Django-does-not-call-get-prep-value-while-using-lookups

[Shahprogrammer]

I have an custom model field in which i have implemented cryptography. I get prefect results when I don't use lookups for filtering data i.e. it calls get_prep_value() But when I filter data using lookups like startswith or endswith. it does not calls get_prep_value().How to solve this problem?