select 2 fields and return a sorted array with their distinct values

Question

Say we have the following docs:

{a: 1, b: 2},
{a: 2, b: 0},
{a: 3, b: 1}

I want a query that will return:

[0, 1, 2, 3]
         


        

Answer 1

You need to $group our documents and use the $push accumulator operator to return an array of "a" and "b" within the collection.

In the $project operator you use the $setUnion operator to filter out the duplicates.

db.coll.aggregate(
    [
        { "$group": { 
            "_id": null, 
            "a": { "$push": "$a" }, 
            "b": { "$push": "$b" } 
        }}, 
        { "$project": {
            "_id": 0, 
            "merged": { "$setUnion": [ "$a", "$b" ] } 
        }} 
    ]
)

which produces:

{ "merged" : [ 3, 2, 0, 1 ] }