CODEWITHSUNDEEP
node.jsmongodbmongoosesum
Joel Langlois
Joel Langlois

Reputation: 55

mongodb aggregate $sum returns extra floating point digits

I am looking to sum the price of some inventory items via their skus on my nodejs server. They are floating-point numbers. My documents look like this

[ 
    { 
        quantity: 1,
        price: 38.95,  
        sku: 'C-2177',
        product: 55b9cfc5fa39110bb644becc,
        _id: 56739191b98d1fde79c3034e 
    },
    { 
        quantity: 1,
        price: 24.95,
        sku: 'C-2188',
        product: 55b9d013fa39110bb645100d,
        _id: 5743870e139e80b9ecb48e02 
    } 
]

All my prices are defined in my schema as Numbers. When I try the $group $sum operators like this:

InventoryItem.model.aggregate([
    { $match: { 'sku': {$in:['C-2177', 'C-2188']}}},
    { $group: { 
        _id: null, subtotal:{$sum:'$price'}
    }}
], function(err, results){
    console.log(results);
});

I get extra floating point digits. Here the sum adds up to $63.90, but I get a response like

[ { _id: null, subtotal: 63.900000000000006 } ]

I'm using mongoose 4.4.2 with the mongodb 2.1.18 driver and my server is running MongoDB 2.6.11. Any help is appreciated!

Upvotes: 4

Views: 1623

Answers (1)

CFrei
CFrei

Reputation: 3627

These are the normal rounding errors of floats. Try in node:

> var a = 38.95, b = 24.95; console.log(a + b)
63.900000000000006

To avoid that, do not store prices in $, store them in cents. If you have really huge numbers, use a library like bigInteger and store the results as strings.

Upvotes: 4

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