How to get the Nth row (or record) with Datamapper

I'm looking for a more datamappery way of doing t开发者_运维技巧he following (since the code below is dependent on sqlite):

id = repository(:default).adapter.query(
  'SELECT id FROM ads ORDER BY random() LIMIT 1'
)
@ad = Ad.get(id)

My hope was to do something like:

@ad = Ad.get(:offset=>rand(Ad.all.count))

or

@ad = Ad.find(:offset=>rand(Ad.all.count), :limit=>1)

Basically I'm just looking for a way to get a row by offset or row number. Apologies if this has been asked already, I'm not sure what vocabulary to search for as I'm new to both Ruby and Datamapper

---

You can use request Ad.first :offset => rand(Ad.count), but it invokes 2 SQL statements:

SELECT COUNT(*) FROM "ads"
SELECT "id" FROM "ads" ORDER BY "id" LIMIT 1

Also you can use raw SQL:

repository.adapter.select "SELECT * FROM ads ORDER BY RANDOM() LIMIT 1"