Support file_fixture in Factory definitions

Related to [factory_bot#1282][]

[rails/rails#45606][] has been merged and is likely to be released as
part of Rails 7.1.

With that addition, the path toward resolving [factory_bot#1282][]
becomes more clear. If factories can pass along [Pathname][] instances
to attachment attributes, Active Support will handle the rest.

Instances of `ActiveSupport::TestCase` provide a [file_fixture][] helper
to construct a `Pathname` instance based on the path defined by
`ActiveSupport::TestCase.file_fixture_path` (relative to the Rails root
directory).

[factory_bot#1282]: thoughtbot/factory_bot#1282 (comment)
[rails/rails#45606]: rails/rails#45606
[Pathname]: https://docs.ruby-lang.org/en/master/Pathname.html
[file_fixture]: https://api.rubyonrails.org/classes/ActiveSupport/Testing/FileFixtures.html#method-i-file_fixture

lib/factory_bot_rails/file_fixture_support.rb

@@ -0,0 +1,9 @@
+module FactoryBotRails
+  module FileFixtureSupport
+    def self.included(klass)
+      klass.class_attribute :file_fixture_support
+
+      klass.delegate :file_fixture, to: :file_fixture_support
+    end
+  end
+end

lib/factory_bot_rails/railtie.rb

@@ -4,13 +4,15 @@
 require "factory_bot_rails/generator"
 require "factory_bot_rails/reloader"
 require "factory_bot_rails/factory_validator"
+require "factory_bot_rails/file_fixture_support"
 require "rails"
 
 module FactoryBotRails
   class Railtie < Rails::Railtie
     config.factory_bot = ActiveSupport::OrderedOptions.new
     config.factory_bot.definition_file_paths = FactoryBot.definition_file_paths
     config.factory_bot.validator = FactoryBotRails::FactoryValidator.new
+    config.factory_bot.file_fixture_support = true
 
     initializer "factory_bot.set_fixture_replacement" do
       Generator.new(config).run
@@ -20,6 +22,22 @@ class Railtie < Rails::Railtie
       FactoryBot.definition_file_paths = definition_file_paths
     end
 
+    config.after_initialize do
+      if config.factory_bot.file_fixture_support
+        FactoryBot::SyntaxRunner.include FactoryBotRails::FileFixtureSupport
+
+        ActiveSupport.on_load :active_support_test_case do
+          setup { FactoryBot::SyntaxRunner.file_fixture_support = self }
+        end
+
+        if defined?(RSpec) && RSpec.respond_to?(:configure)
+          RSpec.configure do |config|
+            config.before { FactoryBot::SyntaxRunner.file_fixture_support = self }
+          end
+        end
+      end
+    end
+
     config.after_initialize do |app|
       FactoryBot.find_definitions
       Reloader.new(app).run

spec/factory_bot_rails/factory_spec.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+require "rails/all"
+require "rspec/rails"
+
+describe "factory extensions" do
+  include FactoryBot::Syntax::Methods
+
+  describe "#file_fixture" do
+    it "delegates to the test harness" do
+      define_model "Upload", filename: :string
+
+      FactoryBot.define do
+        factory :upload do
+          filename { self.file_fixture("file.txt") }
+        end
+      end
+
+      upload = build(:upload)
+
+      expect(upload.filename).to eq(file_fixture("file.txt").to_s)
+    end
+  end
+end

spec/fake_app.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "rails"
+
 module Dummy
   class Application < Rails::Application
     config.eager_load = false

test/factory_bot_rails/factory_test.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+class FactoryBotRails::FactoryTest < ActiveSupport::TestCase
+  include FactoryBot::Syntax::Methods
+
+  Upload = Struct.new(:filename)
+
+  self.file_fixture_path = "test/fixtures/files"
+
+  test "delegates #file_fixture to the test harness" do
+    FactoryBot.define do
+      factory :upload, class: Upload do
+        filename { self.file_fixture("file.txt") }
+      end
+    end
+
+    upload = build(:upload)
+
+    assert_equal file_fixture("file.txt"), upload.filename
+  end
+end

test/test_helper.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+# Configure Rails Environment
+ENV["RAILS_ENV"] = "test"
+
+require_relative "../spec/fake_app"
+require "rails/test_help"
+require "factory_bot_rails"