He's referring to an old "feature" of DataMapper that I changed a while ago. Originally, the only way to access results was through the ->all array. This array was indexed using the rows id.
There were several problems here. First, the query had to include the id, so this led to odd errors if no id was selected. Second, you could never have a query that resulted in more than one copy of a particular item (such as a n-to-many query without DISTINCT).
So, I would personally make sure that all_array_uses_ids is always set to FALSE, and always assume the ->all array is a normal, sequentially indexed array.
Finally, remember that the all array only exists if you call the normal get method, not get_iterated.
That being said, you may be able to override this by setting all_array_uses_ids on the object just before calling get(). But this is not a supported feature, and there is no guarantee it will work on a future version.
