bad.rb

class Bad < ActiveRecord::Migration
  def change
    Post.update_all(active: true) # <-- resolves to Post defined in app/models/post.rb
  end
end

good.rb

class Good < ActiveRecord::Migration
  class Post < ActiveRecord::Base
  end
  
  def change
    Post.update_all(active: true) # <-- resolves to Post defined above
  end
end