对具有ID数组的模型执行单个查询计数？

Question

我有这段代码，一本书可以有很多注释，我想将它们的数量放在数组中

const bookCommentsLoader = new DataLoader(async bookIds => {
  const comments = await Comment.count({
    bookId: { $in: bookIds }
  })
  ...
})

graphql数据加载器的问题在于，返回时，id的结果和长度必须匹配，所以我希望它返回以下内容：

model id-> ['1','3','12'] must return model counts like [5,7,1]

我不想使用循环提示，它将发送多个查询，我想使用单个查询，帮助吗？

Answer 1

https://docs.mongodb.com/manual/reference/operator/aggregation/sum/#grp._S_sum

您将使用汇总

var aggregatePipeline = [
    {$group: {_id: "$bookId", numOfComments: {$sum: 1}}
];


Comments.aggregate(aggregatePipeline, (error, results) => {
    // results comes out to be [{_id: bookId, numOfComments: Number}, ....]
    // From here, you can map the array to the format you wish
    var formattedArray = results.map(o => return(o.numOfComments))

    console.log(formattedArray);
});