Mongo aggregation $subtract between dynamic document value - mongodb

Question

Need to find the difference between two values of attendance,group by ward_id, based on patient id for two dates. The result has dynamic values based on the array. The difference is between two dates. Key would be ward_id, the difference will be between counts of patient's visit to the ward.
Example sample data
{
"_id" : {
"type" : "patient_attendence",
"ts" : ISODate("2015-02-03T21:31:29.902Z"),
"ward_id" : 2561
},
"count" : 4112,
"values" : [
{
"count" : 9,
"patient" : ObjectId("54766f973f35473ffc644618")
},
{
"count" : 19,
"patient" : ObjectId("546680e2d660e2dc5ebfea39")
},
{
"count" : 47,
"patient" : ObjectId("546680e3d660e2dc5ebfea72")
},
{
"count" : 1,
"patient" : ObjectId("546a137bdab5f21e612ea7ef")
},
{
"count" : 93,
"patient" : ObjectId("546680e3d660e2dc5ebfea89")
}
]
}
{
"_id" : {
"type" : "patient_attendence",
"ts" : ISODate("2015-02-03T21:31:29.902Z"),
"ward_id" : 3720
},
"count" : 1,
"values" : [
{
"count" : 1,
"patient" : ObjectId("546a136ddab5f21e612ea6a6")
}
]
}
{
"_id" : {
"type" : "patient_attendence",
"ts" : ISODate("2015-02-04T21:31:29.902Z"),
"ward_id" : 2561
},
"count" : 4112,
"values" : [
{
"count" : 10,
"patient" : ObjectId("54766f973f35473ffc644618")
},
{
"count" : 10,
"patient" : ObjectId("546680e2d660e2dc5ebfea39")
},
{
"count" : 6,
"patient" : ObjectId("5474e9e46606f32570fa48ff")
},
{
"count" : 1,
"patient" : ObjectId("5474e9e36606f32570fa48f2")
},
{
"count" : 1,
"patient" : ObjectId("546680e3d660e2dc5ebfea77")
},
{
"count" : 543,
"patient" : ObjectId("546680e2d660e2dc5ebfea43")
},
{
"count" : 1,
"patient" : ObjectId("5485fdc8d27a9122956b1c66")
}
]
}
{
"_id" : {
"type" : "patient_attendence",
"ts" : ISODate("2015-02-04T21:31:29.902Z"),
"ward_id" : 3720
},
"count" : 1,
"values" : [
{
"count" : 7,
"patient" : ObjectId("546a136ddab5f21e612ea6a6")
}
]
}
Output
{
"ward_id":2561,
"result" : [{"person": ObjectId("54766f973f35473ffc644618"),
"count_1": 9,
"count_1": 10,
"difference":1 },{"person": ObjectId("546680e2d660e2dc5ebfea39"),
"count_1": 19,
"count_1": 10,
"difference":-9 } ....]
},
{
"ward_id":3720,
"result" : [{"person": ObjectId("546a136ddab5f21e612ea6a6"),
"count_1": 9,
"count_1": 10,
"difference":1 },{"person": ObjectId("546680e2d660e2dc5ebfea39"),
"count_1": 1,
"count_1": 7,
"difference":-6 }]
}

Answer 1

you can use the aggregation framework's $subtract operator outlined here: http://docs.mongodb.org/manual/reference/operator/aggregation-arithmetic/
db.wards.aggregate([
{
$match: {id: {$elemMatch: {ward_id: my_ward_id, ts: my_desired_ts}}},
},
{
$limit: 2
},
{
$project: {values: 1}
},
{
$unwind: '$values'
},
{
$match: {patient: my_patient_id}
},
{
$group: {
_id: null,
'count1': {$first: '$values.count'},
'count2': {$last: '$values.count'}
}
},
{
$subtract: ['$count1', '$count2']
}
])
i haven't tested this but it would probably look like something above