class Users
  belongs_to :province
  has_and_belongs_to_many :selected_provinces, :class_name => "Province"
end

I could very easily find out which provinces a user had selected and also which users had selected a specific province. However I didn’t realise that if I accessed a user via the province then the province_id which previously pointed the user’s address province was replaced by the id of the selected province.

province = Province.find_first
province.users.each do |user|
  # do something
  # however user.province_id now points to province
end

I assume the province_id is added by the habtm join so that we can understand which selected_province object this user object comes from however in my case it meant that I lost the original province_id and so if I saved that user then I would end up changing the address province.

This is probably not a bug as such but something that should be watched out for as it took me a while to discover this problem with my application.