title: Default value for field in Django model
body: <p>Suppose I have a model:</p>
  
  <pre><code>class SomeModel(models.Model):
      id = models.AutoField(primary_key=True)
      a = models.CharField(max_length=10)
      b = models.CharField(max_length=7)
  </code></pre>
  
  <p>Currently I am using the default admin to create/edit objects of this type.
  How do I remove the field <strong><code>b</code></strong> from the admin so that each object <em>cannot</em> be created with a value, and rather will receive a default value of <code>0000000</code>?</p>