MongoDB is identifying field with ObjectID as null even thought the value exists

Question

I have created a collection and added an index key by grouping 4 fields as unique.

Here are the example of field and value.

"username":string,
"companyID": ObjectID,
"areaCode": string,
"lotNum":string

The companyID's ObjectID, I'm getting it from another collection's document ID as it's ID.

I tried to insert through MongoDB .Net driver and encountered some problem.

Company company = new Company()
            {
                CompanyName = "xyz" 
            };

await _company.InsertOneAsync(company); //_company is IMongoCollection object

SomeClass someClass = new SomeClass()
           {
                UserName = "James",
                CompanyID = company.ID, //This is the ObjectID generated by MongoDB
                AreaCode = "ABC",
                LotNum = "1234a"
           };
await _someClass.InsertOneAsync(company); //_someClass is IMongoCollection object

So, the document object in MongoDB will look like below. I can actually view the document using Compass.

_id:ObjectID("5b062d5be75ed035f057bf06")
username:"James",
companyID: ObjectID("5b062d5be75ed035f057bf05"),
areaCode: "ABC",
lotNum:"1234a"

Now the problem is when I tried to find the document in SomeClass collection with {companyID:ObjectID("5b062d5be75ed035f057bf05")}, I'm unable to find it. But if I use {companyID:null}. It's returning the document.

And I'm not able to add any new document with same username,areaCode and lotNum with a different companyID, "as duplicate key error collection" occurs eventhou the ID is from a newly created Company object have brand new ID. MongoDB is still saying it is NULL.

Am I doing something wrong? How can I fixed this problem.

Here are my data object for Company and SomeClass

 public class Company
 {
    public ObjectId Id { get; set; }
    public string CompanyName{ get; set;}
}


 public class SomeClass
 {
    public ObjectId Id { get; set; }

    public string UserName { get; set;}

    [BsonRepresentation(BsonType.ObjectId)]
    public string CompanyID { get; set; }

    public string AreaCode{ get; set; }

    public string LotNum{ get; set; }
}

Answer 1

As mentioned by Alex Blex in the comments under my question, it is due to case sensitivity. Here's his answer:

It's case sensitive. "companyID" != "CompanyID". Basically companyID is undefined in any of your documents so all of them match criteria companyID:null