DataMapper ORM v1.8.0

[eluser]WanWizard[/eluser]
Shouldn't it be '$this->post', and not posts?

And if you do this in a model method, try making a clone of $this first, and use that to fetch the first and last post entries.

If you're not sure, use check_last_query() to see what query your code has produced. Usually that will show you where you went wrong in the thought process.

p.s. you might want to look at the nestedsets extension. topics/posts are usually stored in a tree structure, which allows you to keep track of the sequence of replies.
Use a multi-root tree table where every tree is a topic with it's replies. The root can be the topic info. Use a small table with only index columns, and have it point (in a relation) to a messages table containing the actual message (with all its details). This also allows you to insert the same message at multiple points in the tree, which I use for example in forum announcements...