Suppose I am in the usual situation where there're extra fields in the many2many relationship:

class Person(models.Model):
    name = models.CharField(max_length=128)

class Group(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(Person, through='Membership')

class Membership(models.Model):
    person = models.ForeignKey(Person)
    group = models.ForeignKey(Group)
    date_joined = models.DateField()
    invite_reason = models.CharField(max_length=64)

# other models which are unrelated to the ones above..

class Trip(models.Model):
  placeVisited  = models.ForeignKey(Place)
  visitor  = models.ForeignKey(Person)
 pleasuretrip = models.Boolean()

class Place(models.Model):
  name = models.CharField(max_length=128)

I want to add some extra fields in the Membership form that gets displayed through the Inline. These fields basically are a shortcut to the instantiation of another model (Trip). Trip can have its own admin views, but these shortcuts are needed because when my project partners are entering 'Membership' data in the system they happen to have also the 'Trip' information handy (and also because some of the info in Membership can just be copied over to Trip etc. etc.).

So all I want to have is two extra fields in the Membership Inline - placeVisited and pleasuretrip - which together with the Person instance will let me instantiate the Trip model in the background...

I found out I can easily add extra fields to the inline view by defining my own form. But once the data have been entered, how and when to reference to them in order to perform the save operations I need to do?

class MyForm(forms.ModelForm):
 place = forms.ModelChoiceField(required=False, queryset=Place.objects.all(), label="place",)
 pleasuretrip = forms.BooleanField(required=False, label="...")

class MembershipInline(admin.TabularInline):
 model = Membership
 form = MyForm
    def save_model(self, request, obj, form, change):
        place = form.place
        pleasuretrip = form.pleasuretrip
        person = form.person
        ....
        # now I can create Trip instances with those data
        ....
        obj.save()

class GroupAdmin(admin.ModelAdmin):
 model = Group
 ....
 inlines = (MembershipInline,)

This doesn't seem to work... I'm also a bit puzzled by the save_formset method... maybe is that the one I should be using? Many thanks in advance for the help!!!!