MongoDB C# Get latest document from group

Question

I have a group of statuses of pretend payments, each with a payment ID.

I want to get the latest status for each payment ID. The test I have creates some dummy data

Answer 1

There has to be a better way though.

As of 2.5.3, you can access the current group inside of an aggregation. This lets us build a generic accessor that will retrieve the first element from a grouping via a native mongo query.

First, a helper class for Deserialization. KeyValuePair is sealed, so we roll our own.

    /// 

    /// Mongo-ified version of 
    /// 
    class InternalKeyValuePair
    {
        [BsonId]
        public TKey Key { get; set; } 
        public T Value { get; set; }
    }
    //you may not need this method to be completely generic, 
    //but have the sortkey be the same helps
    interface IDateModified
    {
        DateTime DateAdded { get; set; }
    }
    private List GroupFromMongo(string KeyName) where T : IDateModified
    {
        //mongo linq driver doesn't support this syntax, so we make our own bsondocument. With blackjack. And Hookers. 
        BsonDocument groupDoc = MongoDB.Bson.BsonDocument.Parse(@"
         {
                _id: '$" + KeyName + @"',
                Value: { '$first': '$$CURRENT' }
        }");
        //you could use the same bsondocument parsing trick to get a generic 
        //sorting key as well as a generic grouping key, or you could use
        //expressions and lambdas and make it...perfect.
        SortDefinition sort = Builders.Sort.Descending(document => document.DateAdded);
        List intermediateResult = getCol().Aggregate().Sort(sort).Group(groupDoc).ToList();
        InternalResult[] list = intermediateResult.Select(r => MongoDB.Bson.Serialization.BsonSerializer.Deserialize>(r)).ToArray();
        return list.Select(z => z.Value).ToList();
    }

---

Okay..I genericized it with some help from https://stackoverflow.com/a/672212/346272

    /// 

    /// Mongo-ified version of 
    /// 
    class MongoKeyValuePair
    {
        [BsonId]
        public TKey Key { get; set; }
        public T Value { get; set; }
    }
    private MongoKeyValuePair[] GroupFromMongo(Expression> KeySelector, Expression> SortSelector)
    {
        //mongo linq driver doesn't support this syntax, so we make our own bsondocument. With blackjack. And Hookers. 
        BsonDocument groupDoc = MongoDB.Bson.BsonDocument.Parse(@"
         {
                _id: '$" + GetPropertyName(KeySelector) + @"',
                Value: { '$first': '$$CURRENT' }
        }");
        SortDefinition sort = Builders.Sort.Descending(SortSelector);
        List groupedResult = getCol().Aggregate().Sort(sort).Group(groupDoc).ToList();
        MongoKeyValuePair[] deserializedGroupedResult = groupedResult.Select(r => MongoDB.Bson.Serialization.BsonSerializer.Deserialize>(r)).ToArray();
        return deserializedGroupedResult;
    }

    /* This was my original non-generic method with hardcoded strings, PhonesDocument is an abstract class with many implementations */
    public List ListPhoneDocNames() where T : PhonesDocument
    {
        return GroupFromMongo(z=>z.FileName,z=>z.DateAdded).Select(z=>z.Value).ToList();
    }


    public string GetPropertyName(Expression> propertyLambda)
    {
        Type type = typeof(TSource);

        MemberExpression member = propertyLambda.Body as MemberExpression;
        if (member == null)
            throw new ArgumentException(string.Format(
                "Expression '{0}' refers to a method, not a property.",
                propertyLambda.ToString()));

        PropertyInfo propInfo = member.Member as PropertyInfo;
        if (propInfo == null)
            throw new ArgumentException(string.Format(
                "Expression '{0}' refers to a field, not a property.",
                propertyLambda.ToString()));

        if (type != propInfo.ReflectedType &&
            !type.IsSubclassOf(propInfo.ReflectedType))
            throw new ArgumentException(string.Format(
                "Expresion '{0}' refers to a property that is not from type {1}.",
                propertyLambda.ToString(),
                type));

        return propInfo.Name;
    }

---

For bonus points, you can now easily do any of mongos other grouping operations without fighting the linq helpers. See https://docs.mongodb.com/manual/reference/operator/aggregation/group/ for all available grouping operations. Let's add a count.

    class MongoKeyValuePair
    {
        [BsonId]
        public TKey Key { get; set; }
        public T Value { get; set; }
        public long Count { get; set; }
    }

        BsonDocument groupDoc = MongoDB.Bson.BsonDocument.Parse(@"
         {
                _id: '$" + GetPropertyName(KeySelector) + @"',
                Value: { '$first': '$$CURRENT' },
                Count: { $sum: 1 }
        }");

run the aggregation the exact same as before and your count property will be filled in with the amount of documents matching your groupkey. Neat!