猫鼬组和计数 [英] Mongoose group and count

问题描述

下面是我的mongodb结构:

Below is my mongodb structure:

[{
    _id: '111',
    items: [{
        productId: '123'
    }]
}, {
    _id: '222',
    items: [{
        productId: '123'
    }, {
        productId: '456'
    }]
}, {
    _id: '333',
    items: [{
        productId: '123'
    }, {
        productId: '456'
    }, {
        productId: '789'
    }]
}]

我希望对productId进行分组和计数，以便结果为:

And I expect to group and count productId so that the result to be:

[{
    productId: '123',
    count: 3
}, {
    productId: '456',
    count: 2
}, {
    productId: '789',
    count: 1
}]

---

好吧，我尝试像这样使用aggregation，但是我想我错了:

---

Well, I tried using aggregation like this, but I think I got it wrong:

const aggregatorOpts = [{
  $group: {
    _id: "$items.productId",
    count: { $sum: 1 }
  }
}]

Model.aggregate(aggregatorOpts).exec()

我得到了:

result [
  { _id: [ '123' ], count: 1 },
  { _id: [ '123', '456' ], count: 1 },
  { _id: [ '123', '456', '789' ], count: 1 }
]

任何关于如何进行聚合的帮助都将受到赞赏，请不要假设模型有任何变化.

Any help regarding how to do the aggregation probably is appreciated, and please don't assume any change in the model.

提前谢谢！

推荐答案

您需要

You need to $unwind items array before grouping :

const aggregatorOpts = [{
        $unwind: "$items"
    },
    {
        $group: {
            _id: "$items.productId",
            count: { $sum: 1 }
        }
    }
]

Model.aggregate(aggregatorOpts).exec()

给出:

{ "_id" : "789", "count" : 1 }
{ "_id" : "456", "count" : 2 }
{ "_id" : "123", "count" : 3 }

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