Storing composite/nested object graph

问题

I am currently developing a document store in Mongo DB which contains a complete material breakdown of a specific item. The breakdown is calculated and contains a composite structure.

The domain model:

public interface IReagent
{
    int ItemId { get; set; }
    int Quantity { get; set; }
    ConcurrentBag<IReagent> Reagents { get; set; }
}

public class Craft : IReagent
{
    public int ItemId { get; set; }
    public int Quantity { get; set; }
    public int SpellId { get; set; }
    public int Skill { get; set; }
    public Profession Profession { get; set; }
    public ConcurrentBag<IReagent> Reagents { get; set; }
}

public class Reagent : IReagent
{
    public int ItemId { get; set; }
    public int Quantity { get; set; }
    public ConcurrentBag<IReagent> Reagents { get; set; }
}

Now the problem is that a composite structure is not correctly stored. Reagents stays null in mongodb.

/* 28 */
{
  "_id" : ObjectId("4e497efa97e8b617f0d229d4"),
  "ItemId" : 52186,
  "Quantity" : 0,
  "SpellId" : 0,
  "Skill" : 475,
  "Profession" : 8,
  "Reagents" : { }
}

Example of how it should look

{
    "_id" : ObjectId("4e497efa97e8b617f0d229d4"),
    "ItemId" : 52186,
    "Quantity" : 0,
    "SpellId" : 0,
    "Skill" : 475,
    "Profession" : 8,
    "Reagents" : [
        {
            "ItemId" : 521833,
            "Quantity" : 3,
            "SpellId" : 0,
            "Skill" : 400,
            "Profession" : 7,
            "Reagents" : [
                {
                    "ItemId" : 52186,
                    "Quantity" : 3,
                    "SpellId" : 0,
                    "Skill" : 475,
                    "Profession" : 8,
                    "Reagents" : [
                        {
                            "ItemId" : 52183,
                            "Quantity" : 2,
                            "Reagents" : []
                        },
                        {
                            "ItemId" : 521832,
                            "Quantity" : 1,
                            "Reagents" : []
                        }
                    ]
                },
                {
                    "ItemId" : 52386,
                    "Quantity" : 2
                    "SpellId" : 0,
                    "Skill" : 400,
                    "Profession" : 8,
                    "Reagents" : [
                        {
                            "ItemId" : 52383,
                            "Quantity" : 2,
                            "Reagents" : []
                        },
                        {
                            "ItemId" : 523832,
                            "Quantity" : 1,
                            "Reagents" : []
                        }
                    ]
                }
            ]
        }
    ]
}

What could be the problem?

回答1:

The problem is you are using list of abstractions and it cannot serialize these to JSON, so basically you would need to write your own custom serialization. Here is example of custom serialization I wrote:

 public class FieldsWrapper : IBsonSerializable
    {
        public List<DataFieldValue> DataFieldValues { get; set; }


        public object Deserialize(MongoDB.Bson.IO.BsonReader bsonReader, Type nominalType, IBsonSerializationOptions options)
        {
        if (nominalType != typeof(FieldsWrapper)) throw new ArgumentException("Cannot deserialize anything but self");
        var doc = BsonDocument.ReadFrom(bsonReader);
        var list = new List<DataFieldValue>();
        foreach (var name in doc.Names)
        {
            var val = doc[name];
            if (val.IsString)
                list.Add(new DataFieldValue {LocalIdentifier = name, Values = new List<string> {val.AsString}});
            else if (val.IsBsonArray)
            {
                DataFieldValue df = new DataFieldValue {LocalIdentifier = name};
                foreach (var elem in val.AsBsonArray)
                {
                    df.Values.Add(elem.AsString);
                }
                list.Add(df);
            }
        }
        return new FieldsWrapper {DataFieldValues = list};
        }


        public void Serialize(MongoDB.Bson.IO.BsonWriter bsonWriter, Type nominalType, IBsonSerializationOptions options)
        {
            if (nominalType != typeof (FieldsWrapper))
                throw new ArgumentException("Cannot serialize anything but self");
            bsonWriter.WriteStartDocument();
            foreach (var dataFieldValue in DataFieldValues)
            {

                bsonWriter.WriteName(dataFieldValue.LocalIdentifier);
                if (dataFieldValue.Values.Count != 1)
                {
                    var list = new string[dataFieldValue.Values.Count];
                    for (int i = 0; i < dataFieldValue.Values.Count; i++)
                        list[i] = dataFieldValue.Values[i];
                    BsonSerializer.Serialize(bsonWriter, list); 
                }
                else
                {
                    BsonSerializer.Serialize(bsonWriter, dataFieldValue.Values[0]); 
                }
            }
            bsonWriter.WriteEndDocument();
        }

    }

In your case I would write my custom serialization on the level of Reagent class

回答2:

Implenting IBsonSerializer worked :) Did a quick mock up and it worked pretty well :D I got another suggestion from someone who says a List<T> would work, gonna try that later this week. And post the results here.

