MongoDB. Remove subelement from array using parent elements for query

Question

Here is my collection object structure

"_id" : ObjectId("xxxxxxxxxxxxxxxxxxxxxxxx"),
"Name": "HelloWorld",
"OtherFields": "OtherValues",
"Projects" : [ 
    {
        "Project" : {
            "key" : 111
        },
        "Category" : [ 
            {
                "No" : "123"
            }, 
            {
                "No" : "987"
            }
        ]
    },
    {
        "Project" : {
            "key" : 222
        },
        "Category" : [ 
            {
                "No" : "123"
            }, 
            {
                "No" : "987"
            }
        ]
    }
]

I want to delete element with Category No「123」in Project with key 111

Closer thing that I did was

db.project_collection.updateOne({"_id": ObjectId("collection_obj_id")}, {"$pull": { "Projects": {"$and": [ {"Project.key": 111)}, {"Category.No": "123"} ] }} })

Live example

This command removing whole element inside "Projects" object

But I want to delete only "Projects" → "Category" → [0] subelement which has No 123.

When I trying to change "Projects" after "$pull" to something like "Projects.Category" or "Projects.$.Category"

The positional operator did not find the match needed from the query

error occures.

So is it possible to delete subelement without creating own backend logic?

You need to use the Array Update Operator [positional $](https://docs.mongodb.com/v4.2/reference/operator/update/positional/index.html). — prasad_, Nov 06 '20 at 08:24
I already tried Projects.$, Projects.$.Category.$, Projects.Category.$, Projects.$.Category with no luck — rzlvmp, Nov 06 '20 at 08:32
Yes, you can try the answer below by J.F. Another example [here](https://docs.mongodb.com/v4.2/reference/operator/update/pull/#remove-items-from-an-array-of-documents) using `$pull`. — prasad_, Nov 06 '20 at 08:37
One of the answers in the marked duplicates dictates a solution in the form `db.collection.update({ _id: "xxxxxxxxxxxxxxxxxxxxxxxx" }, { $pull: { "Projects.$[elem].Category": { "No": "123" } } }, { arrayFilters: [ { "elem.Project.key": 111 } ] })` — chridam, Nov 06 '20 at 09:37

J.F. · Accepted Answer · 2020-11-06T09:19:36.640

1

Knowing project.key and category.no you can use this query:

db.collection.update({
  "_id": 1,
  "Projects.Project.key": 111
},
{
  "$pull": {
    "Projects.$.Category": {
      "No": "123"
    }
  }
})

With the updated JSON, the $ operator is nedded.

This query basically point to a document with key: 111 and _id: 1 and using $ do the pull. So it will pull the 'pointed' element which match no: "123".

Updated example here

I have added another document with _id: 2 to check that only one with _id:1 is updated.

edited Nov 06 '20 at 09:19

answered Nov 06 '20 at 08:30

J.F.

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no luck :( `WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 0 })` I can use `{"project.key": 111}` for finding element, but even if I find it I can't change – rzlvmp Nov 06 '20 at 08:51
Sorry, JSON data in my question was little bit not complete. Added keys for better understanding. Data in your example little bit differs – rzlvmp Nov 06 '20 at 08:55
Updated answer with the new JSON format. Hope it helps. – J.F. Nov 06 '20 at 09:05
Brilliant! Thank you! Actually I need to update specific object_id, so can you edit your answer query like this https://mongoplayground.net/p/KoMHSFcQyUf And I will check you answer as correct – rzlvmp Nov 06 '20 at 09:15
Answer updated :) – J.F. Nov 06 '20 at 09:19

MongoDB. Remove subelement from array using parent elements for query

1 Answers1