from django.contrib import admin
class ModelBModelAdmin(admin.ModelAdmin):
    rel_dump = {}

    def delete_view(self, request, object_id, extra_context=None):
        """
        handling deletion from change_form
        """

        obj = models.ModelB.objects.get(pk=object_id)
        self.before_delete(obj)
        res = super(ModelBModelAdmin, self).delete_view(request, object_id, extra_context)
        self.after_delete(obj)
        return res

    def response_action(self, request, queryset):
        """
        handling deletion from change_list
        """

        #copy & pasted from super method:
        try: action_index = int(request.POST.get('index', 0))
        except ValueError: action_index = 0
        try: action = request.POST.getlist('action')[action_index]
        except IndexError: action = ''

        if action == 'delete_selected':
            for obj in queryset:
                self.before_delete(obj)
        res = super(ModelBModelAdmin, self).response_action(request, queryset)
        if action == 'delete_selected':
            for obj in queryset:
                self.after_delete(obj)
        return res

    def before_delete(self, obj):
        """
        removing "problem" relation from _meta._m2m_cache and saving them into rel_dump dictionary to restore at the end.
        consequently it will be ignored during the cascade-deletion of all related objects
        """
        self.rel_dump[obj] = {}
        for key in obj._meta._m2m_cache.keys():
            if isinstance(key, generic.GenericRelation) and key.rel.to == ModelA:
                self.rel_dump[obj].update({key: obj._meta._m2m_cache.pop(key)})

    def after_delete(self, obj):
        """
        restoring relations and manual deletion of related objects
        """
        obj._meta._m2m_cache.update(self.rel_dump.pop(obj))
        ModelA.objects.filter(content_type=ContentType.objects.get_for_model(obj.__class__), object_pk=obj.pk).delete()