ManyToMany to_field through and db_table questions

Can anyone tell me if to_field is valid in this context and what is the
distinction between through and db_table? ...

class Sections(models.Model):
""" intermediary table for Document recursive m2m """
document = models.ForeignKey(Document, to_field='ancestor')
section = models.ForeignKey(Document, to_field='ancestor')
comment = models.TextField()

class Document(models.Model):
ancestor = models.IntegerField()
sections = models.ManyToManyField('self',
symmetrical=False,
through=Sections,
db_table=Sections,
related_name='Comprise')

... where 'ancestor' is a surrogate key which survives in descendant
documents - such being both newer revisions of the same document and
other documents more or less based on the ancestral document.

I also wonder how I might have a m2m recursive relationship between
documents without specifying an intermediary table - iow if the comment
column was not needed - is it OK to specify to_field='ancestor' in the
Document model?

Thanks for any help

Mike

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