You're not hitting memcache at all here. You're using a different key format in the post
and get
methods: in get
you use "posts_user" whereas in post
you use "user:posts", so the key is never found and you fall through to the db query. And, of course, the query is not up to date because of eventual consistency, which is presumably the whole reason you're using memcache in the first place.
Fix your memcache keys and this should work.