This is are my models i want to relate. i want for collection to appear in the form of occurrence.

class Collection(models.Model):
    id = models.AutoField(primary_key=True, null=True)
    code = models.CharField(max_length=100, null=True, blank=True)
    address = models.CharField(max_length=100, null=True, blank=True)
    collection_name = models.CharField(max_length=100)

    def __unicode__(self):
        return self.collection_name

    class Meta:
        db_table = u'collection'
        ordering = ('collection_name',)

class Occurrence(models.Model):
    id = models.AutoField(primary_key=True, null=True)
    reference = models.IntegerField(null=True, blank=True, editable=False)
    collection = models.ForeignKey(Collection, null=True, blank=True, unique=True),
    modified = models.DateTimeField(null=True, blank=True, auto_now=True)
    class Meta:
         db_table = u'occurrence'

Every time i go to check the Occurrence object i get this error

 TemplateSyntaxError at /admin/hotiapp/occurrence/
 Caught an exception while rendering: column occurrence.collection_id does not exist
 LINE 1: ...LECT "occurrence"."id", "occurrence"."reference", "occurrenc..

And every time i try to add a new occurrence object i get this error

 ProgrammingError at /admin/hotiapp/occurrence/add/
 column occurrence.collection_id does not exist
 LINE 1: SELECT (1) AS "a" FROM "occurrence" WHERE "occurrence"."coll...

What am i doing wrong? or how does ForeignKey works?