title: Django: Asymmetric ManyToMany Relation
body: <p>I am new to using <code>django</code> and would appreciate your input to the following problem.</p>
  
  <p>Two models are present, namely <code>A</code> and <code>B</code>. Instances of <code>A</code> must belong to at least one instance of <code>B</code>, while a <code>B</code> could contain any number of instances of <code>A</code>s from 0 to infinite. </p>
  
  <pre><code>class A(models.Model):
    name = models.CharField(max_length = 128)
    created_date = models.DateTimeField(auto_now_add = True)
    modified_date = models.DateTimeField(auto_now = True)
  
  class B(models.Model):
    name = models.CharField(max_length = 128)
    created_date = models.DateTimeField(auto_now_add = True)
    modified_date = models.DateTimeField(auto_now = True)
    A_list = models.ManyToManyField(A)
  </code></pre>
  
  <p>Okay. I think, based on the description I have given, <code>ManyToManyField</code> should be used for the two models. The thing is, from what I have read, <code>blank = false</code> must be used to prevent a <code>null</code> assignment for <code>A</code> (since an <code>A</code> must be assigned to at least one <code>B</code>) but a <code>B</code> may not have an assigned <code>A</code> (and thus should not have the <code>blank = false</code> statement. Could anyone asymmetric <code>ManyToMany</code> relation can be implemented in <code>django</code> <code>Model</code>? </p>