IEnumerable.External无法正常工作，那我该怎么办? [英] IEnumerable.Except wont work, so what do I do?

问题描述

我有一个linq to sql数据库.非常简化，我们有3个表格:项目"和用户".有一个称为User_Projects的联接表，将它们联接在一起.

I have a linq to sql database. Very simplified we have 3 tables, Projects and Users. There is a joining table called User_Projects which joins them together.

我已经有了一种为给定用户获取IEnumberable<Project>的有效方法.

I already have a working method of getting IEnumberable<Project> for a given user.

from up in User_Projects
select up.Project;

现在，我想获取用户无关参与的项目.我认为IEnumerable的except方法在这里会很好:

Now I want to get the projects the user isn't involved with. I figured the except method of IEnumerable would be pretty good here:

return db.Projects.Except(GetProjects());

那会编译，但是我遇到一个运行时错误:除Contains()运算符外，不能在查询操作器的LINQ to SQL实现中使用本地序列."

That compiles, however I get a runtime error: "Local sequence cannot be used in LINQ to SQL implementation of query operators except the Contains() operator."

有什么办法可以解决这个问题?

Is there any way to get around this?

一些观点，但没有答案:p

A few views but no answers :p

我已经尝试过:

        IEnumerable<Project> allProjects = db.Projects;
        IEnumerable<Project> userProjects = GetProjects();
        return allProjects.Except(GetProjects());

我知道它与原始语句基本相同-但现在我没有运行时错误.不幸的是，由于某种原因，它实际上并没有做完剩下的部分，只是返回了所有项目

I know it's essentially the same as the original statement - but now i dont get a runtime error. Unfortunately, it doesn't really do the except part and just returns all the projects, for some reason

推荐答案

Linq to Sql无法理解如何使用任意的内存中对象序列.您需要用关系术语来表示，该术语适用于ID:

Linq to Sql doesn't understand how to work with an arbitrary in-memory sequence of objects. You need to express this in relational terms, which works on IDs:

var userProjectIds =
    from project in GetProjects()
    select project.ProjectId;

var nonUserProjects =
    from project in db.Projects
    where !userProjectIds.Contains(project.ProjectId)
    select project;

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