How to get the primary key of the underlying model in a ModelForm?

I have a model, Document, and an associated DocumentForm(forms. ModelForm), and a DocumentAdmin(admin.ModelAdmin). In the clean method of the DocumentForm, I need to get the document_id (ie primary key) of the underlying Document, but it is not included in the field set for that form. I need the primary key, because one of the fields on the form belongs to another model (it is a JSON field), and I need to save that field in the other model when the DocumentForm is saved.

How do I access the primary key of the underlying Model in a ModelForm?

Thanks!

Mark

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