无法创建类型（类型）的常量值在此上下文中仅支持基本类型（如Int32，String和Guid） [英] Unable to create a constant value of type (type) Only primitive types ('such as Int32, String, and Guid') are supported in this context

问题描述

我已阅读全部：

• 无法在Entity Framework中创建类型为System.Object的常量值

实体框架 - 无法创建类型的常量值闭包类型...错误

Entity Framework - Union causes "Unable to create a constant value of type.."

在这种情况下，只支持原始类型（如Int32，String和Guid）。

Only primitive types ('such as Int32, String, and Guid') are supported in this context

并搜索更多，但仍然没有解决方案。我看到这发生在EF 3.5和4.0中，应该支持 Contains 方法，但是我在EF 4中，但是我收到这个错误。我有一个照片库，相册可以有多个不同的照片，每张照片可以属于任何数量的相册。所以这是一个多对多关系。

and searched a bit more, but still no solution. I've seen that this happens on EF 3.5 and in 4.0 the Contains method should be supported, but I'm in EF 4 but I'm getting this error. I have a photo gallery, where albums can have any number of different photos, and each photo can belong to any number of albums. So this is a many-to-many relationship.

我有一个 VisibleObjects 属性被使用约100种其他方法工作得很好，但我正在粘贴它：（我确定问题是不是由于这里的东西）

I've got a VisibleObjects property which is used by about 100 other methods that work well, but I'm pasting it anyway: (I'm definitely sure the problem is not caused by something here)

static IQueryable<GlobalObject> VisibleObjects
    {
        get
        {
            return from obj in db.GlobalObjectSet where obj.IsVisible && !obj.SiteUser.IsDeactivated orderby obj.ID descending select obj;
        }
    }

我尝试了几个不同的查询：

I've tried several different queries:

我有一个 VisiblePhotos 属性：

这不是工作：

static IQueryable<Photo> VisiblePhotos(this Album a)
    {
        return from p in VisibleObjects.OfType<Photo>() where a.Photos.Contains(p) select p;
    }

更改为：

static IQueryable<Photo> VisiblePhotos(this Album a)
    {
        return from p in VisibleObjects.OfType<Photo>() where a.Photos.Any(other => p.ID == other.ID) select p;
    }

仍然没有工作。

这是调用方法：

public static List<Photo> GetLatestPhotosByAlbum(Album alb, int count = 3)
    {
        lock (sync)
        {
            return alb.VisiblePhotos().OrderByDescending(p => p.ID).Take(count).ToList();
        }
    }

不工作，更改为： p>

Wasn't working, changed to this:

public static List<Photo> GetLatestPhotosByAlbum(Album alb, int count = 3)
    {
        lock (sync)
        {
            return (from p in VisibleObjects.OfType<Photo>()
                    where alb.Photos.Any(ph => ph.ID == ph.ID)
                    select p).ToList();
        }
    }

仍然没有工作。抱怨无法创建一个常量的我的照片对象类型，这是一个具有ID属性的实体对象，如果它有帮助。我不知道错误的真正原因，我没有任何其他的疑问想法。我认为方法名称是非常自我解释的：我试图在给定的专辑中获取照片。将相册条目加载到内存中不是解决方案，查询应该在数据库上运行，而不是内存。我需要解释这个异常，为什么它在这里发生，我该如何让我的查询工作。

Still isn't working. Complaining about not being able to create a constant of my Photo object type, which is an Entity object with an ID property, if it helps. I am not sure of the real cause of the error and I don't have any other ideas of queries in my mind. I think the method name is pretty self explanatory: I'm trying to get the photos in a given album. Loading album entries into memory is not a solution, the query should run on database, not memory. I need an explanation of this exception, why it is occuring here, and how can I get my query to work.

推荐答案

它不会工作，因为你想在linq-to-entities查询中使用本地相册。您必须使用p上的导航属性来获取其专辑：

It will not work because you want to use local Album in linq-to-entities query. You must either use navigation property on p to get its album:

var query = from p in VisibleObjects.OfType<Photo>()
            where p.Album.Id == alb.Id
            select p;

或者您必须使用照片和相册之间的一些连接构建复杂的查询。您不能传递本地对象及其与查询的任何关系。只能简单的属性可以传递。

or you must build complex query with some join between photos and albums. You cannot pass local object and any its relation to the query. Only simple properties can be passed.

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