linq to entity 无法识别 System.Collections.Generic.List 方法

Question

我有以下实体：

public class Person
{
    #region Primitive Properties
    public virtual int PersonId {get; set;}
    public virtual string FirstName{get; set;}
    public virtual string LastName { get; set; }
    #endregion

    #region Navigation Projperties
    public virtual ICollection<Address> Addresses { get; set; }
    #endregion
}

public class Address
{
    #region primitive properties
    public virtual int AddressId
    {
        get;
        set;
    }
    public virtual int PersonId
    {
        get;
        set;
    }
    public virtual int AddressSubTypeId
    {
        get;
        set;
    }
    public virtual string CompleteAddress
    {
        get;
        set;
    }
    public virtual bool IsActive
    {
        get;
        set;
    }
    #endregion

}

并有以下型号：

public class Person
{
    public int PersonId { get; set; }
    public string Name { get; set; }
    public string LastName { get; set; }
    public List<Email> Emailes { get; set; }
    public List<Phone> Phones { get; set; }
}

public class Phone
{
    public int PhoneId { get; set; }
    public string Name { get; set; }
}


public class Email
{
    public int EmailId { get; set; }
    public string Name { get; set; }
}

我想用以下代码获取我的人的列表：

 return context.Persons.Select(x => new Model.Person
                        {
                            PersonId = x.PersonId,
                            Name = x.FirstName,
                            LastName = x.LastName,
                            Phones = x.Addresses.Where(a => a.AddressSubTypeId == 1).Select(a => new Model.Phone
                                                {
                                                    PhoneId = a.AddressId,
                                                    Name = a.CompleteAddress
                                                }).ToList(),
                            Emailes = x.Addresses.Where(a => a.AddressSubTypeId == 2).Select(a => new Model.Email
                                                {
                                                    EmailId = a.AddressId,
                                                    Name = a.CompleteAddress
                                                }).ToList()

                        }).ToList();

当我运行上面的表达式时，出现以下错误：

LINQ to Entities 无法识别方法 'System.Collections.Generic.List1[Model.Phone] ToList[Phone](System.Collections.Generic.IEnumerable1[Model.Phone])' 方法，并且该方法无法转换为存储表达式。

Answer 1

你写的东西会尝试翻译

x.Addresses.Where(a => a.AddressSubTypeId == 1).Select(a => new Model.Phone
{
    PhoneId = a.AddressId,
    Name = a.CompleteAddress
}).ToList()

和

Emailes = x.Addresses.Where(a => a.AddressSubTypeId == 2).Select(a => new Model.Email
{
    EmailId = a.AddressId,
    Name = a.CompleteAddress
}).ToList()

在一条 SQL 语句中，它失败了。

您应该首先在内存中获取 Persons，然后应用上述选择。您可以通过调用ToList() 或AsEnumerable() 来获取人员...这些调用将实现查询，然后就可以应用列表。

return context.Persons.ToList().Select(x => new Model.Person
    {
        PersonId = x.PersonId,
        Name = x.FirstName,
        LastName = x.LastName,
        Phones = x.Addresses.Where(a => a.AddressSubTypeId == 1).Select(a => new Model.Phone
            {
                PhoneId = a.AddressId,
                Name = a.CompleteAddress
            }).ToList(),
        Emailes = x.Addresses.Where(a => a.AddressSubTypeId == 2).Select(a => new Model.Email
            {
                EmailId = a.AddressId,
                Name = a.CompleteAddress
            }).ToList()

    }).ToList();

请记住，调用ToList() 将获取内存中的所有记录，这可能会导致服务器内存的大量使用。在您的情况下，您无论如何都会选择所有行，所以这可能不是问题，但您应该考虑在调用 ToList() 之前为普通字段添加一个 Where 子句甚至是一个 Select ，以便得到您想要的需要，而不是整张桌子。

Answer 2

您不能在表达式中调用方法，先具体化您引用的查询，然后在查询中使用具体化列表。