如何投影字段是否存在 [英] How to project whether field exists

问题描述

如果我有类似以下结构的文档.我正在用计算结果更新它们，我想知道结果是否已经插入到文档中.假设我对每个文档运行计算c"和计算d".现在我想显示一个包含所有文档的表格，并显示是否已经执行了计算d".对于这张表，我不关心计算c".

If I have documents with similar structure as below. I am updating them with the results of the computations and I want to know whether the result has already been inserted into a document or not. Let's say for each document I run computation 'c' and computation 'd'. Now I want to display a table of all documents and show whether a computation 'd' has been already carried out. And for this table I do not care about computation 'c'.

{
"_id":1
"a":1,
"resultsOfComputation":{
   "c":{large embedded document},
   "d":{large embedded document}   
   }
}

{
"_id":2
"a":1,
"resultsOfComputation":{
   "c":{large embedded document}
   }
}

我想得到一个结果，告诉我文档是否包含特定字段.例如，我想知道它是否包含字段resultsOfComputation.d"，无论该字段的值是什么.

I would like to get a result that tells me whether a document contains a specific field. For example, I would like to know whether it contains field "resultsOfComputation.d", no matter what is the value of that field.

resultsOfComputation.d"的查询结果示例如下:

An example of the result of the query for "resultsOfComputation.d" would be:

{
"_id":1
"a":1,
"resultsOfComputation":{
   "d":true   
   }
}

{
"_id":2
"resultsOfComputation":{
   "d":false
   }
}

如果resultsOfComputation.d"不在文档中，它也可以是未定义的，这也可以:

If "resultsOfComputation.d" is not in the document it can also be undefined, which is also ok:

{
"_id":1
"a":1,
"resultsOfComputation":{
   "d":true   
   }
}

{
"_id":2
"a":1,
"resultsOfComputation":{}
}

一般来说，这个想法是获取文档的所有根元素，但对于选定的(一个)计算结果只获取 true/false/undefined，因为计算结果是一个大型嵌入文档.

In general, the idea is to get all the root elements of the documents, but only true/false/undefined for the selected (one) result of computation, since the result of computation is a large embedded document.

推荐答案

运行以下聚合管道以获得所需的结果:

Run the following aggregation pipeline to get the desired results:

db.collection.aggregate([
    {
        "$project": {
            "a": 1,
            "resultsOfComputation": {
                "d": { "$gt": ["$resultsOfComputation.d", null] }   
            }
        }
    }
])

样本输出

/* 1 */
{
    "_id" : 1,
    "a" : 1,
    "resultsOfComputation" : {
        "d" : true
    }
}

/* 2 */
{
    "_id" : 2,
    "a" : 1,
    "resultsOfComputation" : {
        "d" : false
    }
}

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