How to pull a range of objects from an array of objects and re-insert them at a new position in the array?

Question

Desired Behaviour

Pull a range of objects from an array of objects and push them back to the array at a new index.

For example, pull objects from the array where their index is between 0 and 2, and push them back to the array at position 6.

For reference, in jQuery, the desired behaviour can be achieved with:

if (before_or_after === "before") {
    $("li").eq(new_position).before($("li").slice(range_start, range_end + 1));
} else if (before_or_after === "after") {
    $("li").eq(new_position).after($("li").slice(range_start, range_end + 1));
}

jsFiddle demonstration

Schema

{
    "_id": ObjectId("*********"),
    "title": "title text",
    "description": "description text",
    "statements": [
    {
        "text": "string",
        "id": "********"
    },
    {
        "text": "string",
        "id": "********"
    },
    {
        "text": "string",
        "id": "********"
    },
    {
        "text": "string",
        "id": "********"
    },
    {
        "text": "string",
        "id": "********"
    }]
}

What I've Tried

I am able to reposition a single object in an array of objects with the code below.

It uses pull to remove the object from the array and push to add it back to the array at a new position.

In order to do the same for a range of objects, I think I just need to modify the $pull and $push variables but:

• I can't figure out how to use $slice in this context, either as a projection or an aggregation, in a $pull operation
• Because I can't figure out the first bit, I don't know how to attempt the second bit - the $push operation

// define the topic_id to search for  
var topic_id = request_body.topic_id;

// make it usable as a search query  
var o_id = new ObjectID(topic_id);

// define the statement_id to search for   
var statement_id = request_body.statement_id;

// define new position
var new_position = Number(request_body.new_position);

// define old position
var old_position = Number(request_body.old_position);

// define before or after (this will be relevant later)
// var before_or_after = request_body.before_or_after;

// define the filter 
var filter = { _id: o_id };

// define the pull update - to remove the object from the array of objects  
var pull_update = {
    $pull: {
        statements: { id: statement_id }  // <----- how do i pull a range of objects here  
    }
};

// define the projection so that only the 'statements' array is returned
var options = { projection: { statements: 1 } };

try {
    // perform the pull update  
    var topic = await collection.findOneAndUpdate(filter, pull_update, options);

    // get the returned statement object so that it can be inserted at the desired index
    var returned_statement = topic.value.statements[old_position];

    // define the push update - to add the object back to the array at the desired position
    var push_update = {
        $push: {
            statements: {
                $each: [returned_statement],
                $position: new_position
            }
        }     // <----- how do i push the range of objects back into the array here  
    };

    // perform the push update  
    var topic = await collection.findOneAndUpdate(filter, push_update);

}

Environments

##### local
  
$  mongod --version
db version v4.0.3

$  npm view mongodb version
3.5.9

$ node -v
v10.16.3

$ systeminfo
OS Name:        Microsoft Windows 10 Home
OS Version:     10.0.18363 N/A Build 18363

##### production
  
$ mongod --version
db version v3.6.3

$ npm view mongodb version
3.5.9

$ node -v
v8.11.4

RedHat OpenShift Online, Linux

Edit

Gradually, figuring out parts of the problem, I think:

Using the example here, the following returns objects from array with index 0 - 2 (ie 3 objects):

db.topics.aggregate([
    { "$match": { "_id": ObjectId("********") } },
    { "$project": { "statements": { "$slice": ["$statements", 0, 3] }, _id: 0 } }
])

Not sure how to use that in a pull yet...

I also looked into using $in (even though i would prefer to just grab a range of objects than have to specify each object's id), but realised it does not preserve the order of the array values provided in the results returned:

Does MongoDB's $in clause guarantee order

Here is one solution to re-ordering results from $in in Node:

https://stackoverflow.com/a/34751295

Answer 1

Here an example with mongo 3.5

const mongo = require('mongodb')

;(async function (params) {
  const client = await mongo.connect('mongodb://localhost:27017')

  const coll = client.db('test').collection('test')

  const from0to99 = Array(100).fill('0').map((_, i) => String(i))
  const from5To28 = Array(24).fill('0').map((_, i) => String(i + 5))

  const insert = { statements: from0to99.map(_ => ({ id: _ })) }
  await coll.insertOne(insert)

  const all100ElementsRead = await coll.findOneAndUpdate(
    { _id: insert._id },
    {
      $pull: {
        statements: {
          id: { $in: from5To28 }
        }
      }
    },
    { returnOriginal: true }
  )
  /**
   * It shows the object with the desired _id BEFORE doing the $pull
   * You can process all the old elements as you wish
   */
  console.log(all100ElementsRead.value.statements)

  // I use the object read from the database to push back
  // since I know the $in condition, I must filter the array returned
  const pushBack = all100ElementsRead.value.statements.filter(_ => from5To28.includes(_.id))

  // push back the 5-28 range at position 72
  const pushed = await coll.findOneAndUpdate(
    { _id: insert._id },
    {
      $push: {
        statements: {
          $each: pushBack,
          $position: 72 // 0-indexed
        }
      }
    },
    { returnOriginal: false }
  )
  console.log(pushed.value.statements) // show all the 100 elements

  client.close()
})()

This old issue helped

Answer 2

if you want "desired behavior" when mutating arrays , you add these to checklist:

• array.length atleast==7 if you want to add ,splice at 6
• creates a new array if u use concat
• mutates orignal if used array.push or splice or a[a.length]='apple'

USE slice() to select between incex1 to index2.

or run a native for loop to select few elements of array or

apply a array.filter() finction.

once you select your elements which needed to be manupulated you mentioned you want to add it to end. so this is the method below. about adding elements at end:

CONCAT EXAMPLE

const original = ['']; //const does not mean its immutable just that it cant be reassigned
let newArray;

newArray = original.concat('');
newArray = [...original, ''];

// Result
newArray; // ['', '']
original; // ['']

SPLICE EXAMPLE:

const zoo = ['', ''];

zoo.splice(
  zoo.length, // We want add at the END of our array
  0, // We do NOT want to remove any item
  '', '', '', // These are the items we want to add
);

console.log(zoo); // ['', '', '', '', '']