Datamapper query error. Can't understand what I am doing wrong

[eluser]WanWizard[/eluser]
First of all, you're violating the rule that models should have the singular name of the table, and the other way around.

The fact that it tries to join on the column '_id' means that, most likely because of the previous statement, it can't find the foreign key in the relationship table.
Foreign keys (and almost everything else) are named in relation to the model, so in this case the FK should be category_model_id.