Start method just once for addition / removal of association elements - ruby-on-rails

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?

Answer 1

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.