how to know the id of user created by python social auth for new and returning users?

Question

I am using python-social-auth for letting user log in through fb, google and linkedin. When a user is created by one of this backends and is stored in database. We have the following: (Using django-mongodb-engine not mongoengine)

user_register_registration(Custom User model)

{
    "_id": {
        "$oid": "abcdxxxxxxx344"
    },
    "username": "something",
    "utype": "AB",
    "email": "abc@abc.com",
    "last_login": {
        "$date": "2014-12-10T07:12:41.848Z"
    },
    "password": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
}

and social_auth_usersocialauth

{
    "_id": {
        "$oid": "yxzxxxxxxx123"
    },
    "extra_data": "access token etc",
    "user_id": {
        "$oid": "abcdxxxxxxx344"
    },
    "uid": "abc@abc.com",
    "provider": "google-oauth2"
}

django_session

{
    "_id": "pqrxxxxxx543",
    "expire_date": {
        "$date": "2014-12-10T08:12:41.853Z"
    },
    "session_data": "session data"
}

How can i know which user was created using these backends.

Method I applied to solve this:

s = UserSocialAuth.objects.last()
ssid = s.user_id

OR

s = Registration.objects.last()
request.session['SSID'] = s.id
ssid = UserSocialAuth.objects.get(user_id=s.id)

This doesn't seem to the right way. I am concerned about concurrency problem and returning users.

This idea fails for returning users

Can we know session key or anything?