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

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
})