null value in column violates not-null constraint

Hi, I'm trying to create a digital marketplace site where users will be able to register and submit their products.
I created the Product model and connected it with User with foreign key.Everything seems  to be alright, but when I submit my product i get the following error:

null value in column violates not-null constraint
 it looks like relation between product and user is not properly configured(regardless of my mixin that is suppose to handle it)

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note that I'm using:

database:postgresql

allauth(for user registration)

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________

models.py

from django.conf import settings
from django.db import models
from django.db.models.signals import pre_save, post_save
from django.core.urlresolvers import reverse
from django.utils.text import slugify
# Create your models here.



class Product(models.Model):
   seller = models.ForeignKey(settings.AUTH_USER_MODEL)
   #user = models.OneToOneField(settings.AUTH_USER_MODEL)
   # user = models.ForeignKey(settings.AUTH_USER_MODEL)
   # managers = models.ManyToManyField(settings.AUTH_USER_MODEL, related_name="managers_products", blank=True)
   title = models.CharField(max_length=30) #owiuerpoajsdlfkjasd;flkiu1p3o4u134123 ewjfa;sd
   slug = models.SlugField(blank=True, unique=True)
   description = models.TextField()
   price = models.DecimalField(max_digits=100, decimal_places=2, default=9.99, null=True,) #100.00


   def __unicode__(self): #def __unicode__(self):
      return self.title

_______
views.py

from django.views import View
from django.views.generic import (
                CreateView,
                )

from .forms import ProductModelForm
from .mixins import FormUserNeededMixin
# Create your views here.


class ProductCreateView(FormUserNeededMixin, CreateView):
    form_class = ProductModelForm
    template_name = 'create_view.html'

_______
forms.py

from django import forms

from django.utils.text import slugify

from .models import Product


class ProductModelForm(forms.ModelForm):
   class Meta:
      model = Product
      fields = [
         "title",
         "description",
         "price",
      ]

________
mixins.py

from django import forms
from django.forms.utils import ErrorList

class FormUserNeededMixin(object):

     def form_valid(self, form):
        if self.request.user.is_authenticated():
            form.instance.user = self.request.user
            return super(FormUserNeededMixin, self).form_valid(form)
        else:
            form._errors[forms.forms.NON_FIELD_ERRORS] = ErrorList(["User must be logged in to continue."])
            return self.form_invalid(form)

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