MongoDB 2.4的“更新后数组中的元素数量限制".使用C#驱动程序? [英] MongoDB 2.4's "Limit Number of Elements in an Array after an Update" using C# driver?

问题描述

MongoDB 2.4添加了新的"Limit 更新后的数组中的元素数" .这是可以通过外壳使用的方式:

MongoDB 2.4 added a new "Limit Number of Elements in an Array after an Update" feature. This is how it can be used through the shell:

db.students.update(
    { _id: 1 },
    { $push: 
        { scores:
            { $each : 
            [
                { attempt: 3, score: 7 },
                { attempt: 4, score: 4 }
            ],
            $sort: { score: 1 },
            $slice: -3
            }
        }
    }
)

如何使用MongoDB的C#驱动程序完成此操作?

推荐答案

下面是一个示例测试，显示了如何在不使用类型化类的情况下执行此操作:

Here is an example test that shows how to do this without using typed classes: https://github.com/mongodb/mongo-csharp-driver/blob/master/MongoDB.DriverUnitTests/Builders/UpdateBuilderTests.cs#L492

您正在寻找的相关代码是:

The relevant piece of code you are looking for is this:

var update = Update.PushEach(
    "name", 
     new PushEachOptions { Slice = -3, Sort = SortBy.Descending("a") }, 
    value1ToPush, 
    value2ToPush);

如果您使用类型化的实体，我们也支持此功能:

We also support this if you are using typed entities: https://github.com/mongodb/mongo-csharp-driver/blob/master/MongoDB.DriverUnitTests/Builders/UpdateBuilderTests.cs#L524

var update = Update<Test>.PushEach(
    x => x.B, 
    args => args.SortDescending(x => x.C).Slice(-3), 
    new[] { new B { C = 0 }, new B { C = 1 } });

最后，像.NET驱动程序中的所有其他内容一样，您始终可以构建一个与上面的结构完全相似的BsonDocument并简单地执行它.

Finally, like everything else in the .NET driver, you can always build up a BsonDocument that looks exactly like your structure above and simply execute it.

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