old_contents: from django.db import models
  
  class Activity(models.Model):
      group = models.CharField(max_length=4)
      grouptype = models.TextField()
      groupdetail = models.TextField()
      details = models.TextField()
      disabled = models.BooleanField()
      time = models.DecimalField(decimal_places=2, max_digits=10)
      unique_together = (("group", "grouptype", "disabled", "time"),)
  
  class ActivityEntry(models.Model):
      user = models.ManyToManyField(
          'tracker.Tbluser',
          related_name="user_foreign"
      )
      activity = models.ManyToManyField(
          Activity,
          related_name="activity_foreign"
      )
      amount = models.BigIntegerField()
  
      def time(self):
          return self.activity.time * self.amount
  
new_contents: from django.db import models
  
  class Activity(models.Model):
      group = models.CharField(max_length=4)
      grouptype = models.TextField()
      groupdetail = models.TextField()
      details = models.TextField()
      disabled = models.BooleanField()
      time = models.DecimalField(decimal_places=2, max_digits=10)
      unique_together = (("group", "grouptype", "disabled", "time"),)
  
  class ActivityEntry(models.Model):
      user = models.OneToOneField(
          'tracker.Tbluser',
          related_name="user_foreign"
      )
      activity = models.OneToOneField(
          Activity,
          related_name="activity_foreign"
      )
      amount = models.BigIntegerField()
  
      def time(self):
          return self.activity.time * self.amount
  
subject: Use a OneToMany field for the activity joiner.