[bug] Fix increment bug for nested array object field

[KaustubhDamania]

Fixes parse-community/parse-server#6687

When I ran the below script,

const Parse = require('parse');

var jsondata = {
  _id: 'someId',
  items: [ { value: 'a', count: 5 }, { value: 'b', count: 1 } ],
  className: 'bug'
}

var object = Parse.Object.fromJSON(jsondata)
object.increment('items.0.count')
console.log(object.get('items'))

I got this output:

{ '0': { value: 'a', count: 6 }, '1': { value: 'b', count: 1 } }

The array was converted into a JSON object. I fixed this behaviour so that the resulting output is as expected:

[ { value: 'a', count: 6 }, { value: 'b', count: 1 } ]

I have modified the estimateAttribute function logic in src/ObjectStateMutations.js and modified relevant tests accordingly.

[ghost]

	Warnings
⚠️	Please add a changelog entry for your changes.

Generated by 🚫 dangerJS

[codecov]

Codecov Report

Merging #1343 (c3d8137) into master (4a75d97) will decrease coverage by 0.70%.
The diff coverage is 42.46%.

❗ Current head c3d8137 differs from pull request most recent head 4c374de. Consider uploading reports for the commit 4c374de to get more accurate results

@@            Coverage Diff             @@
##           master    #1343      +/-   ##
==========================================
- Coverage   99.94%   99.23%   -0.71%     
==========================================
  Files          60       60              
  Lines        5847     5919      +72     
  Branches     1311     1347      +36     
==========================================
+ Hits         5844     5874      +30     
- Misses          3       44      +41     
- Partials        0        1       +1     

Impacted Files	Coverage Δ
src/OfflineQuery.js	86.40% <41.66%> (-13.60%)	⬇️
src/ObjectStateMutations.js	100.00% <100.00%> (ø)

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[dplewis]

The following test fails. Can you add it to the PR?

fit('can increment array nested fields', async () => {
    const obj = new TestObject();
    obj.set('items', [ { value: 'a', count: 5 }, { value: 'b', count: 1 } ]);
    await obj.save();

    obj.increment('items.0.count', 15);
    await obj.save();

    const query = new Parse.Query(TestObject);
    const result = await query.get(obj.id);
    assert.equal(result.get('items')[0].count, 20);
  });

[mtrezza]

How would we differentiate between:

{
  "a": {
    "b": [
      { "c": 0 }
    ]
  }
}

and

{
  "a": {
    "b": {
      "0": {
        "c": 0
      }
    }
  }
}

Edit: I just looked up the MongoDB docs, and it seems MongoDB does not care, it apparently interprets "0" as index if it is an array and as key if it is an object. So that logic would have to be applied internally on the Parse Object as well.

[mtrezza]

Does this PR also address calling other atomic ops on nested keys, such as add or addUnique? I would assume it touches on a common central logic? I don't want to blow up the scope of this PR, but it may be worth considering.

Also, see my analysis point 3; is there a change in Parse Server necessary that accompanies this PR?

Also, I think we need to add a test which increments a nested field for an existing object, that has not been fetched but created with createWithoutData. As far as I can tell this does not currently work, see my analysis point 2, but it should work, otherwise it is not an "atomic" operation on the DB level, which is what one would assume from the Parse docs.