MongoDB .NET Driver - How to access nested element

Question

I use MongoDB Driver in my .NET Core application. I will add data model classes as follows:

namespace Test.Database
{
    public class DocumentFields
    {
        [BsonId]
        [BsonRepresentation(BsonType.ObjectId)]
        public string Id { get; set; }
        [BsonElement("account_id")]
        public string AccountId { get; set; }
        [BsonElement("document_types")]
        public DocumentType[] DocumentTypes { get; set; }
        [BsonElement("document_themes")]
        public DocumentTheme[] DocumentThemes { get; set; }
        [BsonElement("created_at")]
        public DateTime CreatedAt { get; set; }
    }

    public class DocumentType
    {
        [BsonElement("id")]
        public string id { get; set; }
        [BsonElement("name")]
        public string name { get; set; }
    }

    public class DocumentTheme
    {
        [BsonElement("id")]
        public string id { get; set; }
        [BsonElement("name")]
        public string name { get; set; }
    }
}

But when I try to fetch the data using this model class, it gives this error.

System.FormatException: An error occurred while deserializing the DocumentThemes property of class Test.Database.DocumentFields: Element 'id' does not match any field or property of class Test.Database.DocumentTheme.

The document I'm trying to access is something like this.

{
    "_id" : ObjectId("6234410f0f4dd20dd96fb9dd"),
    "account_id" : "96f35500-584e-4db9-94cd-ebd93cce13b7",
    "created_at" : ISODate("2022-03-18T08:21:35.695+0000"),
    "document_themes" : [
        {
            "id" : "6ad3eac9-e44c-4fdd-b54a-f0ff9165ba3e",
            "name" : "Theme 1"
        },
        {
            "id" : "2dd625f2-de57-41f7-9d89-a0e28c62dc65",
            "name" : "Theme 2"
        },
        {
            "id" : "af3347c7-12cc-421a-8945-65ce80bf8a0f",
            "name" : "Theme 3"
        }
    ],
    "document_types" : [
        {
            "id" : "c0934926-9e1d-468c-9a47-97a62d83228d",
            "name" : "Type 1"
        },
        {
            "id" : "331ad071-7d91-4b84-bdf1-733f9868b979",
            "name" : "Type 2"
        },
        {
            "id" : "630bbcad-34ec-4b4d-84e4-89724bd8ae17",
            "name" : "Type 3"
        }
    ]
}

How I can overcome this obstacle?

Answer 1

Late to the answer.

Similar to this question, the .NET/C# driver will bind the property id / Id / _id to the _id field in the document.

Solution 1

Implement the class map to override the default behavior of id property.

BsonClassMap.RegisterClassMap<DocumentType>(cm =>
{
    cm.AutoMap();
    cm.UnmapField(x => x.id);

    cm.MapMember(x => x.id)
        .SetElementName("id");
});

BsonClassMap.RegisterClassMap<DocumentTheme>(cm =>
{
    cm.AutoMap();
    cm.UnmapField(x => x.id);

    cm.MapMember(x => x.id)
        .SetElementName("id");
});

Demo

---

Solution 2

Apply [BsonNoId] attribute to the DocumentType and DocumentTheme classes to suppress the default Id behavior.

[BsonNoId]
public class DocumentType
{
    ...
}

[BsonNoId]
public class DocumentTheme
{
    ...
}

Demo