然后放松MongoDB中的C＃集团聚集 [英] Unwind then Group aggregation in MongoDB C#

问题描述

我有一些麻烦，新的C＃2.0的MongoDB驱动程序和聚合管道。

I'm having some trouble with the new C# 2.0 MongoDB driver and the aggregation pipeline.

基本上，我试图返回内最流行的元素在对象上数组字段。字段类型为：的IList<串GT; FavouritePlaceIds {搞定;组; }

Basically, I'm trying to return the most popular elements within an array field on the object. The field type is: IList<string> FavouritePlaceIds { get; set; }.

我有正按预期以下MongoDB的聚合：

I have the following MongoDB aggregation which is working as expected:

db.users.aggregate([
    { $unwind : "$FavouritePlaceIds" },
    { $group: { "_id": "$FavouritePlaceIds", "count": {$sum: 1}}}, 
    { $sort : { "count": -1 }}
])

不过，这个问题目前正试图翻译成使用新的驱动程序的MongoDB 2.0的C＃代码。我一直在使用下面的链接与聚合管道帮助：的 http://mongodb.github.io/mongo-csharp-driver/2.0/reference/driver/crud/reading/#unwind

However, the issue is now trying to translate that into C# code using the new MongoDB driver 2.0. I've been using the following link for help with the aggregation pipeline: http://mongodb.github.io/mongo-csharp-driver/2.0/reference/driver/crud/reading/#unwind

我已经走到这一步，我的汇聚管道以下内容：

I've got the following so far for my aggregation pipeline:

var pipeline = usersCollection.Aggregate()
                .Unwind(i => i.FavouritePlaceIds)
                .Group(i => i.FavouritePlaceIds, g => new { FavouritePlaceIds = g.Key, Count = g.Count() })
                .SortByDescending(i => i.Count);

当我去编译代码时，我得到以下信息：

When I go to compile that code, I get following message:

BsonDocument'不包含'FavouritePlaceIds的定义，并没有扩展方法'FavouritePlaceIds接受型BsonDocument'的第一个参数可以发现...

'BsonDocument' does not contain a definition for 'FavouritePlaceIds' and no extension method 'FavouritePlaceIds' accepting a first argument of type 'BsonDocument' could be found...

该错误是在第一个参数存在的（ I => i.FavouritePlaceIds ）集团（）方法。

The error is occuring on the first parameter (i => i.FavouritePlaceIds) of the Group() method.

读数下组部分提供的链接笔记，它提到：

Reading the notes at the link provided under the group section, it mentions that:

由于$平仓是一种投射，你必须提供一个返回类型。

Because $unwind is a type of projection, you must provide a return type.

所以，我假设我M没有指定正确的返回类型，这就是为什么它的期待BsonDocument对象，并没有进行编译。

So, I'm assuming that I'm not specifying a correct return type, which is why it's expecting a BsonDocument object, and failing to compile.

所以，我怎么可以指定正确的返回类型的组的方法来使用？

So, how can I specify the correct return type to use in the Group method?

推荐答案

当你让开卷推断它将使用集合类型为 TResult 的类型参数和 BsonDocument 为 TNewResult 。

When you let Unwind infer the type parameters it will use the collection type for TResult and BsonDocument for TNewResult.

如果你想使用特定类型的，而不是 BsonDocument 您需要添加这种类型的参数：

If you want to use a specific type and not BsonDocument you need to add these type parameters:

var pipeline = usersCollection.Aggregate()
    .Unwind<OriginalType, NewResultType>(....

与往常一样，你需要确保该操作不会返回的东西，可以是此类型。

As always, you need to make sure the operation actually returns something that can be of that type.

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