如何定义表达式选择嵌套在实体框架一到一对一的关系 [英] How to define expression selection nested one-to-one relationship in Entity Framework

问题描述

我试图运行一个LINQ到实体包含此表达式进入实体框架。

I am trying to run a Linq-to-Entity that contains this Expression into Entity Framework.

不工作：

//2 seperated expressions, 1st expression calling the 2nd expression
public Expression<Func<User, UserDto>> UserToDtoExpr() {
    var expr = AddressToDtoExpr();
    return x => new UserDto() {
        Id = x.Id,
        Name = x.Name,
        Address = expr.Compile()(x.Address)
    };  
}
public Expression<Func<Address, AddressDto>> AddressToDtoExpr() {
    return x => New AddressDto() {
        Id = x.Id,
        City = x.City,
        Country= x.Country
    };
}

例外：的LINQ表达式节点类型调用不LINQ支持实体。

现在，我如果硬编码和其嵌套在一个表达式，放入LINQ到实体，然后它的工作原理：

Now, I if hardcode and nest it into one Expression and put it into Linq-to-Entity then it works:

//hardcode combined together into 1 expression with nested object
public static Expression<Func<User, UserDto>> UserToDtoExpr() {
    return x => new UserDto() {
        Id = x.Id,
        Name = x.Name,
        Address = New AddressDto() {
            Id = x.Address.Id,
            City= x.Address.City,
            Country = x.Address.Country
        }
    };  
}

但我不想硬编码像第二种方法，因为我要modularise和重用这些表达式功能。如何解决了第一个方法，使工作？谢谢你。

But I don't want to hardcode it like the 2nd way, because I want to modularise and reuse these Expression functions. How do I fix up the 1st way to make it work? Thanks.

推荐答案

您需要使用的 LinqKit 的 AsExpandable（）来实现这一目标。

You need to use LinqKit's AsExpandable() to achieve this.

第一，改变你的表达式中使用LinqKit的的invoke（）（扩展方法）如下：

First, change your expression to use LinqKit's Invoke() (extension method) as follows:

Address = expr.Invoke(x.Address) // instead of expr.Compile()(x.Address)

然后使用 AsExpandable（）在 DbSet< T> ：

var results = context.Users.AsExpandable().Select(UserToDtoExpr());

阅读的这个来了解与嵌套表达式的问题。

Read this to understand the problem with nested expressions.

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