Probably the easiest thing to do is to add a test to spec/acceptance/callbacks_spec.rb. You'll want to make sure that before(:build) is called before anything is built.

Spitballing here, something like:

class TitleSetter
  def self.title=(new_title)
    @@title = new_title
  end

  def self.title
    @@title
  end
end

define_model("Article", title: :string)

FactoryBot.define do
  factory :article_with_before_callbacks, class: :article do
    before(:build) { TitleSetter.title = "title from before build" }
    after(:build) { TitleSetter.title = "title from after build" }

    title { TitleSetter.title }
  end
end

it "runs the before callback" do
  article = FactoryBot.build(:article_with_before_callbacks)
  expect(article.title).to eq("title from before build")
end

@mike-burns Thank you for your suggestion 🙏
but won't title from after build in the after(:build) callback always overwrite the title from before build? why is this scenario expected to pass?

also would something like this do it?

define_model("User", full_name: :string)

FactoryBot.define do
  factory :user_with_before_callbacks, class: :user do
    before(:build) { user.full_name = "first }
    after(:build) { user.full_name = "#{user.full_name} last" }
  end
end

it "runs the before callback" do
  article = FactoryBot.build(:user_with_before_callbacks)
  expect(user.full_name).to eq("first last")
end

We need to make sure before(:build) is called before any building happens.

My theory on why my suggestion works is this timeline:

1. FactoryBot.build(:article)
2. before(:build) { TitleSetter.title = "title from before build" }
3. TitleSetter.title = "title from before build"
4. now build happens
5. internal: resource = Article.new
6. internal: resource.title = (-> { TitleSetter.title }).call
7. TitleSetter.title # => "title from before build"
8. internal: done building, returns resource
9. after(:build) { TitleSetter.title = "title from after build" }
10. TitleSetter.title = "title from after build"
11. resource.title has not changed since step (6), because we already called #call.

(For what it's worth, I'm not sure your current code calls the callback before the object is built.)

@mike-burns you are right the approach was not correct and it was actually called after the build, thanks for raising the hand!
I changed the approach here, now it is being called before the object is built and your test passes.

Hey @mike-burns , sorry to ping you again 😅 I am just wondering if the new approach is okay or do I need to investigate other options? Thank you

Oh hey, this approach is good, thanks for taking it on.

I need to find time to do a final review and merge it. I can do small tweaks from here and give you credit.

Thank you, much appreciated 🙏