SQL 'UNION ALL' like implementation in MongoDB

Question

There are two collections:

Sales

{
  \"_id\" : ObjectId(\"5ba0bfb8d1acdc0de716e839\"),
  \"invoiceNumber\" : 1,
  \"saleDate\" : ISODate(\"2018-09-01T00:         


        

Answer 1

You can try below aggregation

db.sales.aggregate([
  { "$limit": 1 },
  { "$facet": {
    "collection1": [
      { "$limit": 1 },
      { "$lookup": {
        "from": "sales",
        "pipeline": [
          { "$match": {
            "date": { "$gte": ISODate("2018-09-01"), "$lte": ISODate("2018-09-10") },
            "customer.name": customerName
          }},
          { "$project": {
            "_id":0, "dated": "$saleDate", "heading": "Sale", "particulars": "$invoiceNumber",
            "amount": "$totalAmount", "modeOfPayment": null
          }}
        ],
        "as": "collection1"
      }}
    ],
    "collection2": [
      { "$limit": 1 },
      { "$lookup": {
        "from": "transactions",
        "pipeline": [
          { "$match": {
            "transactionDate": { "$gte": ISODate("2018-09-01"), "$lte": ISODate("2018-09-10") },
            "userId": userId, "partyName": customerName
          }},
          { "$project": {
            "_id":0, "dated": "$transactionDate", "heading": "Payment","particulars": "$transactionNumber",
            "amount": "$amount", "paymentMode": "$transactionMode"
          }}
        ],
        "as": "collection2"
      }}
    ]
  }},
  { "$project": {
    "data": {
      "$concatArrays": [
        { "$arrayElemAt": ["$collection1.collection1", 0] },
        { "$arrayElemAt": ["$collection2.collection2", 0] },
      ]
    }
  }},
  { "$unwind": "$data" },
  { "$replaceRoot": { "newRoot": "$data" } },
  { "$sort": { "dated": -1 }}
])

Answer 2

Disclaimer: The technique presented below is not exactly advisable. ;) This is true in particular when dealing with big collections. However, it can be used to achieve the identical effect as a SQL UNION ALL from MongoDB v3.6 onwards.

Given a collection first and a collection second:

db.first.aggregate([{
    $group: { // create an array to hold all documents of the first collection
        "_id": null,
        "first": {
            $push: "$$ROOT"
        }
    }
}, {
    $lookup: { // perform some kind of ridiculous lookup which will return all documents from the second collection in an array
        from: "second",
        let: { /* we do not need any variables */ },
        pipeline: [ { $match: { /* this filter will match every document */ } } ],
        as: "second"
    }
}, {
    $project: {
        "all": { $concatArrays: [ "$first", "$second" ] } // merge the two collections
    }
}, {
    $unwind: "$all" // flatten the resulting array
}, {
    $replaceRoot: { "newRoot": "$all" } // move content of "all" field all the way up 
}], {
    allowDiskUse: true // make sure we do not run into memory issues
})

Answer 3

I dont think so that you can perform a union on 2 different collections in mongodb.

However you can get data from both collections using q.all and afterwards make a union from them using your own function or may be third party module like lodash.

The only way you can perform union in mongodb is as mentioned here. https://docs.mongodb.com/manual/reference/operator/aggregation/setUnion/