title: django modelForm, need to save only selected fields
body: <p>how can i save only selected field in my modelForm? Documentation doesn't help me
  my form:</p>
  
  <pre><code>class ProfileForm(forms.ModelForm):
      class Meta:
          model = CustomUser
         fields = ['avatar', 'firstname', 'lastname',
              'date_of_birth',]
  
          widgets = {
          'date_of_birth': SelectDateWidget(years=range(1920, 2015)),
          }
  </code></pre>
  
  <p>my view:</p>
  
  <pre><code>def cabinet(request):
  
      profile_user = CustomUser.objects.filter(pk = request.user.pk)
      profile = CustomUser.objects.get(pk = request.user.pk)
      form = ProfileForm(request.POST, request.FILES or None, instance=profile)
  
      if request.POST and form.is_valid():
          obj = form.save(commit=False)
          obj.profile_user = CustomUser.objects.get(pk=request.user.pk)
          obj.save()
          return redirect(reverse(cabinet))
  
      return render(request, 'cabinet.html', 
      {'form':form, 'profile_user': profile_user,}, )
  </code></pre>
  
  <p>in html code it's just:</p>
  
  <pre><code>        &lt;form action="{% url 'cabinet' %}" method="post" enctype="multipart/form-data"&gt;
              {% csrf_token %}
              {{ form|crispy }}
              &lt;button class="btn btn-default" type="submit"&gt;Submit&lt;/button&gt;
          &lt;/form&gt;
  </code></pre>
  
  <p>So, if I want only change my 'name' field, i save other fields by empty values.  </p>
  
  <p>And other problem here, at the top of my avatar field i have this:
  <a href="https://i.stack.imgur.com/G6Sv4.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/G6Sv4.png" alt="enter image description here"></a></p>
  
  <p>Documentation doesn't help for both of this problems</p>
  
  <p>UPD  My form view:
  <a href="https://i.stack.imgur.com/2vTL8.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/2vTL8.png" alt="enter image description here"></a></p>
  
  <p>UPD my CustomUser model:</p>
  
  <pre><code>class CustomUser(AbstractBaseUser, PermissionsMixin):
  
      alphanumeric = RegexValidator(r'^[0-9a-zA-Z]*$', message='Only                     alphanumeric characters are allowed.')
  
      username = models.CharField(unique=True, max_length=30, validators=[alphanumeric])
  
      email = models.EmailField(
      'Email',
      max_length=255,
      unique=True,
      db_index=True
      )
      avatar = models.ImageField(
      'Avatar',
      blank=True,
      null=True,
      upload_to='user/avatar',
      default='user/default.jpg'
      )
      firstname = models.CharField(
      'First name',
      max_length=40,
      blank=True,
      null=True
      )
      lastname = models.CharField(
      'Last name',
      max_length=40,
      blank=True,
      null=True
      )
      date_of_birth = models.DateField(
      'Date of birth',
      blank=True,
      null=True
      )
      register_date = models.DateField(
      'Register date',
      auto_now_add=True
      )
      is_active = models.BooleanField(
      'Active',
      default=True
      )
      is_admin = models.BooleanField(
      'Superuser',
      default=False
      )
  
      @property
      def is_staff(self):
          return self.is_admin
  
      def get_short_name(self):
          return self.email
  
      def __str__(self):
          return self.email
  
      def has_perm(self, perm, obj=None):
          return True
  
      def has_module_perms(self, app_label):
          return True
  
      USERNAME_FIELD = 'email'
      REQUIRED_FIELDS = ['username']
  
      objects = UserManager()
  
      class Meta:
          verbose_name = 'User'
          verbose_name_plural = 'Users'
  </code></pre>