title: Django model manytomany field count occurrence
body: <p>models.py</p>
  <pre><code>class Region(models.Model):
      name = models.CharField(blank=False, unique=True)
      count = models.IntegerField(blank=True, null=True) # This should be the occurrence count in ETF
  
  class ETF(models.Model):
      ticker = models.CharField() 
      region = models.ManyToManyField(Region, blank=True)
  </code></pre>
  <p>Example: If there is 100 ETF objects in ETF model have U.S.A. as region, then count field in U.S.A. object in Region model should be 100.</p>
  <p>How do I automatically update the count field in Region Model everytime there are changes in the database? Sorry if there is duplicate question,</p>
  <p>I just can't find the keywords to search for the solution.</p>