I'm trying to build a toolkit of Func classes built on the fuzzystrmatch postgres extension.

For instance I have this wrapper which takes in an Expression and a search term and returns the levenshtein distance:

class Levenshtein(Func):
    """This function calculates the Levenshtein distance between two strings:"""
    template = "%(function)s(%(expressions)s, '%(search_term)s')"
    function = "levenshtein"

    def __init__(self, expression, search_term, **extras):
        super(Levenshtein, self).__init__(
            expression,
            search_term=search_term,
            **extras
        ) 

Called like this, using an F Expression:

Author.objects.annotate(lev_dist=Levenshtein(F('name'),'JRR Tolkien').filter(lev_dist__lte=2)

However if the 'name' field here is greater than 255 it throws an error:

Both source and target can be any non-null string, with a maximum of 255 characters.

I can truncate the name when I annotate using Substr:

Author.objects.annotate(clipped_name=Substr(F('name'),1,250))

But I can't seem to figure out how to place that logic inside the func, which I'm placing inside an ExpressionWrapper and setting the output_field as per the docs:

class Levenshtein(Func):
    """This function calculates the Levenshtein distance between two strings:"""
    template = "%(function)s(%(expressions)s, '%(search_term)s')"
    function = "levenshtein"

    def __init__(self, expression, search_term, **extras):
        super(Levenshtein, self).__init__(
            expression=ExpressionWrapper(Substr(expression, 1, 250), output_field=TextField()),
            search_term=search_term,
            **extras
        )