Force Haystack to index first letter of Charfield

I have a model开发者_如何转开发 with CharField()

class MyModel(models.Model)
   field = models.CharField(...)

How can I configure Haystack to index first letter of this field in SearchIndex?

class MyIndex(SearchIndex):
   starts_with = CharFiled(?)

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I found out that Haystack has something like Django`s forms clean functions which is called prepare.

So in my case to get first letter from field I can do:

class MyIndex(SearchIndex): starts_with = CharField()

def prepare_starts_with(self, obj):
    return obj.field[:1]