MongoDB with C# Driver: How to filter on field within nested array of objects

Question

I'm new to MongoDB and have been struggling with this a bit. Let's say I have a document with an id, name, and array of objects:

{
    "_id": {
        "$oid": "614ba49b46727413d60b99e2"
    },
    "Name": 1,
    "ObjectArray": [
    {
        "ObjectId": {
            "$oid": "614ba49b46727413d60b99e3"
        },
        "Value": "some value", 
    }, 
    {
        "ObjectId": {
            "$oid": "614ba49b46727413d60b99e5"
        },
        "Value": "other value",
    }
    ],
}

I have a C# class that maps to this document and everything works in that regard. Let's say I want to add another object to the object array only if the new object's Id doesn't already exist in the array? I'm using C# Driver for this and have tried several things.

I've tried several ideas including the following --

var filter = FilterBuilder.Eq(x => x.Id, id) & 
             FilterBuilder.Nin(x => x.ObjectArray[-1].ObjectId, new[] { newDoc.ObjectId});

This unfortunately only checks the first object in the object array. So as asked before how do I only add a new object to an array if a condition exists all within one filter?

Thanks for your time.

*** Solution ***

Used Elematch in conjunction with Not to do the filtering all in one.

var filter = FilterBuilder.Eq(x => x.Id, id) & 
            FilterBuilder.Not(FilterBuilder.ElemMatch(x => x.ObjectArray, 
                 Builders<ObjectClass>.Filter.Eq(y => y.ObjectId, newDoc.ObjectId)));

Yong Shun · Accepted Answer · 2021-09-25 07:33:43Z

2

You can use $elemMatch to access and filter nested property. And compare the value with $eq.

{
  "ObjectArray": {
    $elemMatch: {
      "ObjectId": {
        $eq: <value>
      }
    }
  }
}

Complete MongoDB aggregation query

db.collection.aggregate([
  {
    $match: {
      $and: [
        {
          "_id": {
            // With Exist _id
            $eq: ObjectId("614ba49b46727413d60b99e2")
          }
        },
        {
          "ObjectArray": {
            $elemMatch: {
              "ObjectId": {
                // With unexist ObjectArray.ObjectId
                $eq: ObjectId("614ba49b46727413d60b99e3")
              }
            }
          }
        }
      ]
    }
  }
])

Sample Mongo Playground

For MongoDB .NET/C# driver

var filter = FilterBuilder.Eq(x => x.Id, id) & 
             FilterBuilder.ElemMatch(x => x.ObjectArray, 
                 Builders<ObjectClass>.Filter.Eq(y => y.ObjectId, newDoc.ObjectId);

Replace ObjectClass with the class name for the Object in ObjectArray.

Remarks:

With the above solution, it will return the document when the parsed ObjectId is existed in ObjectArray. Hence you should do the handling for the existed ObjectId case.

And add the document to ObjectArray only when no document is queried.

References

edited Sep 25, 2021 at 7:33

answered Sep 25, 2021 at 2:57

Yong Shun

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1 Comment

SchwarzePete Over a year ago

Finally got it working with elematch for the c# driver. Had to use the Not function in conjunction with elematch. Thanks for your answer.

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