Allow specification of default assocations depending on context

Problem this feature will solve

FactoryBot allows us to easily define associations that should automatically be constructed when creating or building a model. These associations are statically defined in the Factory.

If I want to change which object is used in an association I can overwrite the association when constructing an object, but I have to do that each time I want to create something. To illustrate, consider these factories:

factory :post do 
  text { 'text' }
end 

factory :comment do
  text { 'comment' }
  association :post
end

It may be fine to automatically construct a new post for every comment most of the time, but there are contexts (e.g. a small set of tests) where I always want to use a specific post. I then have to specify the association manually in each test:

def setup 
 @post = create(:post)
end

test 'first test'  do 
  create(:comment, post: post)
  create(:comment, post: post) 
  ...
end

# Imagine tons of similar tests like that below...

I would like a solution that allows greater flexibility than what is currently there.

Desired solution

TestProf provides a Factory Default helper.

It should be possible to specify a set of associations to be used in a context, e.g. a block or a test class / Rspec context, using a helper method use_factory_defaults (or similar)

The tests above could then be rewritten as:

def setup 
 @post = create(:post)
 use_factory_defaults(post: @post)
end

test 'first test'  do 
  create(:comment) # These then use @post automatically
  create(:comment) 
  ...
end

or alternatively, for a single test:

test 'first test'  do 
  use_factory_defaults(post: create(:post)) do 
    create(:comment) # These then use @post automatically
    create(:comment) 
  end
end

Alternatives considered

It is of course possible to simply use FactoryProf. However, I feel that this specific functionality would work nicely in FactoryBot itself.

As outlined above, create(:comment, post: post) is verbose when many similar models are created.

I have considered that one may be able to hack something together with traits, but I'm not even sure how. I couldn't come up with anything 😅

Additional context

TestProf provides additional helpers such as create_default, which are essentially shortcuts to the functionality outlined above.

There are parts of this feature that are strictly RSpec/Minitest specific (e.g. cleaning up defaults after a test suite is done using framework specific hooks). Including those in FactoryBot is not required.

I already have some helpers cobbled together for Minitest (since TestProf only comes with Rspec integration), and I'd be prepared to port the TestProf functionality to FactoryBot if desired 🙂

Start by reading TestProf’s lib/test_prof/recipes/rspec/factory_default.rb, which is linked as the reference implementation. Compare its context and cleanup behavior with FactoryBot’s association handling, then define the supported use_factory_defaults API. Done means scoped defaults work for the shown create(:comment) calls without per-call overrides, with coverage for the supported usage.