How to flatten a object containing list to an array in mongodb-CodePudding

I'm using mongodb aggregation and added a facet stage. Here is the output of this stage.

{
    "a": [{x: 1}, {x: 2}],
    "b": [{x: 3}, {x: 4}]
}

Now I want to get rid of the a and b keys and create a list of objects from it. So final result should be like the following:

[
    {x: 1}, 
    {x: 2},
    {x: 3}, 
    {x: 4}
]

What should I use in to aggregation pipeline for this?

CodePudding user response：

One option is:

db.collection.aggregate([
  {$project: {_id: 0, data: {$concatArrays: ["$a", "$b"]}}}
])

See how it works on the playground example

If you want an array and not an object, add to it:

 {$unwind: "$data"},
 {$replaceRoot: {newRoot: "$data}}