How to find match in documents in Mongo and Mongo aggregation? - mongodb

Question

I have following json structure in mongo collection-
{
"students":[
{
"name":"ABC",
"fee":1233
},
{
"name":"PQR",
"fee":345
}
],
"studentDept":[
{
"name":"ABC",
"dept":"A"
},
{
"name":"XYZ",
"dept":"X"
}
]
},
{
"students":[
{
"name":"XYZ",
"fee":133
},
{
"name":"LMN",
"fee":56
}
],
"studentDept":[
{
"name":"XYZ",
"dept":"X"
},
{
"name":"LMN",
"dept":"Y"
},
{
"name":"ABC",
"dept":"P"
}
]
}
Now I want to calculate following output.
if students.name = studentDept.name
so my result should be as below
{
"name":"ABC",
"fee":1233,
"dept":"A",
},
{
"name":"XYZ",
"fee":133,
"dept":"X"
}
{
"name":"LMN",
"fee":56,
"dept":"Y"
}
Do I need to use mongo aggregation or is it possible to get above given output without using aggregation???

Answer 1

What you are really asking here is how to make MongoDB return something that is actually quite different from the form in which you store it in your collection. The standard query operations do allow a "limitted" form of "projection", but even as the title on the page shared in that link suggests, this is really only about "limiting" the fields to display in results based on what is present in your document already.
So any form of "alteration" requires some form of aggregation, which with both the aggregate and mapReduce operations allow to "re-shape" the document results into a form that is different from the input. Perhaps also the main thing people miss with the aggregation framework in particular, is that it is not just all about "aggregating", and in fact the "re-shaping" concept is core to it's implementation.
So in order to get results how you want, you can take an approach like this, which should be suitable for most cases:
db.collection.aggregate([
{ "$unwind": "$students" },
{ "$unwind": "$studentDept" },
{ "$group": {
"_id": "$students.name",
"tfee": { "$first": "$students.fee" },
"tdept": {
"$min": {
"$cond": [
{ "$eq": [
"$students.name",
"$studentDept.name"
]},
"$studentDept.dept",
false
]
}
}
}},
{ "$match": { "tdept": { "$ne": false } } },
{ "$sort": { "_id": 1 } },
{ "$project": {
"_id": 0,
"name": "$_id",
"fee": "$tfee",
"dept": "$tdept"
}}
])
Or alternately just "filter out" the cases where the two "name" fields do not match and then just project the content with the fields you want, if crossing content between documents is not important to you:
db.collection.aggregate([
{ "$unwind": "$students" },
{ "$unwind": "$studentDept" },
{ "$project": {
"_id": 0,
"name": "$students.name",
"fee": "$students.fee",
"dept": "$studentDept.dept",
"same": { "$eq": [ "$students.name", "$studentDept.name" ] }
}},
{ "$match": { "same": true } },
{ "$project": {
"name": 1,
"fee": 1,
"dept": 1
}}
])
From MongoDB 2.6 and upwards you can even do the same thing "inline" to the document between the two arrays. You still want to reshape that array content in your final output though, but possible done a little faster:
db.collection.aggregate([
// Compares entries in each array within the document
{ "$project": {
"students": {
"$map": {
"input": "$students",
"as": "stu",
"in": {
"$setDifference": [
{ "$map": {
"input": "$studentDept",
"as": "dept",
"in": {
"$cond": [
{ "$eq": [ "$$stu.name", "$$dept.name" ] },
{
"name": "$$stu.name",
"fee": "$$stu.fee",
"dept": "$$dept.dept"
},
false
]
}
}},
[false]
]
}
}
}
}},
// Students is now an array of arrays. So unwind it twice
{ "$unwind": "$students" },
{ "$unwind": "$students" },
// Rename the fields and exclude
{ "$project": {
"_id": 0,
"name": "$students.name",
"fee": "$students.fee",
"dept": "$students.dept"
}},
])
So where you want to essentially "alter" the structure of the output then you need to use one of the aggregation tools to do. And you can, even if you are not really aggregating anything.