If your function only requires one variable from an object passed into it, require that variable and not the whole object. Example:

def delete_cache_key(user)
  cache.delete(user.username)
  cache.delete("all_users_" + user.username)
  cache.delete("special_users_" + user.username)
end

NO! WRONG! Better:

def delete_cache_key(username)
  cache.delete(username)
  cache.delete("all_users_" + username)
  cache.delete("special_users_" + username)
end

/rant