"File unchanged! Either the supplied flag value not found or the content has already been inserted!"

I am working on a Ruby on Rails 8.0 application with Ruby 3.4.5 and the factory_bot_rails 6.5.0 gem. When I run rails generate model MyObject some_string:string, the generator creates the migration, models, and test files, but it does not stub out a factory in test/factories.rb, instead outputting this error:

File unchanged! Either the supplied flag value not found or the content has already been inserted!

I didn't expect any warning and a stub factory to be added to test/factories.rb, but the file was not modified. My factories.rb file presently looks like this; it does not contain any factory definition with the my_object name:

FactoryBot.define do # rubocop:disable Metrics/BlockLength

  factory :user do
    email { "user@example.com" }
    name { "John Doe" }
  end

  # ... 450 lines of other existing factories ...
end

As you noted your factories.rb file contains this line:

FactoryBot.define do # rubocop:disable Metrics/BlockLength

Which according to your answer was "a rubocop control comment recently added".

The reason you are seeing this warning is because the the ModelGenerator provided as part of the factory_bot_rails gem, is trying to insert the new factory into the existing factories.rb file, and is unable to determine where that insertion should occur due to the above noted line.

This occurs in ModelGenerator#insert_factory_into_existing_file (Source)

def insert_factory_into_existing_file
  insert_into_file(
    factories_file,
    factory_definition,
    after: "FactoryBot.define do\n"
  )
end

You can see here that the generator is looking to insert the new factory definition after the explicit line "FactoryBot.define do\n".

The insert_into_file method is provided by the thor gem (Source).

It uses the supplied after String as a RegExp to find the appropriate location in the file to insert the new content.

In the event that it cannot find this line in the file, it outputs the warning you are experiencing (in ANSI red where supported)

Long story short:

"FactoryBot.define do\n".match?("FactoryBot.define do # rubocop:disable Metrics/BlockLength\n")
#=> false