    def generate_the_pdf
      # do your one-time logic here
    end

    def assign_args
      args.each do |k,v| 
        class_eval do 
          attr_accessor k
        end
        send("#{k}=",v)
      end
    end

end

You would call it something like:

UpdateCompositionAuthorsService.call(composition: @composition, authors: @authors)

I got sick of remembering what args to send to my service classes, so I created a module called ActsAs::CallingServices. When included in a class that wants to call services, the module provides a method called call_service that lets me do something like:

class FooClass
  include ActsAs::CallingServices

  def bar
    call_service UpdateCompositionAuthorsService
  end

end

Then, in the service class, I include some additional class-level data, like this:

class UpdateCompositionAuthorsService

  SERVICE_DETAILS = [
    :composition,
    :authors
  ].freeze

    ...

    def call
      do_stuff_to_update_authors
      generate_the_pdf
    end

    ...

end

The calling class (FooClass, in this case) uses UpdateCompositionAuthorsService::SERVICE_DETAILS to build the appropriate arguments hash (detail omitted).

I also have a method called good_to_go? (detail omitted) that is included in my service classes, so my call method typically looks like:

class UpdateCompositionAuthorsService

    ...

    def call
      raise unless good_to_go?
      do_stuff_to_update_authors
      generate_the_pdf
    end

    ...

end

So, if the argument set is bad, I know right away instead of bumping into a nil error somewhere in the middle of my service.

Start method just once for addition / removal of association elements

1 Answers1