Pull from nested Array in MongoDB

Question

I have a document structure like this:

{
  "name": "Example",
  "description": "foo",
  "vocabulary": [
    ["apple", "pomme"],
    ["hello", "bonjour"],
    ["bus", "bus"]
  ]
}

Now I want to pull an array inside the vocabulary array by specifying the first item, a.E.:

{"$pull": {"vocabulary.$": ["apple"]}

Which should remove the array ["apple", "pomme"] from vocabulary, but this doesn't work.

I tried this ($pull from nested array), but it did not work, it threw

pymongo.errors.WriteError:
The positional operator did not find the match needed from the query., full error: {'index': 0, 'code': 2, 'errmsg': 'The positional operator did not find the match needed from the query.'}

Yong Shun · Accepted Answer · 2021-11-21 12:00:12Z

0

Very tricky question.

I think for this case $ positional operator is not suitable.

Instead, you need an aggregation pipeline in update query. Query ($filter) the values with the "apple" word is not ($not) existed ($in) in the vocabulary array field. Then $set to the vocabulary field.

db.collection.update({},
[
  {
    "$set": {
      "vocabulary": {
        $filter: {
          input: "$vocabulary",
          cond: {
            $not: {
              $in: [
                "apple",
                "$$this"
              ]
            }
          }
        }
      }
    }
  }
])

Sample Mongo Playground

answered Nov 21, 2021 at 12:00

Yong Shun

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1 Comment

AXOME Over a year ago

Thanks for the answer! Unfortunately, that doesn't work on my server, but it works in the playground, which is pretty odd...

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