belongs_to with :primary_key raises exception on rendering _form

Created by: iarie

Administrate 0.4 and Rails 5.02.

setup:

class User
  belongs_to :group, foreign_key: :group_code, primary_key: :code
end

class Group
  has_many :users,  foreign_key: :group_code, primary_key: :code
end 

and in UserDashboard we have

ATTRIBUTE_TYPES = {
  ...
  group: Field::BelongsTo.with_options(primary_key: :code),
  ...
}

Showing {path to gem}/administrate-0.4.0/app/views/fields/belongs_to/_form.html.erb where line #23 raised:

undefined method `group_id' for #<User:0x007fa338a048c0>
Did you mean?  group
               group=

The cause of problem is hardcoded id at: https://github.com/thoughtbot/administrate/blob/master/lib/administrate/field/belongs_to.rb#L7

def self.permitted_attribute(attr)
  :"#{attr}_id"
end

I didn't find a quick workaround, but i think it must be something like this :"#{attr}_#{primary_key}" or I'm missing something.