title: Django - manytomany relation between User.Profile and another model, ValueError: Cannot add
body: <p>user/models.py</p>
  
  <pre><code>from django.db import models
  from django.contrib.auth.models import User
  from django.db.models.signals import post_save
  from django.dispatch import receiver
  
  
  class Bread(models.Model):
      title = models.CharField(max_length=40)
  
      def __str__(self):
          return self.title
  
  
  class Profile(models.Model):
      user = models.OneToOneField(User, on_delete=models.CASCADE)
      bio = models.TextField(max_length=500, blank=True)
      location = models.CharField(max_length=30, blank=True)
      birth_date = models.DateField(null=True, blank=True)
      picture = models.ImageField(upload_to="photos/", default="photos/none/default.png")
      bread = models.ManyToManyField(Bread)
  
  
  @receiver(post_save, sender=User)
  def create_user_profile(sender, instance, created, **kwargs):
      if created:
          Profile.objects.create(user=instance)
  
  
  @receiver(post_save, sender=User)
  def save_user_profile(sender, instance, **kwargs):
      instance.profile.save()
  </code></pre>
  
  <p>I trying to add a simple object to Profile model, but it does not work.</p>
  
  <pre><code>&gt;&gt;&gt; from user.models import Bread
  &gt;&gt;&gt; from user.models import Profile
  &gt;&gt;&gt; from django.contrib.auth.models import User
  &gt;&gt;&gt; p1 = Bread(title="Example")
  &gt;&gt;&gt; p1.save()
  &gt;&gt;&gt; Profile(User).bread.add(p1)
  ValueError: Cannot add "&lt;Bread: Example&gt;": instance is on database "None", value is on database "default"
  &gt;&gt;&gt; u1 = Profile(User)
  &gt;&gt;&gt; u1.save()
  TypeError: _prepare() takes 1 positional argument but 2 were given
  </code></pre>
  
  <p>Seeing similar previous questions I tried to save profile model, as u can see, I know its related with User and his id, but i have no idea how to fix it easily</p>