MongoDB .NET Driver - Using ElemMatch with FilterDefiniton

Question

I'm trying to search for a time interval by ElemMatch. My fields of interest consist of simple objects or arrays of objects, like the following:

{
  "ID": "123456789",
  "field1": {
      "item1": "string",
      "item2": 123
  },
  "field2": [
      {
        "item11": 11.11,
        "myDatetime": {
           ts: 2022-10-23T23:14:55
         }
      },
      {
        "item11": 12.22,
        "myDatetime": {
           ts: 2022-08-23T23:14:55
         }
      }
   ]
}

and my model is:

public record MongoCollection
{
    public string ID { get; init; }
    public object field1 { get; init; }

    public List<object> field2 { get; init; }
}

I'm trying to implement ElemMatch in this way:

var filter = Builders<MongoCollection>.Filter.ElemMatch("field1",
  Builders<MongoCollection>.Filter.And(
    Builders<MongoCollection>.Filter.Gte("myDatetime.ts", fromDate),
    Builders<MongoCollection>.Filter.Lte("myDatetime.ts", tillDate))
);

The error that I receive on this part of code is:

Unable to cast object of type 'MongoDB.Bson.Serialization.Serializers.ObjectSerializer' to type 'MongoDB.Bson.Serialization.IBsonSerializer

I also tried this approach:

var field = new StringFieldDefinition<MongoCollection, List<object>>("field1");  

And

FieldDefinition<MongoCollection,List<object>> field = "field1";

And I put the field in ElemMatch like this:

var filter = Builders<MongoCollection>.Filter.ElemMatch(field,
  Builders<MongoCollection>.Filter.And(
    Builders<MongoCollection>.Filter.Gte("myDatetime.ts", fromDate),
    Builders<MongoCollection>.Filter.Lte("myDatetime.ts", tillDate))
);

and I received the same error. How can I solve it?

Answer 1

1. You should refer to field2 but not field1.

2. As in the MongoCollection class the item in field2 is object type, the filter for the nested object should be in object type by using Builders<object>.

var filter = Builders<MongoCollection>.Filter.ElemMatch("field2",
    Builders<object>.Filter.And(
        Builders<object>.Filter.Gte("myDatetime.ts", fromDate),
        Builders<object>.Filter.Lte("myDatetime.ts", tillDate))
);