This past week or team ran into a rather annoying bug in Mongoid. It seems that Mongoid has issues creating a query that performs a find against a scope created using the `_id` field.
Suppose you create a scope like `users = User.in(id: array_of_ids)`. If you're interested in finding a specific user within that scope, you might try a query like `users.find(id: user_id)`. However, there's a problem with this approach. When Mongoid executes the query, the find kills the previous scope and performs a find against the entire collection. Much to our dismay, we found the same issue when using a `where` query for the initial scope.
After investigating the query logs, it became evident that Mongoid was doing a merge operation when the `_id` field was involved. We found a way to drop down to the driver and manually created a query that used a combination of `$and` and `$in`. This query allows us to achieve the desired result using Mongoid.
Or conclusion is that in order for Mongoid to respect scope while performing a find, you have to use a scope like this: `users = User.and(:id.in=> array_of_ids)`. From that scope, you can then execute the find `users.find(id)`.
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