Ensure correctness in riak?

It seems you cannot do transactions in riak. How to ensure the correctness of the data?

Suppose we want to insert comments. In redis, I would do

commentId=incr commentCount
multi
SET comment_post:commentId postId //for later use when we flag comments. We'll need to know where in the db it is
RPUSH post_comment:postId binaryValue //contains commentId in it + comment body
exec

In sql, I insert a new line into the comment table with text and id message. Both use more than one statement. How do I insert a comment body and link the post to the comment in riak, since it has no transactions?

Another problem is that I am modifying the message. How to update message and update tag list after using this tag