Move elements within MongoDB document

Question

Background:

A customer is an object that has a name field.

A line is an object that has the following fields:

inLine - an array of customers
currentCustomer - a customer
processed - an array of customers

The collection 'line' contains documents that are line objects.

Problem:

I'm trying to implement a procedure which would do the following:

Push currentCustomer to processed
Set currentCustomer to the 1st element in inLine
Pop the 1st element of inLine

Since the new value of a field depends on the previous value of another, atomicity is important here.

What I tried so far:

Naive approach

db.collection('line').findOneAndUpdate({
    _id: new ObjectId(lineId),
}, {
    $set: {
        currentCustomer: '$inLine.0',
    },
    $pop: {
        inLine: -1,
    },
    $push: {
        processed: '$currentCustomer',
    },
});

However, currentCustomer is set to a string which is literally "$inLine.0" and processed has a string which is literally "$currentCustomer".

Aggregation approach

db.collection('line').findOneAndUpdate({
    _id: new ObjectId(lineId),
}, [{
    $set: {
        currentCustomer: '$inLine.0',
    },
    $pop: {
        inLine: -1,
    },
    $push: {
        processed: '$currentCustomer',
    },
}]);

However, I got the following error:

MongoError: A pipeline stage specification object must contain exactly one field.

Multi-stage aggregation approach

db.collection('line').findOneAndUpdate({
    _id: new ObjectId(lineId),
}, [{
    $set: {
        currentCustomer: '$inLine.0',
    },
}, {
    $pop: {
        inLine: -1,
    },
}, {
    $push: {
        processed: '$currentCustomer',
    },
}]);

However, $pop and $push are Unrecognized pipeline stage names.

I tried making it using only $set stages, but it ended up very ugly and I still couldn't get it to work.

[Update MongoDB field using value of another field](https://stackoverflow.com/questions/3974985/update-mongodb-field-using-value-of-another-field) — Ivan Rubinson, Nov 16 '20 at 16:19

Ivan Rubinson · Accepted Answer · 2020-12-15T23:33:14.157

Based on turivishal's answer, it was solved like so:

db.collection('line').findOneAndUpdate({
    _id: new ObjectId(lineId),
}, [{
    $set: {
        // currentCustomer = inLine.length === 0 ? null : inLine[0]
        currentCustomer: {
            $cond: [
                { $eq: [{ $size: '$inLine' }, 0] },
                null,
                { $first: '$inLine' },
            ],
        },
        // inLine = inLine.slice(1)
        inLine: {
            $cond: [
                { $eq: [{ $size: '$inLine' }, 0] },
                [],
                { $slice: ['$inLine', 1, { $size: '$inLine' }] },
            ],
        },
        // if currentCustomer !== null then processed.push(currentCustomer)
        processed: {
            $cond: [
                {
                    $eq: ['$currentCustomer', null],
                },
                '$processed',
                {
                    $concatArrays: [
                        '$processed', ['$currentCustomer'],
                    ],
                }
            ],
        },
    },
}]);

Worth noting: This currently doesn't work on MongoDB M0 Sandbox (which is the free one) because it runs version 4.2, but [`$first` is new in 4.4](https://stackoverflow.com/a/64937528/6517320). — Ivan Rubinson, Dec 15 '20 at 23:31

turivishal · Answer 2 · 2020-11-16T17:20:57.007

1

I don't think its possible with simple update using $push or $pop.

As per your experiment, the aggregation can not support direct $push, $pop stage in root level, so I have corrected your query,

currentCustomer check condition if size of inLine is 0 then return null otherwise get first element from inLine array using $arrayElemAt,
inLine check condition if size of inLine is 0 then return [] otherwise remove first element from inLine array using $slice and $size
processed concat both arrays using $concatArrays, $ifNull to check if field is null then return blank array, check condition if currentCustomer null then return [] otherwise return currentCustomer

db.collection('line').findOneAndUpdate(
  { _id: new ObjectId(lineId), }, 
  [{
    $set: {
      currentCustomer: {
        $cond: [
          { $eq: [{ $size: "$inLine" }, 0] },
          null,
          { $arrayElemAt: ["$inLine", 0] }
        ]
      },
      inLine: {
        $cond: [
          { $eq: [{ $size: "$inLine" }, 0] },
          [],
          { $slice: ["$inLine", 1, { $size: "$inLine" }] }
        ]
      },
      processed: {
        $concatArrays: [
          { $ifNull: ["$processed", []] },
          {
            $cond: [
              { $eq: ["$currentCustomer", null] },
              [],
              ["$currentCustomer"]
            ]
          }
        ]
      }
    }
  }]
);

Playground

edited Nov 16 '20 at 17:20

answered Nov 16 '20 at 16:30

turivishal

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If `currentCustomer` is null, this pushes null in to `processed`. – Ivan Rubinson Nov 16 '20 at 16:40
If `inLine` is empty then it errors with "Third argument of $slice must be positive: 0" – Ivan Rubinson Nov 16 '20 at 16:43
If `inLine` is empty, it should remain empty. – Ivan Rubinson Nov 16 '20 at 16:51
`[]` is empty array. – Ivan Rubinson Nov 16 '20 at 16:56
This still pushes `null` in to `processed` if `currentCustomer` is null. Did you mean to instead wrap `$currentCustomer` in `$ifNull`? – Ivan Rubinson Nov 16 '20 at 16:59
This query playground indeed solves the issue with `inLine`, but `processed` still gets `null` if `currentCustomer` is `null`. – Ivan Rubinson Nov 16 '20 at 17:07
I have clear the comments and updated latest answer, it will helpful for someone as well. – turivishal Nov 16 '20 at 17:22