I think thats a great idea, however I actually do (somewhat embarassingly) have a situation where I don't have a uniqueness constraint on my `order_column`. I think this suggests the constraint of uniqueness is good to recommend but I don't think it is critical for LHM to run so I almost don't want to enforce it. 

We don't have one for speed reasons, and because the gem managing the table does a kind of upsert style thing where it will always use an existing row for a given order_column value or just create it if it doesn't exist. That said my case is definitely special so I would be willing to add the constraint and then somehow remove it in my own code if you think it should definitely make it in.