MongoDB - transform array field to set

Question

I have documents with a field containing an array of values which can be duplicated. I want to transform these documents with an extra field corresponding to unique values of this array. I tried aggregate + addToSet without success.

Data:

{..., "random_integers" : [1, 1, 2, 2, 3, 3]},
{..., "random_integers" : [2, 3, 4, 4, 5, 6]},
{..., "random_integers" : [9, 9, 8, 8, 7, 7]}

Expecting:

{
    ...
    "random_integers" : [1, 1, 2, 2, 3, 3],
    "unique_integers" : [1, 2, 3],
},
{
    ...
    "random_integers" : [2, 3, 4, 4, 5, 6],
    "unique_integers" : [2, 3, 4, 5, 6],

},
{
     ...
    "random_integers" : [9, 9, 8, 8, 7, 7],
    "unique_integers" : [7, 8, 9],
}

Try with aggregate + addToSet():

# Query
db.getCollection().aggregate([
    {
        $group: {
            _id: '$_id',
            unique_integers: {$addToSet: '$random_integers' }
        }
    }
])

# Results
{..., "unique_integers" : [[1, 1, 2, 2, 3, 3]]},
{..., "unique_integers" : [[2, 3, 4, 4, 5, 6]]},
{..., "unique_integers" : [[9, 9, 8, 8, 7, 7]]}

$addToSet add the whole list into a set, instead of each element of the array. I tried to combine $addToSet with $each but it is not recognize by mongo on a group:

# Query
db.getCollection().aggregate([
    {
        $group: {
            _id: '$_id',
            unique_integers: {$addToSet: { $each: '$random_integers' }}
        }
    }
])

# Error
Error: command failed: {
    "ok" : 0,
    "errmsg" : "Unrecognized expression '$each'",
    "code" : 168,
    "codeName" : "InvalidPipelineOperator"
} : aggregate failed

You can just do the $setIntersection on the same array field to get the array with unique values (and no need for the $group). — prasad_
– prasad_, Commented Nov 19, 2019 at 3:43

prasad_ · Accepted Answer · 2019-11-19 09:02:31Z

7

db.ints.aggregate( [
  { $project: { 
        random_integers: 1,
        unique_integers: { $setIntersection: [ "$random_integers", "$random_integers" ] },  
       _id: 0 
  } }
] )

answered Nov 19, 2019 at 9:02

prasad_

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1 Comment

GuiTib Over a year ago

This is working with $setIntersection: "$random_integers" too

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