Annotate with a filtered Count

[Casey Greene]

I am trying to figure out how to annotate with a count that is filtered. 
 Here is a quick example of the situation and what I'm looking for:

models.py:

class A(models.Model):
    name = models.CharField(...)
    ...

class B(models.Model):
    date = models.DateField(null=False)
    ...

---
views.py
(proper imports)

def myview(request):
    <process/validate form>
    #here I would like to filter based on a date field from the form
    #let's call the form 'form' and the field 'date'
    a_qset = A.objects.annotate(before_val=Count(
                      'b__date__lte='+form.cleaned_data['date']))

---

Basically I want the number of B as of the date 
form.cleaned_data['date'] accessible in a queryset of As.

Now I know this code isn't correct (since I don't know the proper way to 
approach it) and is vastly simplified, but hopefully it gives an idea of 
what I want to do.  Any suggestions are greatly appreciated.

Thanks!
Casey

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