Setter method being called before create action - ruby-on-rails

Question

I have a new form that creates an Item (all the codes are obviously simplified):
<%= simple_form_for #item do |f| %>
<%= f.input :brand_name %>
<%= f.button :submit %>
<% end %>
The current user will create an item and link it to a new or to an existing brand.
This field doesn't exist in the database; it'll be used as a way to associate all models. Hence, I create its getter and setter.
def Item < ActiveRecord::Base
belongs_to :user
belongs_to :brand
attr_accessible :brand_name
def brand_name
brand.try :name
end
def brand_name=(name)
if name.present?
brand = user.brands.find_or_initialize_by_name(name)
brand if brand.save
end
end
end
class ItemsController < ApplicationController
def new
#item = current_user.items.build
end
def create
#item = current_user.items.build(params[:item])
if #item.save
...
end
end
end
The problem is that when the form is submitted, I get this error, which lies in the product_name=() method. I've done some debugging through Rails' console and it goes all fine, but in the browser the setter method is called before the create action. That is, the record doesn't even have a user associated to it. I tried leaving the create method empty, for example, but nothing different happens.
undefined method `brands' for nil:NilClass
What is really weird is that this was working a couple of weeks ago (I've checked my git commits and the code is identical).
I though about calling the before_create callback, but there's no way to know which user should be linked.
UPDATE
I'm using Sorcery as the authentication handler. Everything always works fine, except for this create action.
class User < ActiveRecord::Base
authenticates_with_sorcery!
belongs_to :company
has_many :items
end
class Company < ActiveRecord::Base
has_many :users, dependent: :destroy
has_many :brands, dependent: :destroy
end