How to cahce only by single parameter in the method argument

[vijaykr...@gmail.com]

    public List<CoverageAgreement> retrieveCoveragesByPlanId( Integer planId, DateTime dateFrom, DateTime dateTo)  --> In this method i want to cache the return type only based on the PLAN ID argument. Is there any way i can do that.
I did the below and its not working. Can any one please help.

@Cacheable(cacheName = "contractByPlanIdCache", keyGenerator = @KeyGenerator(name = "StringCacheKeyGenerator"))
    public List<CoverageAgreement> retrieveCoveragesByPlanId(@PartialCacheKey Integer planId, DateTime dateFrom, DateTime dateTo) {