如何编写LINQ查询，选择一个属性，该属性是一个嵌套实体属性集合的字符串连接

试图提高我的查询效率，但不确定如何将这一切写为一个查询。我有一个域模型:

public class UserReport : Entity
{
    public string Name { get; set; }
    public List<string> Statuses { get; set; }
    public List<GroupModel> Groups { get; set; }
    public string Email { get; set; }
    public string OfficeStates {get; set;} //Comma delimited list
}
public class GroupModel : Entity
{
    public string Name {get; set;}
    public string Type {get; set;
}

这是一个"复合实体"，如果你愿意。表示这些集合的标准实体是具有User对象的M2M关系实体:

public class User : Entity
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public ICollection<Status> Statuses { get; set; }
    public ICollection<Group> Groups { get; set; }
    public ICollection<Office> Offices { get; set; }
}
public class Office : Entity
{
    //other properties
    public State State { get; set; }
}
public class State : Entity
{
    public string Name { get; set; }
    public string Abbreviation { get; set; }
}

到目前为止，我得到了:

Context.DbSet<User>.Select(user => new UserReport()
    {
       Name = user.FirstName + ", " + user.LastName,
       Email = user.Email,
       Statuses = user.Statuses.Select(status => status.Name).ToList(),
       Groups = user.Groups.Select(group => new GroupModel(){ Name = group.Name, Type = group.Type.Name}).ToList(),
       OfficeStates = string.Join(",", user.Offices.Select(office => office.State.Abbreviation).ToList())
    }).ToList();

抛出错误:

LINQ to Entities不能识别方法System。字符串连接(系统。字符串，System.Collections.Generic.IEnumerable ' 1[System.String])'方法，此方法不能转换为存储表达式。

我完全明白这是说什么，我知道如果我调用。tolist()之前我的选择它会起作用，但我需要有这是一个查询，所以我仍然可以有一个IQueryable应用过滤以后。当然，LINQ中一定有一些方法可以做到这一点。它可以在SQL中完成:将许多行连接到单个文本字符串中?如何通过LINQ实现?

Context.DbSet<User>.Select(user => new // note, anonymous type here
{
   Name = user.FirstName + ", " + user.LastName,
   Email = user.Email,
   Statuses = user.Statuses.Select(status => status.Name), // no ToList - this won't work
   Groups = user.Groups.Select(group => new GroupModel(){ Name = group.Name, Type = group.Type.Name}), // no ToList()
   OfficeStates = user.Offices.Select(office => office.State.Abbreviation) // no ToList(), no String.Join()
}).ToList() // here we materizized, and now we can concatenate strings by hand
.Select(c => new UserReport {
     Name = c.Name,
     Email = c.Email,
     Statuses = c.Statuses.ToList(),
     Groups = c.Groups.ToList(),
     OfficeStates = String.Join(",", c.Offices)
});