I have tried several codes but it didn't work. Example from the database, one has a prerequisite and one does not have prerequisites and I would like to find the total number of the subject with no prerequisites :
db.Subject.insert(
{
"_id":ObjectId(),
"subject":{
"subCode":"CSCI321",
"subTitle":"Final Year Project",
"credit":6,
"type":"Core",
"assessments": [
{ "assessNum": 1,
"weight":30,
"assessType":"Presentation",
"description":"Prototype demonstration" },
{ "assignNum": 2,
"weight":70,
"assessType":"Implementation and Presentation",
"description":"Final product Presentation and assessment of product implementation by panel of project supervisors" }
]
}
}
)
db.Subject.insert(
{
"_id":ObjectId(),
"subject":{
"subCode":"CSCI203",
"subTitle":"Algorithm and Data Structures",
"credit":3,
"type":"Core",
"prerequisite": ["csci103"]
}})
one of the few codes that I tried using :
db.Subject.aggregate({$group:{"prerequisite":{"$exists": null}, count:{$sum:1}}});
Results :
_getErrorWithCode@src/mongo/shell/utils.js:25:13
doassert@src/mongo/shell/assert.js:18:14
_assertCommandWorked@src/mongo/shell/assert.js:534:17
assert.commandWorked@src/mongo/shell/assert.js:618:16
DB.prototype._runAggregate@src/mongo/shell/db.js:260:9
DBCollection.prototype.aggregate@src/mongo/shell/collection.js:1062:12
@(shell):1:1
You can use $match to eliminate unwanted documents and $group to calculate sum
db.collection.aggregate([
{
$match: {
"subject.prerequisite": {
"$exists": false
}
}
},
{
$group: {
_id: null,
total: {
$sum: 1
}
}
}
])
Working Mongo playground
この記事はインターネットから収集されたものであり、転載の際にはソースを示してください。
侵害の場合は、連絡してください[email protected]
コメントを追加