blocks|key|2722209|text|您可以将与+stage一起使用，如下所示：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2722210|db.collection.aggregate([
++++{+"$addFields":+{+"subdoc.a":+"$a"+}+},
++++{+"$replaceRoot":+{+"newRoot":+"$subdoc"+}++}
])|code-block|syntax|javascript|2722211|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You can use <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/replaceRoot/" rel="noreferrer"><code>$replaceRoot</code></a> with a <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/addFields/#pipe._S_addFields" rel="noreferrer"><code>$addFields</code></a> stage as follows:

<pre><code>db.collection.aggregate([
 { "$addFields": { "subdoc.a": "$a" } },
 { "$replaceRoot": { "newRoot": "$subdoc" } }
])
</code></pre>

blocks|key|41271|text|我们可以使用$replaceWith、$replaceRoot的别名和$mergeObjects操作符来实现这一点。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|41272|let+pipeline+=+[
+++{
++++++"$replaceWith":+{
+++++++++"$mergeObjects":+[+{+"a":+"$a"+},+"$subdoc"+]
++++++}
+++}
];|code-block|syntax|javascript|41273|或|41274|let+pipeline+=+[
++++{
++++++++"$replaceRoot":+{
++++++++++++"newRoot":+{
++++++++++++++++"$mergeObjects":+[{+"a":+"$a"+},+"$subdoc"+]
++++++++++++}
++++++++}
++++}
];

db.collection.aggregate(pipeline)|41275|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.mongodb.com/master/reference/operator/aggregation/replaceWith/|1|https://docs.mongodb.com/manual/reference/operator/aggregation/replaceRoot|2|https://docs.mongodb.com/manual/reference/operator/aggregation/mergeObjects/^0|6|C|J|C|Z|D|6|C|0|J|C|1|Z|D|2|0|0|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@$9|11|A|12|B|C]|$9|13|A|14|B|C]|$9|15|A|16|B|C]]|D|@$9|17|A|18|1|19]|$9|1A|A|1B|1|1C]|$9|1D|A|1E|1|1F]]|E|$]]|$1|F|3|G|5|H|7|1G|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|1H|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|1I|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|1J|8|@]|D|@]|E|$]]]|P|$Q|$5|R|S|T|E|$U|V]]|W|$5|R|S|T|E|$U|X]]|Y|$5|R|S|T|E|$U|Z]]]]

We can do this with <a href="https://docs.mongodb.com/master/reference/operator/aggregation/replaceWith/" rel="noreferrer"><code>$replaceWith</code></a> an alias for <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/replaceRoot" rel="noreferrer"><code>$replaceRoot</code></a> and the <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/mergeObjects/" rel="noreferrer"><code>$mergeObjects</code></a> operator.

<pre><code>let pipeline = [
 {
 "$replaceWith": {
 "$mergeObjects": [ { "a": "$a" }, "$subdoc" ]
 }
 }
];
</code></pre>

or

<pre><code>let pipeline = [
 {
 "$replaceRoot": {
 "newRoot": {
 "$mergeObjects": [{ "a": "$a" }, "$subdoc" ]
 }
 }
 }
];

db.collection.aggregate(pipeline)
</code></pre>

blocks|key|2722286|text|从Mongo+4.2开始，可以使用$replaceWith聚合操作符将一个文档替换为另一个文档(在我们的例子中是一个子文档)，作为@chridam提到的$replaceRoot的语法糖。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2722287|因此，我们可以首先在子文档中包含根字段，以继续使用$set运算符(也是在Mongo+4.2中引入的，作为$addFields的别名)，然后使用$replaceWith的子文档替换整个文档|2722288|//+{+a:+1,+subdoc:+{+b:+2,+c:+3+}+}
db.collection.aggregate([
++{+$set:+{+"subdoc.a":+"$a"+}+},
++{+$replaceWith:+"$subdoc"+}
])
//+{+b:+2,+c:+3,+a:+1+}|code-block|syntax|javascript|2722289|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.mongodb.com/master/reference/operator/aggregation/replaceWith/#pipe._S_replaceWith|1|https://docs.mongodb.com/master/reference/operator/aggregation/set/index.html^0|1|9|H|C|24|C|H|C|0|0|P|4|10|9|1G|A|1Z|C|P|4|1|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@$9|13|A|14|1|15]]|E|$]]|$1|F|3|G|5|6|7|16|8|@$9|17|A|18|B|C]|$9|19|A|1A|B|C]|$9|1B|A|1C|B|C]|$9|1D|A|1E|B|C]]|D|@$9|1F|A|1G|1|1H]]|E|$]]|$1|H|3|I|5|J|7|1I|8|@]|D|@]|E|$K|L]]|$1|M|3|-4|5|6|7|1J|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]|U|$5|P|Q|R|E|$S|V]]]]

Starting <code>Mongo 4.2</code>, the <a href="https://docs.mongodb.com/master/reference/operator/aggregation/replaceWith/#pipe._S_replaceWith" rel="nofollow noreferrer"><code>$replaceWith</code></a> aggregation operator can be used to replace a document by another (in our case by a sub-document) as syntaxic sugar for the <code>$replaceRoot</code> mentioned by @chridam.