/* 3131 */
{
  "_id" : ObjectId("4e4d58df77d2cf00691aaef2"),
  "ItemId" : 28432,
  "Quantity" : 0,
  "Reagents" : [{
      "ItemId" : 23448,
      "Quantity" : 0,
      "Reagents" : [{
          "ItemId" : 23447,
          "Quantity" : 0,
          "Reagents" : [{
              "ItemId" : 23427,
              "Quantity" : 2,
              "Reagents" : []
            }]
        }, {
          "ItemId" : 23445,
          "Quantity" : 0,
          "Reagents" : [{
              "ItemId" : 23424,
              "Quantity" : 2,
              "Reagents" : []
            }]
        }]
    }, {
      "ItemId" : 23572,
      "Quantity" : 8,
      "Reagents" : []
    }, {
      "ItemId" : 28431,
      "Quantity" : 0,
      "Reagents" : [{
          "ItemId" : 23571,
          "Quantity" : 0,
          "Reagents" : [{
              "ItemId" : 21885,
              "Quantity" : 1,
              "Reagents" : []
            }, {
              "ItemId" : 21884,
              "Quantity" : 1,
              "Reagents" : []
            }, {
              "ItemId" : 22451,
              "Quantity" : 0,
              "Reagents" : [{
                  "ItemId" : 21885,
                  "Quantity" : 1,
                  "Reagents" : []
                }]
            }, {
              "ItemId" : 22452,
              "Quantity" : 1,
              "Reagents" : []
            }, {
              "ItemId" : 22457,
              "Quantity" : 0,
              "Reagents" : [{
                  "ItemId" : 21884,
                  "Quantity" : 1,
                  "Reagents" : []
                }]
            }]
        }, {
          "ItemId" : 22456,
          "Quantity" : 0,
          "Reagents" : [{
              "ItemId" : 21885,
              "Quantity" : 1,
              "Reagents" : []
            }]
        }, {
          "ItemId" : 23573,
          "Quantity" : 0,
          "Reagents" : [{
              "ItemId" : 23446,
              "Quantity" : 0,
              "Reagents" : [{
                  "ItemId" : 23425,
                  "Quantity" : 2,
                  "Reagents" : []
                }]
            }]
        }, {
          "ItemId" : 23448,
          "Quantity" : 0,
          "Reagents" : [{
              "ItemId" : 23447,
              "Quantity" : 0,
              "Reagents" : [{
                  "ItemId" : 23427,
                  "Quantity" : 2,
                  "Reagents" : []
                }]
            }, {
              "ItemId" : 23445,
              "Quantity" : 0,
              "Reagents" : [{
                  "ItemId" : 23424,
                  "Quantity" : 2,
                  "Reagents" : []
                }]
            }]
        }]
    }]
}

Serialization impl:

public void Serialize(MongoDB.Bson.IO.BsonWriter bsonWriter, Type nominalType, MongoDB.Bson.Serialization.IBsonSerializationOptions options)
{
    if (nominalType != typeof(IReagent) && nominalType != typeof(Reagent) && nominalType != typeof(Craft))
    {
        throw new ArgumentException("Cannot serialize anything but self");
    }

    bsonWriter.WriteStartDocument();

    bsonWriter.WriteInt32("ItemId", this.ItemId);
    bsonWriter.WriteInt32("Quantity", this.Quantity);

    if (this.Reagents != null)
    {
        bsonWriter.WriteName("Reagents");
        bsonWriter.WriteStartArray();

        foreach (var r in this.Reagents)
        {
            BsonSerializer.Serialize(bsonWriter, r.GetType(), r, options);
        }

        bsonWriter.WriteEndArray();
    }

    bsonWriter.WriteEndDocument();

}

来源：https://stackoverflow.com/questions/7105274/storing-composite-nested-object-graph