Comparing an element's field in array with a field in MongoDB - arrays

Question

I have a collection Group like this:
{
"_id" : ObjectId("5822dd5cb6a69ca404e0d93c"),
"name" : "GROUP 1",
"member": [
{
"_id": ObjectId("5822dd5cb6a69ca404e0d93d")
"user": ObjectId("573ac820eb3ed3ea156905f6"),
"task": ObjectId("5822ddecb6a69ca404e0d942"),
},
{
"_id": ObjectId("5822dd5cb6a69ca404e0d93f")
"user": ObjectId("57762fce5ece6a5d04457bf9"),
"task": ObjectId("5822ddecb6a69ca404e0d943"),
}
],
curTask: {
"_id": ObjectId("5822ddecb6a69ca404e0d942"),
"time": ISODate("2016-01-01T01:01:01.000Z")
}
}
{
"_id" : ObjectId("573d5ff8d1b7b3b32e165599"),
"name" : "GROUP 2",
"member": [
{
"_id": ObjectId("574802e031e70b503eabe195")
"user": ObjectId("573ac820eb3ed3ea156905f6"),
"task": ObjectId("5775f1a74b41037e246a51d1"),
},
{
"_id": ObjectId("574802e031e70b503eabe198")
"user": ObjectId("573ac79beb3ed3ea156905f4"),
"task": ObjectId("576cfa042c0a4054794dd242"),
}
],
curTask: {
"_id": ObjectId("577249a2f9dba0c750ef705b"),
"time": ISODate("2016-01-01T01:01:01.000Z")
}
}
{
"_id" : ObjectId("574802e031e70b503eabe194"),
"name" : "GROUP 3",
"member": [
{
"_id": ObjectId("574be0a2bf16234f5a752f83")
"user": ObjectId("573ac79beb3ed3ea156905f4"),
"task": ObjectId("5822ddecb6a69ca404e0d942"),
},
{
"_id": ObjectId("574d397d6e9f07d64d1e4e40")
"user": ObjectId("57762fce5ece6a5d04457bf9"),
"task": ObjectId("5822ddecb6a69ca404e0d943"),
}
],
curTask: {
"_id": ObjectId("5822ddecb6a69ca404e0d942"),
"time": ISODate("2016-01-01T01:01:01.000Z")
}
}
And I want to be able to find all group where user with objectId 573ac820eb3ed3ea156905f6 (1st user in group 1) do not do the same task as currentTask. So far I've wrote this query:
db.getCollection('groups').find({"member":{ "$elemMatch": {"user": ObjectId("573ac820eb3ed3ea156905f6")
, "task": { "$ne":"this.curTask._id"}}}})
But this didn't seem to work as it still return the group where user 573ac820eb3ed3ea156905f6 having his task === curTask._id. The first half of elemMatch seem to work fine (only find group with user with objectid 573ac820eb3ed3ea156905f6 in member, the query only return group 1 and 2 since group 3 don't have that user.) but I cant seem to make mongodb compare a field in the object of the array with another field of the document. Anyone have any idea how do I make this comparison?

Answer 1

There are two solutions to the problem -
First - Using $where. By using $where you can use Javascript code inside mongodb queries. Makes the code flexible, but the shortcoming is that it runs slow since Javascript code has to run rather than more optimized mongoDB C++ code.
db.getCollection('groups').find({
$where: function () {
var flag = 0;
for(var i=0; i<obj.member.length;i++) {
if(obj.member[i].user.str == ObjectId("573ac820eb3ed3ea156905f6").str && obj.member[i].task.str != obj.curTask._id.str ){flag = 1; break;}
}
return flag;
}
})
Second - Using an aggregation pipeline. Here I am unwinding the array, doing matches as described, and finally recreating the array as it was needed. If the not matching elements in the member array are not needed, one can omit the last grouping part.
[
{$match: {'member.user': ObjectId("573ac820eb3ed3ea156905f6")}},
{$unwind: '$member'},
{$project: {
name: 1,
member: 1,
curTask: 1,
ne: {$and: [{$ne: ['$member.task', '$curTask._id']}, {$eq: ['$member.user', ObjectId("573ac820eb3ed3ea156905f6")]}]}
}},
{$group: {
_id: '$_id',
member: {$push: '$member'},
curTask: {$first: '$curTask'},
name: {$first: '$name'},
check: {$sum: {$cond: ['$ne', 1, 0]}}
}},
{$match: {check: {$gt: 0}}}
]