We can thus first include within the sub-document the root field to keep using the <a href="https://docs.mongodb.com/master/reference/operator/aggregation/set/index.html" rel="nofollow noreferrer"><code>$set</code></a> operator (also introduced in <code>Mongo 4.2</code> as an alias for <code>$addFields</code>) and then replace the whole document by the sub-document using <code>$replaceWith</code>:

<pre><code>// { a: 1, subdoc: { b: 2, c: 3 } }
db.collection.aggregate([
 { $set: { "subdoc.a": "$a" } },
 { $replaceWith: "$subdoc" }
])
// { b: 2, c: 3, a: 1 }
</code></pre>

blocks|key|2512079|text|您还可以在$mergeObjects中使用$$ROOT|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2512080|{+$replaceWith:+{
++++++++$mergeObjects:+[+"$$ROOT",+"$subdoc"+]
}+}|code-block|syntax|javascript|2512081|entityMap^0|5|D|L|6|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|L|$]]

You can also use <code>$$ROOT</code> in <code>$mergeObjects</code>
<pre><code>{ $replaceWith: {
 $mergeObjects: [ &quot;$$ROOT&quot;, &quot;$subdoc&quot; ]
} }
</code></pre>

blocks|key|2722301|text|如果您想查询嵌套数组元素并将其显示在根级别，如果您有两个嵌套数组，则应该使用$unwind两次|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2722302|db.mortgageRequests.aggregate(+[
+++++++{+$unwind:+"$arrayLevel1s"+},
+++++++{+$unwind:+"$arrayLevel1s.arrayLevel2s"+},
+++++++{+$match:+{+"arrayLevel1s.arrayLevel2s.documentid"+:+{+$eq:+"6034bceead4ed2ce3951f38e"+}+}+},
+++++++{+$replaceRoot:+{+newRoot:+"$arrayLevel1s.arrayLevel2s"+}+}
++++]+)|code-block|syntax|javascript|2722303|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

If you want to query nested array element and display it at root level
You should use $unwind twice if you have 2 nested array
<pre><code>db.mortgageRequests.aggregate( [
 { $unwind: &quot;$arrayLevel1s&quot; },
 { $unwind: &quot;$arrayLevel1s.arrayLevel2s&quot; },
 { $match: { &quot;arrayLevel1s.arrayLevel2s.documentid&quot; : { $eq: &quot;6034bceead4ed2ce3951f38e&quot; } } },
 { $replaceRoot: { newRoot: &quot;$arrayLevel1s.arrayLevel2s&quot; } }
 ] )
</code></pre>

blocks|key|40326|text|我想最简单的方法是在结果返回后使用map修改结果。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|40327|collection.mapFunction+=+function(el)+{
++el.b+=+el.subdoc.b;
++el.c+=+el.subdoc.c
++delete(el.subdoc);
++return+el;
}

...

var+result+=+await+collection.aggregate(...).toArray();
result+=+result.map(collection.mapFunction);|code-block|syntax|javascript|40328|entityMap^0|H|3|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

I guess the easiest way is altering the result after it returns, using <code>map</code>.

<pre><code>collection.mapFunction = function(el) {
 el.b = el.subdoc.b;
 el.c = el.subdoc.c
 delete(el.subdoc);
 return el;
}

...

var result = await collection.aggregate(...).toArray();
result = result.map(collection.mapFunction);
</code></pre>

blocks|key|2722192|text|您可以使用和来完成此操作|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2722193|db.collection.update({},{+$set:+{+"b":+2,+"c":+3+},+$unset:+{+"subdoc":+1+}+})|code-block|syntax|javascript|2722194|当然，如果集合中有很多文档，那么就像上面一样，您需要循环集合文档才能获得值。MongoDB无法单独在更新操作中引用字段的值：|2722195|db.collection.find({+"subdoc":+{"$exists":+1+}+}).forEach(function(doc)+{
++++var+setDoc+=+{};
++++for+(+var+k+in+doc.subdoc+)+{
++++++++setDoc[k]+=+doc.subdoc[k];
++++}
++++db.collection.update(
++++++++{+"_id":+doc._id+},
++++++++{+"$set":+setDoc,+"$unset":+"subdoc"+}
++++);
})|2722196|这也使用了一些的安全用法，以确保您选择的是"subdoc“键实际在那里的文档。|2722197|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

You can do this with the use of <a href="http://docs.mongodb.org/manual/reference/operator/update/set/" rel="nofollow"><code>$set</code></a> and <a href="http://docs.mongodb.org/manual/reference/operator/update/unset/" rel="nofollow"><code>$unset</code></a>



<pre class="lang-js prettyprint-override"><code>db.collection.update({},{ $set: { "b": 2, "c": 3 }, $unset: { "subdoc": 1 } })
</code></pre>

Of course if there are lots of documents in the collection, then much as above you need to loop the collection documents in order to get the values. MongoDB has no way of referring to a value of a field in an update operation alone:

<pre class="lang-js prettyprint-override"><code>db.collection.find({ "subdoc": {"$exists": 1 } }).forEach(function(doc) {
 var setDoc = {};
 for ( var k in doc.subdoc ) {
 setDoc[k] = doc.subdoc[k];
 }
 db.collection.update(
 { "_id": doc._id },
 { "$set": setDoc, "$unset": "subdoc" }
 );
})
</code></pre>

That also employs some safe usage of <a href="http://docs.mongodb.org/manual/reference/operator/query/exists/" rel="nofollow"><code>$exists</code></a> in order to make sure you are selecting documents where the "subdoc" key is actually there.

For example, if I have a document like this
<pre><code>{
 a: 1,
 subdoc: {
 b: 2,
 c: 3
 }
}
</code></pre>
How can I convert it into a format like this? (without using <code>project</code>)
<pre><code>{
 a: 1,
 b: 2,
 c: 3
}
</code></pre>

How to flatten a subdocument into root level in MongoDB?

例如，如果我有一个像这样的文档{  a: 1,  subdoc: {    b: 2,    c: 3  }}如何将其转换为这样的格式？(不使用 project**)**{  a: 1,  b: 2,  c: 3}

问如何在MongoDB中将子文档展平到根级别？
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问如何在MongoDB中将子文档展平到根级别？EN

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问如何在MongoDB中将子文档展平到根级别？
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