Django m2m and saving objects - django

Question

I have a couple of simple objects that have a many-to-many relationship. Django has joined them using obj1_obj2 table and it looks like this in mysql;
id | person_id | nationality_id
-----------------------------------
1 | 1 | 1
2 | 1 | 2
Now when I save obj1 (which shows obj2 in as Multi-select in its form) the ids in the obj1_obj2 table increase even thow I have not changed them. For example I change a basic character field for obj1 on its form and save it and the the data in the joining table appears to be deleted and re-saved giving the entries new ids.
In fact I don't have to change anything all I have to do is save the form and the same thing happens.
All I am doing in the view is form.save(), nothing special. Is that the normal way that it works?
EDIT: Added Models, Views, Forms
class Person(models.Model):
name = models.CharField()
birthdate = models.CharField()
nationality = models.ManyToMany(Nationality)
class Employee(Person):
employeeNum = models.CharField()
class FamilyMember(Person):
employee = models.ForeignKey(Employee)
relationship = models.CharField()
class Nationality(models.Model):
abbrev = models.CharField()
country = models.CharField()
class FamilyMemberDetailsForm(forms.ModelForm):
class Meta:
model = FamilyMemeber
exclude = ['employee']
def editFamilyMember(request, familyMember_id):
familyMember = get_object_404(FamilMember, familyMember_id)
if request.method == 'POST':
form = FamilyMemberDetailsForm(request.POST, instance=familyMember)
if form.is_valid():
form.save()
else:
form = FamilyMemberDetailsForm(instance=familyMember)
return render_to_response(editForm.html, {'form':form},
context_instance(RequestContext(request))
This is a cut down version of the models, but the same thing happens for saving an employee or familyMember. The FamilyMember I have shown because it is as simple as this I create the modelForm and then make changes and then save it. For the employee I do some more manipulation in the init of Form for the Nationality, mainly for presentation, and at first I thought it was this manipulation that was causing it, but as I said the same thing happens with the FamilyMember where I do nothing except save.
The Nationality is presented on the form as a multiselect box with a list and the user can select 1 or more from the list. If I just present the populated form and then save it without changing anything the id for the many-to-many table entry changes.
I have changed the example table titles also.
Thanks,
Andrew

Answer 1

Yes, the deletion of any existing rows in appname_obj1_obj2 is expected behavior when saving a form for an object that has a ManyToManyField.
You can see the clear() before the add(**values) in ReverseManyRelatedObjectsDescriptor and ManyRelatedObjectsDescriptor in django/db/models/fields/related.py.
Pop open a shell and take a look at the queries yourself. Something like this should show you the DELETE before the INSERT in the raw sql.
from django.db import connection
fm = FamilyMember.objects.get(pk=1)
form = FamilyMemberDetailsForm(instance=fm)
data = form.initial
data['name'] = "z%s" % data['name']
form = FamilyMemberDetailsForm(data, instance=fm)
connection.queries = [] # clearing to limit the queries you have to look at
form.save()
for q in connection.queries:
print("%s\n" % q['sql'])