Mongodb: Push element in multiple nested array with condition

Question

How can I push value into multiple nested array with specific conditions?
I have a document like this

[
  {
    "_id": "class_a",
    "students": [
      {
        "_id": "1a",
        "name": "John",
        "grades": []
      },
      {
        "_id": "1b",
        "name": "Katie",
        "grades": []
      },
      {
        "_id": "1c",
        "name": "Andy",
        "grades": []
      },
      
    ]
  }
]

Query to insert into nested array. (Not sure what is missing here)

db.collection.update({
  "_id": "class_a",
  "students": {
    $elemMatch: {
      "_id": {
        "$in": [
          "1a",
          "1b"
        ]
      }
    }
  }
},
{
  $push: {
    "students.$.grades": "A+"
  }
})

Got the following result. But I was expecting both John and Katie have A+ in grades

[
  {
    "_id": "class_a",
    "students": [
      {
        "_id": "1a",
        "grades": ["A+"],
        "name": "John"
      },
      {
        "_id": "1b",
        "grades": [],
        "name": "Katie"
      },
      {
        "_id": "1c",
        "grades": [],
        "name": "Andy"
      }
    ]
  }
]

Expected result

[
  {
    "_id": "class_a",
    "students": [
      {
        "_id": "1a",
        "grades": ["A+"],
        "name": "John"
      },
      {
        "_id": "1b",
        "grades": ["A+"],
        "name": "Katie"
      },
      {
        "_id": "1c",
        "grades": [],
        "name": "Andy"
      }
    ]
  }
]

Mongo playground to test the code

nimrod serok · Accepted Answer · 2022-12-06 11:52:16Z

1

You can use $[<identifier>] to update only the items that match a condition. Your first {} is to find the relevant documents, while the arrayFilters is to find the relevant items inside the document nested array:

db.collection.update(
  {_id: "class_a", students: {$elemMatch: {_id: {$in: ["1a", "1b"]}}}},
  {$push: {"students.$[item].grades": "A+"}},
  {arrayFilters: [{"item._id": {$in: ["1a", "1b"]}}], upsert: true}
)

See how it works on the playground example

answered Dec 6, 2022 at 11:52

nimrod serok

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5 Comments

Bsonjin Over a year ago

This is a good solution also, I would have mark this is the answer if I can mark more than one. Thank you for your help!

nimrod serok Over a year ago

This is the first answer and a better one

Bsonjin Over a year ago

Ok then, can you justify how is this solution better? Like is this going to run faster in terms of performance?

nimrod serok Over a year ago

It is justified in the explanation and on the comments of the other answer...

Bsonjin Over a year ago

After reading the second time, I agree that your solution might be better performance-wise.

dangarfield · Accepted Answer · 2022-12-06 11:57:32Z

1

You should really use arrayFilters for these otherwse it'll only match the first entity. You don't need to use $elemMatch at all.

Playground - https://mongoplayground.net/p/_7y89KB83Ho

db.collection.update({
  "_id": "class_a"
},
{
  $push: {
    "students.$[students].grades": "A+"
  }
},
{
  "arrayFilters": [
    {
      "students._id": {
        "$in": [
          "1a",
          "1b"
        ]
      }
    }
  ]
})

answered Dec 6, 2022 at 11:57

dangarfield

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3 Comments

nimrod serok Over a year ago

There is a need to use $elemMatch in order to find the relevant document. Your answer will try the arrayFilters on all items on all documents

dangarfield Over a year ago

Are you sure? I'm happy to be wrong and learn, but I was under the impression that an arrayFilter would be all that is needed. The playground version works as expected. Ah, I see what you mean, the initial match (_id:"class_a") should limit the ATTEMPT to apply the $push and $arrayFilters, eg, if there were no record of 1a or 1b, it would still ATTEMPT to apply the array filters. I would be interested to know the actual performance impact of this. I think I would prefer this option in terms of readability anyway. But I get what you're saying. Thanks

Bsonjin Over a year ago

In terms of readability, this solution is better. Didn't knew we can use arrayFilter like this. Thank you so much!

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