title: django 'DateTimeField' has no default value error
body: <p>i am novice at django development. i am creating a class A which might have multiple class B assigned to it:</p>
  
  <pre><code>class A(models.Model):        
      name = models.CharField(max_length=200)    
  
      def __unicode__(self):
          self.name
  
  
  class B(models.Model):
      a = models.ForeignKey(A)
      name = models.CharField(max_length=200)
      mydate = models.DateTimeField('party date')
  </code></pre>
  
  <p>When i am trying to create a new "A" element at the admin page, and creating a matching element B for it and then <code>save()</code> , i am getting the Warning:
  <strong>Field 'mydate' doesn't have a default value</strong></p>
  
  <p>If i move the "mydate",element to class A, then when hitting <code>save()</code> i am getting a message <strong>This field is require</strong> from Django, requires me to fill the field!</p>
  
  <p>how can i make this required message appear also when date field is part of B!!!</p>
  
  <p>Thanks</p>