SQLite3::Error de excepción de ocupación

Recibo este error cuando se crea una nueva cuenta de usuario:

ActiveRecord::StatementInvalid in SessionsController#create

SQLite3::SQLException: cannot start a transaction within a transaction: begin transaction

Estoy usando koala y omniauth gem para autenticar y obtener amigos de los usuarios. ¿Por qué no puedo iniciar dos transacciones, por qué revierte la migración de amigos?

¿Cómo se arregla esto?

Este es mi controlador de sesión:

class SessionsController < ApplicationController
  def create
    user = User.from_omniauth(env['omniauth.auth'])
    session[:user_id] = user.id
    redirect_to root_url, notice: "Signed in!"
  end

Este es mi modelo de usuario:

class User < ActiveRecord::Base
  has_many :friends

  def self.from_omniauth(auth)
    where(auth.slice(:provider, :uid)).first_or_initialize.tap do |user|
      user.provider = auth["provider"]
      user.uid = auth["uid"]
      user.name = auth["info"]["name"] unless auth["info"].blank?
      user.first_name = auth["info"]["first_name"] unless auth["info"].blank?
      user.last_name = auth["info"]["last_name"] unless auth["info"].blank?
      user.image = auth["info"]["image"] unless auth["info"].blank?
      user.email = auth["info"]["email"] unless auth["info"].blank?
      user.gender = auth["extra"]["raw_info"]["gender"] if (!auth["extra"].blank? && !auth["extra"]["raw_info"].blank?)
      user.location = auth["extra"]["raw_info"]["location"]["name"] if (!auth["extra"].blank? && !auth["extra"]["raw_info"].blank? && !auth["extra"]["raw_info"]["location"].blank?)          
      user.token = auth["credentials"]["token"] unless auth["credentials"].blank?


        # highschool data
        user.highschool_name = auth["extra"]["raw_info"]["education"][0]["school"]["name"] if (!auth["extra"].blank? && !auth["extra"]["raw_info"]["education"][0]["school"].blank?)
        user.highschool_year = auth["extra"]["raw_info"]["education"][0]["year"]["name"] if (!auth["extra"].blank? && !auth["extra"]["raw_info"]["education"][0]["year"].blank?)
        # graduate school data
        user.graduateschool_name = auth["extra"]["raw_info"]["education"][1]["school"]["name"] if (!auth["extra"].blank? && !auth["extra"]["raw_info"]["education"][1]["school"].blank?)
        user.graduateschool_year = auth["extra"]["raw_info"]["education"][1]["year"]["name"] if (!auth["extra"].blank? && !auth["extra"]["raw_info"]["education"][1]["year"].blank?)
      user.add_friends  
      user.save!
      user
    end 
  end

  def add_friends
    facebook.get_connections("me", "friends").each do |hash|
      self.friends.where(:name => hash['name'], :uid => hash['id']).first_or_create
    end
  end

  private 

  def facebook
    @facebook ||= Koala::Facebook::API.new(token)
  end

end