MongoDB Custom Serializer 避免 _t 被添加集合，抛出 ReadEndArray 错误？

Question

Situation: Language: C# using the C# Driver I have a model that contains a List as a property. That List can contain one of 3 different models that all inherit the BaseModelClass. To assist in serialization of this situation Mongo adds _t to identify which of the models is actually being used. For us this is a problem due to the amount of space that _t is taking up. I am a lowly dev, I have asked for more space and ram and they have told me to solve it without the additional space. So I sat down to writing a custom serializer that handled the different types without writing a _t to the BSONDocument. I thought all was great until I started doing my unit testing of the serialization. I started getting "ReadEndArray can only be called when ContextType is Array, not when ContextType is Document."

[BsonCollectionName("calls")]
[BsonIgnoreExtraElements]
public class Call
{
    [BsonId]
    public CallId _id { get; set; }

    [BsonElement("responses")]
    [BsonIgnoreIfNull]
    public IList<DataRecord> Responses { get; set; }
}

Answer 1

Also asked here: https://groups.google.com/forum/#!topic/mongodb-user/iOeEXbUYbo4

I think your better bet in this situation is to use a custom discriminator convention. You can see an example of this here: https://github.com/mongodb/mongo-csharp-driver/blob/v1.x/MongoDB.DriverUnitTests/Samples/MagicDiscriminatorTests.cs . While this example is based on whether a field exists in the document, you could easily base it on what type the field is (BsonType.Int32, BsonType.Date, etc...).

Answer 2

Basing on @Craig Wilson answer, to get rid off all discriminators, you can:

public class NoDiscriminatorConvention : IDiscriminatorConvention
    {
        public string ElementName => null;

        public Type GetActualType(IBsonReader bsonReader, Type nominalType) => nominalType;

        public BsonValue GetDiscriminator(Type nominalType, Type actualType) => null;
    }

and register it:

BsonSerializer.RegisterDiscriminatorConvention(typeof(BaseEntity), new NoDiscriminatorConvention());

Answer 3

This problem occurred in my case when I was adding a Dictionary<string, object> and List entities to database. The following link helped me in this regard: C# MongoDB complex class serialization . For your case, I would suggest, following the link given above, as follows:

using System;
using Newtonsoft.Json;
using Newtonsoft.Json.Linq;

[JsonConverter(typeof(DataRecordConverter))]
public abstract class DataRecord
{
    public string Key { get; set; }
    public string DataRecordType {get;}
}

public class DataRecordInt : DataRecord
{
    public int Value { get; set; }
    public override string DataRecordType => "Int"
}

public class DataRecordDateTime : DataRecord
{
    public DateTime? Value { get; set; }
    public override string DataRecordType => "DateTime"
}


public class DataRecordConverter: JsonConverter
{
  public override bool CanWrite => false;
  public override bool CanRead => true;
  
  public override bool CanConvert(Type objectType)
  {
     return objectType == typeof(DataRecord);
  }
 
  public override void WriteJson(
              JsonWriter writer, 
              object value, 
              JsonSerializer serializer)
  {}

  public override object ReadJson(
              JsonReader reader,
              Type objectType,
              object existingValue,
              JsonSerializer serializer)
  {
     var jsonObject = JObject.Load(reader);
     var dataRecord = default(DataRecord);

     switch (jsonObject["DataRecordType"].Value<string>())
     {
         case "Int":
            dataRecord = new DataRecordInt();
            break;
         case "DateTime":
            dataRecord = new DataRecordDataTime();
            break;

     }

     serializer.Populate(jsonObject.CreateReader, dataRecord);
     return dataRecord;
  }
}

[BsonCollectionName("calls")]
[BsonIgnoreExtraElements]
public class Call
{
    [BsonId]
    public CallId _id { get; set; }

    [JsonIgnore]
    [BsonElement("responses")]
    [BsonIgnoreIfNull]
    public BsonArray Responses { get; set; }
}

You can add populate the BsonArray using:

var jsonDoc = JsonConvert.SerializeObject(source);
var bsonArray = BsonSerializer.Deserialize<BsonArray>(jsonDoc);

Now, you can get deserialized List from Mongo using:

var bsonDoc = BsonExtensionMethods.ToJson(source);
var dataRecordList = JsonConvert.DeserializeObject<List<DataRecord>>(bsonDoc, new DataRecordConverter())

Hope this helps, again, thanks to C# MongoDB complex class serialization for this.