title: Django error: [<class 'decimal.InvalidOperation'>]
body: <p>I have done the following signal in my project:</p>
  
  <pre><code>@receiver(pre_save, sender=group1)
  @disable_for_loaddata
  def total_closing_group1(sender,instance,*args,**kwargs):
      total_group_closing_deb_po = instance.master_group.filter(ledgergroups__Closing_balance__gte=0,balance_nature='Debit').aggregate(the_sum=Coalesce(Sum('ledgergroups__Closing_balance'), Value(0)))['the_sum']
      total_group_closing_deb_neg = instance.master_group.filter(ledgergroups__Closing_balance__lt=0,balance_nature='Debit').aggregate(the_sum=Coalesce(Sum('ledgergroups__Closing_balance'), Value(0)))['the_sum']
      total_group_closing_po_cre = instance.master_group.filter(ledgergroups__Closing_balance__gte=0,balance_nature='Credit').aggregate(the_sum=Coalesce(Sum('ledgergroups__Closing_balance'), Value(0)))['the_sum']
      total_group_closing_neg_cre = instance.master_group.filter(ledgergroups__Closing_balance__lt=0,balance_nature='Credit').aggregate(the_sum=Coalesce(Sum('ledgergroups__Closing_balance'), Value(0)))['the_sum']
      total_closing_deb_po = instance.ledgergroups.filter(Closing_balance__gte=0,group1_Name__balance_nature='Debit').aggregate(the_sum=Coalesce(Sum('Closing_balance'), Value(0)))['the_sum']
      total_closing_deb_ne = instance.ledgergroups.filter(Closing_balance__lt=0,group1_Name__balance_nature='Debit').aggregate(the_sum=Coalesce(Sum('Closing_balance'), Value(0)))['the_sum']
      total_closing_cre_po = instance.ledgergroups.filter(Closing_balance__gte=0,group1_Name__balance_nature='Credit').aggregate(the_sum=Coalesce(Sum('Closing_balance'), Value(0)))['the_sum']
      total_closing_cre_ne = instance.ledgergroups.filter(Closing_balance__lt=0,group1_Name__balance_nature='Credit').aggregate(the_sum=Coalesce(Sum('Closing_balance'), Value(0)))['the_sum']
      if total_group_closing_deb_po != None and total_group_closing_neg_cre != None and total_closing_deb_po != None and total_closing_cre_ne != None:
          instance.positive_closing = total_group_closing_deb_po + abs(total_group_closing_neg_cre) + total_closing_deb_po + abs(total_closing_cre_ne)
      if total_group_closing_po_cre != None and total_group_closing_deb_neg != None and total_closing_cre_po != None and total_closing_deb_ne != None:    
          instance.negative_closing = total_group_closing_po_cre + abs(total_group_closing_deb_neg) + total_closing_cre_po + abs(total_closing_deb_ne)
  </code></pre>
  
  <p>My models are:</p>
  
  <pre><code>class Group1(models.Model):   
      group_name = models.CharField(max_length=32)    
      master = models.ForeignKey("self",on_delete=models.CASCADE,related_name='master_group',null=True)    
      negative_closing = models.DecimalField(max_digits=10,default=0,decimal_places=2,null=True)    
      positive_closing = models.DecimalField(max_digits=10,default=0,decimal_places=2,null=True)
  
  
  class Ledger1(models.Model):
      name            = models.CharField(max_length=32)
      group1_name     = models.ForeignKey(Group1,on_delete=models.CASCADE,null=True,related_name='ledgergroups')
      closing_balance = models.DecimalField(default=0.00,max_digits=10,decimal_places=2,blank=True)
  </code></pre>
  
  <p>It was working fine in the beginning but all of a sudden when I am increasing the load of the database by putting datas into fields.</p>
  
  <p>It is throwing me the error <code>[&lt;class 'decimal.InvalidOperation'&gt;]</code>.</p>
  
  <p>What this error implies?</p>
  
  <p>Any idea anyone</p>
  
  <p>Thank you</p>