My question is a combination of:

This question: Removing the array element in mongoDB based on the position of element

And this question: mongodb set object where key value dynamically changes

I know you can define the field (array) dynamically like this:

{ [arrayName]: { <condition> } }

But, I then want to remove a certain element in a dynamically defined array by specifying the position (which is also defined dynamically). In other words, the function that processes this query is coming in which two parameters: the array's name and the index of the element to remove.

The options given by the selected answer were the following:

Option 1, does not work (in general), adapted to my case this looks like:

{ $pull : { [arrayName] : { $gt: index-1, $lt: index 1 } } }

Option 2, I cannot use dynamically defined values in field selectors with quotation marks (as far as I am aware):

{ $pull : "[arrayName].[index]" } 

or

{ $pull : "[arrayName].$": index }

Option 3, is different method but can't use it for the same reason:

{ $unset: { "[arrayName].[index]": 1 } } // Won't work
{ $pull: { [arrayName]: null } } // Would probably work

The only workarounds I can think of right now involve significantly changing the design which would be a shame. Any help is appreciated!

PS: I'm using mongoose as a driver on the latest version as of today (v6.3.5) and MongoDB version 5.0.8

CodePudding user response：

On Mongo version 4.2 You can use pipelined updates to achieve this, you can get it done in multiple ways, here is what I consider the easiest two ways:

1. using $slice and $concatArrays to remove a certain element:

db.collection.update({},
[
  {
    $set: {
      [arrayName]: {
        $concatArrays: [
          {
            $slice: [
              `$${arrayName}`,
              index,
            ]
          },
          {
            $slice: [
              `$${arrayName}`,
              index   1,
              {
                $size: `$${arrayName}`
              }
            ]
          }
        ]
      }
    }
  }
])

Mongo Playground

2. using $filter and $zip to filter out based on index:

db.collection.updateOne(
{},
[
  {
    "$set": {
      [arrayName]: {
        $map: {
          input: {
            $filter: {
              input: {
                $zip: {
                  inputs: [
                    {
                      $range: [
                        0,
                        {
                          $size: `$${arrayName}`
                        }
                      ]
                    },
                    `$${arrayName}`
                  ]
                }
              },
              cond: {
                $ne: [
                  {
                    "$arrayElemAt": [
                      "$$this",
                      0
                    ]
                  },
                  index
                ]
              }
            }
          },
          in: {
            $arrayElemAt: [
              "$$this",
              1
            ]
          }
        }
      }
    }
  }
])

Alternatively you can just prepare