Django models Manager isn't accessible via model instances
<p>I get this error 'Manager isn't accessible via Stage instances'. I have spent few good hours trying to work it out but I have got no clue what I am doing wrong. </p>

<p>models.py</p>

<pre><code>class StageManager(models.Manager):
    def get_queryset(self):
        return super(StageManager, self).get_queryset().filter(stage=self.id, stage__zone=self.zone).aggregate(Sum('substage_value'))


class Stage(models.Model):
    tablerow_stage = models.ForeignKey(TRStage)
    stage_value = models.PositiveSmallIntegerField(default=0)
    slug = models.SlugField(unique=True)
    zone = models.ForeignKey(Zone)

    objects = models.Manager() # The default manager
    subs_objects = StageManager()

    def __str__(self):
        return '%s.%s.%s' % (self.zone.project,
                             self.tablerow_stage.stage_num,
                             self.zone.zone_num)
</code></pre>

<p>line of code from template:</p>

<pre><code>&lt;td style="width:40px" align="center"&gt;{{ stage.subs_objects }}&lt;/td&gt;
</code></pre>