Fetch data from 2 collections in mongodb in single query - mongodb

Question

I wanted to fetch data from 2 independent collections and sort the results based on date through a single query. Is that even possible in mongodb? I have collections:
OrderType1
{
"id": "1",
"name": "Hello1",
"date": "2016-09-23T15:07:38.000Z"
},
{
"id": "2",
"name": "Hello1",
"date": "2015-09-23T15:07:38.000Z"
}
OrderType2
{
"id": "3",
"name": "Hello3",
"date": "2012-09-23T15:07:38.000Z"
},
{
"id": "4",
"name": "Hello4",
"date": "2018-09-23T15:07:38.000Z"
}
Expected Result
[
{
"id": "3",
"name": "Hello3",
"date": "2012-09-23T15:07:38.000Z"
},
{
"id": "2",
"name": "Hello1",
"date": "2015-09-23T15:07:38.000Z"
},
{
"id": "1",
"name": "Hello1",
"date": "2016-09-23T15:07:38.000Z"
},
{
"id": "4",
"name": "Hello4",
"date": "2018-09-23T15:07:38.000Z"
}
]
Now, I want to fetch both types of orders in a single query sorted by date.

Answer 1

You can try below aggregation with mongodb 3.6 and above but I think you should use two queries because for the large data set $lookup pipeline will breach BSON limit of 16mb. But also It depends upon your $match condition or $limit. If they are applied to the $lookup pipeline then your aggregation would work perfectly.
db.OrderType1.aggregate([
{ "$limit": 1 },
{ "$facet": {
"collection1": [
{ "$limit": 1 },
{ "$lookup": {
"from": "OrderType1",
"pipeline": [{ "$match": { } }],
"as": "collection1"
}}
],
"collection2": [
{ "$limit": 1 },
{ "$lookup": {
"from": "OrderType2",
"pipeline": [{ "$match": { } }],
"as": "collection2"
}}
]
}},
{ "$project": {
"data": {
"$concatArrays": [
{ "$arrayElemAt": ["$collection1.collection1", 0] },
{ "$arrayElemAt": ["$collection2.collection2", 0] },
]
}
}},
{ "$unwind": "$data" },
{ "$replaceRoot": { "newRoot": "$data" } }
])