'Mongoose, conditional atmoic update

Using mongoose, I want to decrease the value of count by one, unless the value is already 0. I also need this operation to be atomic, so I'm using findOneAndUpdate. I've tried the following code, however have had no luck. Any recommendations?

Item.findOneAndUpdate({
        _id : req.query.item_id,
        user : req.user._id
    },{
        $inc: {
            count : {
                $cond: { 
                    if: { count : {$gt : 0} }, 
                    then: -1,
                    else: 0
                }
            }
        }
    })

Provides the following error message:

MongoServerError: Cannot increment with non-numeric argument: {count: { $cond: { if: { count: { $gt: 0 } }, then: -1, else: 0 } }}
node.jsmongodbmongoose


Solution 1:[1]

My solution to this problem was a workaround, instead of conditionally $inc count, I just filter to only return results with a count greater than 0.

Item.findOneAndUpdate({
    _id : req.query.item_id,
    user : req.user._id,
    count : {$gt : 0}
},{
    $inc: { count : -1}
})

Sources

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Source: Stack Overflow

Solution Source
Solution 1 Ian Wise

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