store a calculated value in the datastore or just calculate it on the fly
What about storing the ranking as a property in the post.  That would make sense for querying/sorting wouldn't it.
If you store the ranking at the same time (meaning in the same entitiy) as you store the votes/comment count, then the only increase in write cost would be for the index.  (ok initial write cost too but that is what 2 [very small anyway]).
You need to do a database operation everytime anyone votes or comments on the post anyway right!?!  How else can to track votes/comments?
Actually though, I imagine you will get into use text search to find data in the posts. If so, I would look into maybe storing the ranking as a property in the search index and using it to rank matching results.
Don't we need to consider how you are selecting the posts to display.  Is ranking by votes and comments the only criteria?
I writing an app in python for google app engine where each user can submit a post and each post has a ranking which is determined by its votes and comment count.  The ranking is just a simple calculation based on these two parameters.  I am wondering should I store this value in the datastore (and take up space there) or just simply calculate it every time that I need it.  Now just fyi the posts will be sorted by ranking so that needs to be taken into account.
I am mostly thinking for the sake of efficiency and trying to balance if I should try and save the datastore room or save the read/write quota.
I would think it would be better to simply store it but then I need to recalculate and rewrite the ranking value every time anyone votes or comments on the post.
Any input would be great.