bash

Version4::Foo.last.bars.first.class
=> Version4::Bar

Version5::Foo.last.bars.first.class
=> Version4::Bar

version4_bar.rb

class Version4::Bar < ApplicationRecord
end

version4_foo.rb

class Version4::Foo < ApplicationRecord
  has_many :bars
end

version5_bar.rb

class Version5::Bar < Version4::Bar
end

version5_foo.rb

class Version5::Foo < Version4::Foo
end