将C#MongoDB LINQ与鉴别符一起使用 [英] Using C# MongoDB LINQ with discriminator

问题描述

我有一个单一的MongoDB集合，其中包含三个不同类(A，B，C)的文档，这些文档都继承自相同的类D.

I have a single MongoDB collection holding documents of three different classes (A,B,C) which all inherit from a common class D.

使用正式的C#驱动程序，我插入了所有三种类型(A，B，C)的文档，并且它们都正确地显示了_t鉴别符，并且在我的代码中注册了它们的类映射.

Using the official C# driver, I have inserted documents of all three types (A,B,C) and they all appear correctly with the _t discriminator, and in my code their class maps are registered.

如果我发出如下所示的LINQ查询(使用VB):

If I issue a LINQ query such as the following (using VB):

dim Result = database.GetCollection("mycol").AsQueryable(Of C).Where(some where clause)

如果我计算得出的结果，则会收到错误消息元素'来自类A的元素名称'与类C的任何字段或属性都不匹配."

If I count the results of this, I am getting an error "Element 'an element name from class A' does not match any field or property of class C."

识别符不是要在AsQueryable(Of C)代码中插入吗?看来，当我发出.Count时，我的Where子句(特定于C类的元素)将应用于A，B和C的文档.

Isn't the discriminator meant to kick in here in the AsQueryable(Of C) code? It appears that when I issue .Count my Where clause, which is specific to elements of class C, is being applied to documents of A,B, and C.

我尝试添加.OfType(Of C)无效，尝试使用.ToList首先转换为列表，但是我仍然遇到相同的错误.有任何想法吗?

I have tried adding .OfType(Of C) with no effect, have tried converting to a List first with .ToList, but I continue to get the same error. Any ideas?

作为背景，我的客户代码通常将处理类型D的对象.A，B，C共享许多从D继承的通用属性，因此我希望将它们放在索引上，因此将它们放在单个集合中.但是，在特殊情况下，有时我需要直接引用类型为A，B或C的对象.

As background, my client code will usually deal with objects of type D. A, B, C share a lot of common properties inherited from D which I want to put indexes on, hence I put them in a single collection. However, occassionally I need to directly reference an object of types A, B, or C in special circumstances.

推荐答案

在使用多态类型层次结构时，您的集合变量和LINQ查询应从基类开始.例如，要从数据库中读取所有类型为A的文档，您应编写:

When working with a polymorphic type hierarchy your collection variable and your LINQ queries should start in terms of the base class. For example, to read all the documents of type A back from the database you would write:

var collection = database.GetCollection<D>("mycol");
var query = collection.AsQueryable<D>().OfType<A>();
foreach (var a in query)
{
    // process document of type A
}

出于诊断目的，您可以使用以下命令查看相应的本机MongoDB查询:

For diagnostic purposes you can see the corresponding native MongoDB query using:

var json = ((MongoQueryable<A>)query).GetMongoQuery().ToJson();

请注意，由于OfType()调用更改了IQueryable的类型，因此必须将查询强制转换为MongoQueryable< A>(而不是MongoQueryable< D>).

Note that you have to cast query to a MongoQueryable<A> (not MongoQueryable<D>) because the OfType() call changed the type of the IQueryable.

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