无法识别的表达式'$ first' [英] Unrecognized expression '$first'

问题描述

我需要从数组中提取第一项并将其添加到其自己的对象中，所以我发现$ first确实做到了 https://docs.mongodb.com/manual/reference/operator/aggregation/first-array-element/#example
但是，我得到了错误无法识别的表达式'$ first'，并认为这是我的查询无法正常工作，因此我克隆了他们在文档中的确切示例，但它给了我相同的答案错误.

I need to extract the first item from an array and add it to its own object, so I found $first the does exactly that https://docs.mongodb.com/manual/reference/operator/aggregation/first-array-element/#example
However, I was getting the error Unrecognized expression '$first' and thought it was my query that wasn't working, so I cloned the exact example they have in their docs, but it gives me the same error.

db.runninglog.insertMany([
 { "_id" : 1, "team" : "Anteater", log: [ { run: 1, distance: 8 }, { run2: 2, distance: 7.5 }, { run: 3, distance: 9.2 } ] },
 { "_id" : 2, "team" : "Bears", log: [ { run: 1, distance: 18 }, { run2: 2, distance: 17 }, { run: 3, distance: 16 } ] },
 { "_id" : 3, "team" : "Cobras", log: [ { run: 1, distance: 2 } ] }
])

db.runninglog.aggregate([
 { $addFields: { firstrun: { $first: "$log" }, lastrun: { $last: "$log" } } }
])

预期:文档期望什么.
收到:无法识别的表达式'$ first'

Expected: What the doc is expecting.
Received: Unrecognized expression '$first'

修改

我发现了使用 {$ addFields:{firstrun:{$ arrayElemAt:[" $ log'，0]}}} 的另一种方法，但我仍然很好奇关于以上

I found out another way to do the same thing with { $addFields: { firstrun: { $arrayElemAt: [ "$log", 0 ] } } } but still I'm curious about the above

推荐答案

来自文档:版本4.4中的新内容

From the Documentation : new in version 4.4

请注意，您尝试使用的是$ first数组元素运算符，它不同于 $ first聚合累加器在$ group阶段可用.

Be careful, the one you try to use is the $first array element operator, different from the $first aggregation accumulator available in $group stage.

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