Devise Validations with Associations - ruby-on-rails

Question

I have a User model generated using Devise. This User model has 2 types (Buyer or Seller) created using Polymorphic Associations.
class Buyer < ActiveRecord::Base
has_one :user, as: :owner, dependent: :destroy
accepts_nested_attributes_for :user
validates_presence_of :user
validates_associated :user
class User < ActiveRecord::Base
after_commit :transaction_success
after_rollback :transaction_failed
devise :database_authenticatable, :registerable,
:recoverable, :rememberable, :trackable, :validatable
belongs_to :owner, polymorphic: true
validates :email, presence: true
validates :first_name, presence: true
private
def transaction_success
logger.info "Transfer succeed for Account #{self.to_param}"
end
def transaction_failed
logger.warn "Transfer failed for Account #{self.to_param}"
end
end
When an User is Registered, he does it from an specific registration link (/buyer/new or /seller/new), calling the create method from the controller:
def create
#buyer = Buyer.new(buyer_params)
#user = User.new(user_params)
respond_to do |format|
begin
User.transaction do
#buyer.save
#user.owner_id = #buyer.id
#user.owner_type = "Buyer"
#user.save
end
format.html { sign_in_and_redirect #user, notice: 'User was successfully created.' }
format.json { render :show, status: :created, location: #user }
rescue ActiveRecord::Rollback
format.html { render :new }
format.json { render json: #user.errors, status: :unprocessable_entity }
end
end
end
The problem is that sometimes the transaction fails but saves a Buyer without an User. Also, the Devise validations, or any validation that I include in the User Model doesn't make any difference, allowing users to be created without any email, password etc.
How can I fix the transaction and make sure that the validations work??