MongoDB Aggregation - Merge arrays of an object

Question

I would like to get the union of all the arrays in an object in MongoDB 3.4 using the aggregation framework:

This is the input:

{ 
  _id: "001",
  name: "something",
  important_part: {
    foo: [1,2,3],
    bar: [4,5],
    x: [6,7]
  }
}

This should be the output:

{ 
  _id: "001",
  name: "something",
  merged_arrays: [1,2,3,4,5,6,7]
}

The tricky part is, that the fields in the important_part object is dynamic, and I don't think the $setUnion operator can be used, as it needs the exact list of array fields.

Could someone please help me?

Thx in advance

What is your version ?

s7vr
– s7vr

2018-03-09 16:19:20 +00:00
Commented Mar 9, 2018 at 16:19 — s7vr
– s7vr, Commented Mar 9, 2018 at 16:19
docs.mongodb.com/manual/reference/operator/aggregation/…

chridam
– chridam

2018-03-09 16:21:30 +00:00
Commented Mar 9, 2018 at 16:21 — chridam
– chridam, Commented Mar 9, 2018 at 16:21

s7vr · Accepted Answer · 2018-03-09 16:30:44Z

3

You can use below aggregation in 3.4.

$objectToArray to transform object into array of key value pairs and $reduce to $concatArrays.

db.col.aggregate({
  "$addFields":{
    "merged_arrays":{
      "$reduce":{
        "input":{"$objectToArray":"$important_part"},
        "initialValue":[],
        "in":{"$concatArrays":["$$value", "$$this.v"]}
      }
    }
  }
})

edited Mar 9, 2018 at 16:30

answered Mar 9, 2018 at 16:25

s7vr

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