django foreign key error Cannot assign must be a instance

# This error is because you're trying to assign an instance.id instead of 
# the instance itself. If its a foreign key field you assigned the instance i.e.
yourTable.yourCol = relatedTable.objects.get(relatedTableColumn = yourColValue)
#-or-
Orders.Dept_id = Department.objects.get(id = yourValueHere)
Orders.save()

Here is what the above code is Doing:
1. You’re creating a new instance of the Orders class.
2. You’re assigning the Dept_id field to the Department instance with the id of yourValueHere.
3. You’re saving the Orders instance.

You’re not assigning the Department instance to the Dept_id field. You’re assigning the id of the Department instance to the Dept_id field.

You need to do this:
Orders.Dept_id = Department.objects.get(id = yourValueHere)
Orders.save()