title: Django Validate M2M Form Save
body: <p>I am trying to creating cron application my model look like :</p>
  
  <pre><code>class Crontab(models.Model):
      #Some filelds 
      ...
      ...
      #m2m fields From here prob starts
      minute = models.ManyToManyField(Mst_Cron_Minute, db_table="crontab_minute_map")
      hour = models.ManyToManyField(Mst_Cron_Hour, db_table = "crontab_hour_map")
      day_of_month = models.ManyToManyField(Mst_Cron_Day_Of_Month, db_table="crontab_day_of_month_map")
      month = models.ManyToManyField(Mst_Cron_Month, db_table="crontab_month_map")
      day_of_week = models.ManyToManyField(Mst_Cron_Day_Of_Week, db_table="crontab_day_of_week_map") 
      description = models.CharField(max_length=250, blank=True)
      is_active = models.IntegerField(choices=YES_NO_CHOICES)
      reason_for_deactivate = models.CharField(max_length=250, blank=True)
  </code></pre>
  
  <p>I need to validate the cron before it saves it into database.</p>
  
  <ol>
  <li>Should not have duplicate cron data saved on this model.</li>
  <li>if already a data in db say  5 1 * * * then for new record in the position of * none of the value allowed. 
  Example 5 1 * * 1 or 5 1 * 1 * or 5 1 1 1 1 not allowed.</li>
  <li>if i have a data data 5 1 * 1 * is in DB the new data * 1 * 1 * or 5 * * 1 * ... not allowed, it mean if there is a digit in the position the * not allowed.</li>
  </ol>
  
  <p>How can i validate this ? where my code should go ? Please help me .</p>