Optimize saving FK and M2M relationships in django

See also http://stackoverflow.com/questions/2846029/django-set-foreign-key-using-integer
    employee = Employee(first_name="Name", last_name="Name") 
  employee.type_id = 4 
  employee.save(update_fields = ["type",])

ForeignKey and M2M fields store their value in an attribute with _id at the end, which you can access directly to avoid visiting the database.

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