c# - 通过接口(interface)访问时，DbContext 在查询时抛出异常

Question

我已经创建了我的 DbContext 类实现的接口(interface)，这使我能够为单元测试创​​建一个假的数据库上下文。到目前为止，这适用于我所有的 LINQ 查询，但有一个查询出现以下异常:

Unable to create a constant value of type 'DemoApp.Member'. Only primitive types ('such as Int32, String, and Guid') are supported in this context.

通过接口(interface)执行 LINQ 查询会抛出上述异常，但是当直接在我的 DBContext 上执行完全相同的查询时，查询 100% 有效。这是接口(interface)和相关的演示代码定义:

    interface IDemoContext : IDisposable
{
    IDbSet<Member> Members { get; set; }
    IDbSet<Team> Teams { get; set; }
}

public partial class DemoContext : DbContext, IDemoContext
{
    public DemoContext() : base("name=DemoContext"){}

    public IDbSet<Member> Members { get; set; }
    public IDbSet<Team> Teams { get; set; }
}

public partial class Member
{
    public Member()
    {
        this.SecondaryTeams = new HashSet<Team>();
    }

    public int ID { get; set; }
    public string Name { get; set; }
    public int? PrimaryTeamID { get; set; }

    public virtual Team PrimaryTeam { get; set; }
    public virtual ICollection<Team> SecondaryTeams { get; set; }
}

public partial class Team
{
    public Team()
    {
        this.PrimaryMembers = new HashSet<Member>();
        this.SecondaryMembers = new HashSet<Member>();
    }

    public int ID { get; set; }
    public string Name { get; set; }

    public virtual ICollection<Member> PrimaryMembers { get; set; }
    public virtual ICollection<Member> SecondaryMembers { get; set; }
}

每个成员都可能属于一个主要团队，也可能属于多个次要团队。以下演示代码抛出异常:

using (IDemoContext dbi = new DemoContext())
        {
            var members =
                (from member in dbi.Members
                select new
                {
                    Name = member.Name,
                    Team = member.PrimaryTeam.Name,
                    SecondaryTeams = from secondaryTeam in member.SecondaryTeams
                        join primaryMember in dbi.Members
                        on secondaryTeam.ID equals primaryMember.PrimaryTeamID
                        into secondaryTeamMembers
                        select new
                        {
                            Name = secondaryTeam.Name,
                            Count = secondaryTeamMembers.Count()
                        }
                }).ToList();
        }

如果我将第一行更改为:

using (DemoContext dbi = new DemoContext())

然后查询完美执行。

所以我的问题是:

1. 为什么它通过 DemoContext 而不是 IDemoContext 工作？
2. 如何更改 IDemoContext 以便此查询通过界面工作？

Answer 1

尝试用查询中的局部变量替换 dbi.Members。

    using (IDemoContext dbi = new DemoContext())
    {
        IQueryable<Member> memberSet = dbi.Members;

        var members =
            (from member in memberSet
            select new
            {
                Name = member.Name,
                Team = member.PrimaryTeam.Name,
                SecondaryTeams = from secondaryTeam in member.SecondaryTeams
                    join primaryMember in memberSet
                    on secondaryTeam.ID equals primaryMember.PrimaryTeamID
                    into secondaryTeamMembers
                    select new
                    {
                        Name = secondaryTeam.Name,
                        Count = secondaryTeamMembers.Count()
                    }
            }).ToList();
    }