如何在Mongo的聚合管道的$ unwind阶段保留零长度值？ [英] How can I retain zero-length values during the $unwind phase of the aggregate pipeline in Mongo?

问题描述

我正在使用聚合管道编写Mongo查询。在聚合期间，我需要 $ unwind 其中一个字段。但是，我不希望 $ unwind 排除具有该字段的零长度数组的条目，因为我仍然需要它们进一步沿着管道。

I'm writing a Mongo query using the aggregate pipeline. During the aggregation, I need to $unwind one of the fields. However, I don't want the $unwind to exclude entries that have zero-length arrays for that field, because I still need them further down the pipeline.

我的字段名为 items ，它是一个对象数组，每个对象包含两个值： quantity 和价格。到目前为止，这是我的Mongo查询的片段：

My field is called items and it's an array of objects, which each contain two values: quantity and price. Here's a snippet of my Mongo query so far:

db.MyCollection.aggregate([
    {$match: ... },
    {$project: ... },
    // put another $project here to retain zero-length values? how to format it?
    {$project: {
            _id: "$$ROOT",
            items: {
                $ifNull: [
                    {
                        $literal: {
                            quantity: 0,
                            price: 0
                        }
                    }
                ]
            }
        }
    },
    {$unwind: "$items"},
    {$group: ... },
    {$project: ... },
    ...
]);

如您所见，这已经处理了文档没有<$ c的情况$ c> items 字段完全存在，在这种情况下，它会将其添加并为其提供一个空值来展开。

As you can see, this already handles the case where a document doesn't have the items field present at all, in which case it adds it in and gives it an empty value to unwind on.

什么是但是，处理的是已经存在 items 字段的文档，但它是空白的。我以为我可以使用 $ cond 和 $ size 的某种组合来明确检查另一个的大小为零 $ project ，然后在这种情况下替换相同的文字，但 $ if 需要布尔值，所以我不是非常确定如何格式化。

What isn't handled, however, are documents that do already have that items field present, but it's blank. I was thinking I could use some combination of $cond and $size to check explicitly for a size of zero within another $project, and then substitute the same literal in that case, but $if expects a boolean so I'm not quite sure how to format that.

推荐答案

修改你的项目阶段管道如下：

Modify your project stage in the pipeline as below:

投影阶段做了以下两件事：

The projection stage does the following two things:

• 如果items数组 null 或不存在 ，计算
  项目字段值为 [] （一个空数组。）


• 接下来检查<$ c刚刚计算的项目字段的$ c> size 是 0 ，如果
  它是 0 ，然后将其值更改为具有默认对象的数组。

• If the items array is null or does not exist, calculates the items field value as [](an empty array.)
• Next checks if the size of the just calculated items field is 0, if it is 0, then changes its value to an array with default object.

阶段代码：

db.MyCollection.aggregate([
...
{$project:{"_id":"$$ROOT",
           "items":{$cond:[
                          {$eq:[{$size:{$ifNull:["$items",[]]}},0]},
                          [{"quantity":0,"price":0}],
                          "$items"
                          ]}}},
{$unwind:"$items"},
...
])

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