Django InlineModelAdmin - set inline field from request on save (set user field automatically) (save_formset vs save_model)

Question

I have two models, a MainModel and a related InlineModel that i'd like to show as an inline in the admin. This InlineModel can be used for, say, making notes about the model and should track the logged in admin user making changes. While this seems simple (and indeed, the docs show an example for this when the user field is part of the MainModel), I can't seem to grasp it when the field is on the Inline.

To be specific, my goal is:

1. User edits MainModel
2. User adds an InlineModel, not filling in the user field
3. User presses save
4. Code fills in the user field for newly created InlineModel instances
5. (Bonus! user field is readonly for existing instances and hidden for new inlines)

And my questions:

1. Is this correct? Its too bas save_model isn't called for InlineModelAdmin instances
2. Does doing it this way allow me to save without causing an error? (user is required, validation flags it)
3. How can I hide the user input field for new inlines, and have it readonly for existing inlines?

Here are my current ideas:

---

#models.py
class MainModel(models.Model):
    some_info = models.IntegerField()

class InlineModel(models.Model):
    main = models.ForeignKey(MainModel)
    data = models.CharField(max_length=255)
    user = models.ForeignKey('auth.User')

#admin.py
class InlineModelInline(admin.TabularInline):
    model = InlineModel
    fields = ('data', 'user')
    #readonly_fields = ('data', 'user') #Bonus question later

class MainModelAdmin(admin.ModelAdmin):
    list_display = ('id', 'some_info')
    inlines = [InlineModelInline]

    #def save_model(self, request, obj, form, change):
        #http://docs.djangoproject.com/en/dev/ref/contrib/admin/#django.contrib.admin.ModelAdmin.save_model
        #Only called for MainModel, not for any of the inlines
        #Otherwise, would be ideal

    def save_formset(self, request, form, formset, change):
        #http://docs.djangoproject.com/en/dev/ref/contrib/admin/#django.contrib.admin.ModelAdmin.save_formset
        #Experimenting showd this is called once per formset (where the formset is a group of inlines)
        #See code block at http://code.djangoproject.com/browser/django/tags/releases/1.2.1/django/contrib/admin/options.py#L894
        if not isinstance(formset.model, InlineModel):
            return super(MainModelAdmin, self).save_formset(request, form, formset, change)
        instances = formset.save(commit=False)
        for instance in instances:
            if not instance.pk:
                instance.user = request.user
        instance.save()
        formset.save_m2m()

Answer 1

I have solved the first half of my question:

def save_formset(self, request, form, formset, change):
    if formset.model != InlineModel:
        return super(MainModelAdmin, self).save_formset(request, form, formset, change)
    instances = formset.save(commit=False)
    for instance in instances:
        if not instance.pk:
            instance.user = request.user
        instance.save()
    formset.save_m2m()

Now i'm interested in the bonus behavior:

1. I'm required to select a user when adding a new inline due to validation rules. My best guess is to not include the 'user' field in my InlineModelInline.fields tuple, but then this won't show the author for existing InlineModel instances. (Edit: adding 'user' to readonly_fields works here)

2. (Edit) How can I make the existing inlines render 'data' as readonly, but still be able to edit it when adding a new inline?