error_message partial not rendered in Rails

I have problems displaying error_messages.html.erb partial when building custom validation.

I have for instance, a form to make a transaction, which starts like:

<%= form_for @transaction do |f| %>
  <%= render 'shared/error_messages', :object => f.object %>

I have a validation which prevents a user from making a transaction with himself:

validate :self_transaction_not_possible

with

    def self_transaction_not_possible
      bel = Belonging.find_by_id(self.belonging.id)
      unless bel.nil?
        if self.user_id == bel.user_id
          errors.add(:base, "You cannot perform a transaction with yourself")
        end
      end
    end 

Then I ran such an example in the browser (initiate a transaction on a belonging I own), and I placed a debugger inside self_transaction_not_possible. I checked that the validation happens, and indeed, inside the debugger, I see that typing: self.errors.full_messages give me my error message, so validation seems to be performed...

I would then have expected, as the object was not valid, that the error message would appear in my view, but no ... putting the debugger inside the error_messages.html.erb partial showed me that object.errors.any remains == {} :-/ ...

The user is then redirected towards the page it should see when the saving process fails (validation has been performed correctly), but he never sees the error message ...

Am I missing something obvious ? ...