Bads results from queryset

Key is unique.

First case:

Foo.objects.filter(key="ABCD") 
>> [<Foo: Foo object>] 

Foo.objects.filter(key="A") 
>>[] 

Second case (Xapian) , after doing rebuild_index:

sqs = SearchQuerySet()
search = sqs.models(CertificationApplicant)

search.filter(key="ABCD")
>> [<SearchResult: foo.foo (pk=62)>] #only one it's ok

search.filter(key__exact="ABCD")
>> [<SearchResult: foo.foo (pk=62)>] #only one it's ok

search.filter(key=Exact("ABCD"))
>> []   #WHY no result ???

search.filter(key="A")
>> [<SearchResult: foo.foo (pk=62)>, [<SearchResult: foo.foo (pk=124)>, u'...(remaining elements truncated)...'] 

search.filter(key__exact="A")
>> [<SearchResult: foo.foo (pk=62)>,  [<SearchResult: foo.foo (pk=124)>, u'...(remaining elements truncated)...']  #WHY multiple results ????

search.filter(key_exact=Exact("A"))
>> [] # WHY ?

I would like same behavior of "Foo.objects.filter(key="ABCD") " but with Xapian.

Configuration

• Operating system version: MacOsx 10.11.5
• Search engine version: xapian-haystack==2.0.0
• Python version: 2.7.1
• Django version: 1.4.2
• Haystack version:

Start by reproducing the Foo.objects.filter and SearchQuerySet.filter examples with xapian-haystack 2.0.0, Django 1.4.2, and Python 2.7.1. Trace how key, key__exact, and Exact("...") are translated for the Xapian backend. Done means exact filtering matches Django queryset behavior without returning partial or multiple results.