如何解决如何在mongo查询中获取子数组的长度
以下是示例数据:
db.infos.find()
{
"groupId": "1111","customerId": "A100","tracks": [{
"trackId": "234","infos": [{
"location": {
"address": "street1","city": "test","country": "US"
}
},{
"location": {
"address": "street2",{
"location": {
"address": "street3","country": "US"
}
}
]
}]
}
{
"groupId": "2222","tracks": [{
"trackId": "345","infos": [{
"location": {
"address": "street4",{
"location": {
"address": "street5","tracks": [{
"trackId": "666","country": "US"
}
}
]
}]
}
我们需要一个查询来获取 groupId 级别的“infos”子数组的长度。在上面的示例数据中,我们需要以下输出: 1111,3 2222,6
在下面尝试过,但不起作用:
db.infos.aggregate([{"$project": {"groupId": "$groupId","samples": "$tracks.infos"}},{"$group": {"_id": "$groupId","samples": {"$push": "$samples"}}},{"$project": {"_id": 1,"samples": {"$size": "$samples"}}}],{ "allowdiskUse": true });
还有大量数据,上面的查询抛出一个 ExceededMemoryLimit 异常:
2021-07-10T07:20:14.415+0000 E QUERY [js] Error: command Failed: {
"ok" : 0,"errmsg" : "$push used too much memory and cannot spill to disk. Memory limit: 104857600 bytes","code" : 146,"codeName" : "ExceededMemoryLimit"
} : aggregate Failed :
_getErrorWithCode@src/mongo/shell/utils.js:25:13
doassert@src/mongo/shell/assert.js:18:14
_assertCommandWorked@src/mongo/shell/assert.js:580:17
assert.commandWorked@src/mongo/shell/assert.js:673:16
DB.prototype._runAggregate@src/mongo/shell/db.js:260:9
DBCollection.prototype.aggregate@src/mongo/shell/collection.js:1074:12
@(shell):1:1
解决方法
这应该适用于放松:
db.collection.aggregate([
{
$unwind: "$tracks"
},{
$group: {
"_id": "$groupId","count": {
"$sum": {
"$size": "$tracks.infos"
}
}
}
}
])
即使您的轨道阵列有多个对象,这也会将它们相加。 Playground
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