title: Django filter queryset if a field exists
body: <p>I want to filter my queryset if and only if <code>loan</code> exists in my model <code>ShgGroupLoanMarking</code>.</p>
  <pre><code>class ShgGroupLoanMarking(models.Model):
   shg = models.ForeignKey(Shg, null=True)
   category = models.CharField(max_length=25, choices=GROUP_CATEGORY, default=&quot;shg_group&quot;)
   msss = models.ForeignKey(Msss, null=True)
   shgmember = models.ForeignKey(ShgMember, null=True)
   loan = models.ForeignKey(Loan, null=True)
   date_of_marking = models.DateField(null=True)
   paid = models.CharField(max_length=100, null=True)
   loan_amount = models.CharField(max_length=100, null=True)
   service_charge = models.CharField(max_length=100, null=True)
   description = models.CharField(max_length=100, null=True)
   status = models.CharField(max_length=100, null=True)
   print_status = models.BooleanField(default=False)
   sent_to_bank_status = models.BooleanField(default=False)
   receipt_number = models.IntegerField(default=0)
  </code></pre>
  <p>How do I implement this?</p>