Aggregation in C# does return null for some properties with MongoDB C# Driver 2.0

Question

I'm writing a MongoDB application and I'm using the aggregation pipeline in C# fow complex queries. When I copy the aggregation generated by C# in the shell, everything seems fine. However, when the aggregation is executed in C#, some properties are being set to null. Please see below for more information.

First, let me show you my model:

public class Smartphone
{
    #region Properties

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

    [BsonElement("name")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public string Name { get; set; }

    [BsonElement("description")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public string Description { get; set; }

    [BsonElement("typenr")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public string Type { get; set; }

    [BsonElement("props")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public List<SmartphoneProperty> Properties { get; set; }

    #endregion
}

public class UnwindedSmartphone
{
    #region Properties

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

    [BsonElement("name")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public string Name { get; set; }

    [BsonElement("description")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public string Description { get; set; }

    [BsonElement("typenr")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public string Type { get; set; }

    [BsonElement("props")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public SmartphoneProperty Property { get; set; }

    #endregion
}

public class SmartphoneProperty
{
    #region Properties

    [BsonElement("type")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public string Type { get; set; }

    [BsonElement("value")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public string Value { get; set; }

    #endregion
}

A single document in my collection does look like the following:

{
    "_id" : ObjectId("55d45c0285afc59c146f66f0"),
    "name" : "LG Nexus 6",
    "description" : "A Nexus 6 device, created by Google.",
    "typenr" : "LG-NEX-5/BLACK",
    "props" : [ 
        {
            "type" : "os",
            "value" : "Android"
        }, 
        {
            "type" : "storage",
            "value" : "8"
        }, 
        {
            "type" : "storage",
            "value" : "16"
        }, 
        {
            "type" : "storage",
            "value" : "32"
        }, 
        {
            "type" : "storage",
            "value" : "64"
        }
    ]
}

The aggregation command that needs to be executed is the following:

// Get all the amount of filters that are defined.
db.smartphones.aggregate([
    // Unwind the "props".
    { "$unwind" : "$props" },

    // Grouping phase.
    // Group by unique properties, add a count for the amount of articles, and add articles to an element named "articles".
    // We use the function "$addToSet" here to ensure that only unique articles are being added.
    { 
        "$group" : { 
            "_id" : "$props", 
            count : { "$sum" : 1 }, 
            articles: { 
                "$addToSet": { 
                    name: "$name", 
                    description: "$description", 
                    typenr: "$typenr" 
                } 
            } 
        } 
    },

    // Sort the results based on the "_id" field.
    { "$sort" : { "_id" : 1 } }
]);

A single result of this aggregation returns me the following:

"_id" : {
    "type" : "storage",
    "value" : "128"
},
"count" : 1.0000000000000000,
"articles" : [ 
    {
        "name" : "Apple iPhone 6",
        "description" : "An iPhone 6 device, created by Apple.",
        "typenr" : "APP-IPHONE-6/BLACK"
    }
]

Now, in C#, I've written the aggregation in the following way:

var aggregation = collection.Aggregate()
    .Unwind<Smartphone, UnwindedSmartphone>(x => x.Properties)
    .Group(key => key.Property, g => new
    {
        Id = g.Key,
        count = g.Count(),
        articles = g.Select(x => new
        {
            name = x.Name,
            description = x.Description,
            typenr = x.Type
        }).Distinct()
    })
    .SortBy(x => x.Id);

If I check the command that this aggregation is transformed to, it appears to be the following:

// Get all the amount of filters that are defined.
db.smartphones.aggregate([
    // Unwind the "props".
    { "$unwind" : "$props" }, 

    // Grouping phase.
    // Group by unique properties, add a count for the amount of articles, and add articles to an element named "articles".
    // We use the function "$addToSet" here to ensure that only unique articles are being added.
    { 
        "$group" : { 
            "_id" : "$props", 
            "count" : { "$sum" : 1 }, 
            "articles" : { 
                "$addToSet" : { 
                    "name" : "$name", 
                    "description" : "$description", 
                    "typenr" : "$typenr" 
                } 
            } 
        } 
    }, 
    { "$sort" : { "_id" : 1 } }
])

So, it's the same as the original aggregation which I tried to convert into C# code, apart that properties are enclosed with double quotes, but that's not an issue.

If I execute this aggregation in the shell, the result is fine. However, when it's executed in C#, the articles properties have a null value for name, description and typenr.

Anyone who knows why this is happening?

Answer 1

Ok,

Thanks to @BlakesSeven who pointed me in the right direction (see his comment on my question, I'm now aware why I'm having that particular issue).

I need to convert the select statement in the group may not return an anonymous type. Rather than that, I need to return a typed object.

This means that my code needs to change to the following:

var aggregation = collection.Aggregate()
    .Unwind<Smartphone, UnwindedSmartphone>(x => x.Properties)
    .Group(key => key.Property, g => new
    {
        Count = g.Count(),
        Articles = g.Select(x => new AggregatedSmartphoneArticle
        {
            Name = x.Name,
            Description = x.Description,
            Type = x.Type
        }).Distinct()
    });

Now everything is working. Thanks again. Using typed code is so much clearer than using those BsonDocument.