I have two models implemented like

class A(models.Model):
    a_name = models.CharField(max_length=50)

class B(models.Model):
    a = models.ForeignKey(A)
    b_tag = models.CharField(max_length=50)
    user=models.ForeignKey(User)             # stores username

Now I define an A admin and register it with B as a subclass to TabularInline. I wonder if it is possible somehow to filter the list of B objects before the inline formset is rendered, so not all B objects related to A get into the formset, only ones whose user parameter matches the currently logged in user shows up!

Use the get_queryset method: https://docs.djangoproject.com/en/stable/ref/contrib/admin/#django.contrib.admin.ModelAdmin.get_queryset

Should look like:

class BAdmin(admin.TabularInline):
    ...

    def get_queryset(self, request):
        qs = super(BAdmin, self).get_queryset(request)
        return qs.filter(user=request.user)