I want to find those users who has not invited any user by a single query (using aggregate).
for Example : I have 4 users in DB
{
"_id" : ObjectId("581a18d41b6c5c752f11c87a"),
"name": "aaa",
"invitedBy": ObjectId("5808f53d28c14ee470856d8b")
},
{
"_id" : ObjectId("581a1a671b6c5c752f11c87b"),
"name": "bbb",
"invitedBy": ObjectId("581a18d41b6c5c752f11c87a")
},
{
"_id" : ObjectId("581a1a671b6c5c752f11c87c"),
"name": "ccc",
"invitedBy": ObjectId("581a18d41b6c5c752f11c87a"),
},
{
"_id" : ObjectId("581a1a671b6c5c752f11c87d"),
"name": "ddd",
"invitedBy": ObjectId("581a1a671b6c5c752f11c87b"),
}
Here
1- aaa invited bbb and ccc user
2- bbb invited ddd user
3- but ccc and ddd user not invited any one
so I want to pic ccc and ddd users
it would be better if can by using aggregate because I need perform some operation on selected data.
CodePudding user response:
You can use the $lookup
operator to perform a left join to the collection itself.
The users who have not sent any invitation are those with empty invitation array. To retrieve only those users, simply filter the documents in a $match
stage using the $exists
operator and numeric array indexing.
db.users.aggregate(
[
{ "$lookup": {
"from": "users",
"localField": "_id",
"foreignField": "invitedBy",
"as": "invitation"
}},
{ "$match": { "invitation.0": { "$exists": false } } }
]
)
which yields:
{
"_id" : ObjectId("581a1a671b6c5c752f11c87c"),
"name" : "ccc",
"invitedBy" : ObjectId("581a18d41b6c5c752f11c87a"),
"invitation" : [ ]
}
{
"_id" : ObjectId("581a1a671b6c5c752f11c87d"),
"name" : "ddd",
"invitedBy" : ObjectId("581a1a671b6c5c752f11c87b"),
"invitation" : [ ]
}
CodePudding user response:
There is another method to do the same. But this method might end up being less efficient than @Styvane's answer. This should work if there are lesser elements.
[
{$group: {_id: null, senders: {$addToSet: '$invitedBy'}, everyone: {$addToSet: '$_id'}}},
{$project: {_id: 0, res: {$setDifference: ['$everyone', '$senders']}}}
]
In case other information is needed, that can also be obtained (sorry but it becomes bit dirty here)
[
{$group: {_id: null, senders: {$addToSet: '$invitedBy'}, everyone: {$addToSet: {id: '$_id', name: '$name'}}}},
{$project: {_id: 0, everyone: 1, res: {$setDifference: ['$everyone.id', '$senders']}}},
{$unwind: '$everyone'},
{$unwind: '$res'},
{$project: {flag: {$eq: ['$everyone.id', '$res']}, everyone: 1}},
{$match: {flag: true}}
]