LINQ查询在FirstOrDefault方法上引发异常

Question

I'm using EF core, and I have a many-to-many relationship between two entity

IotaProject <--> User

Here's entities & dto related to the question

public class IotaProject
{
    [Key]
    public int Id { get; set; }
    [Required]
    public string ProjectName { get; set; }
    [Required]
    public DateTime Create { get; set; }

    public ICollection<ProjectOwnerJoint> Owners { get; set; } = new List<ProjectOwnerJoint>();

}

public class ProjectOwnerJoint
{
    public int IotaProjectId { get; set; }
    public IotaProject IotaProject { get; set; }

    public int UserId { get; set; }
    public User User { get; set; }

}

public class User
{
    [Key]
    public int Id { get; set; }
    [Required]
    public string FullName { get; set; }
    [Required]
    public string ShortName { get; set; }
    [Required]
    public string Email { get; set; }

    public ICollection<ProjectOwnerJoint> OwnedProjects { get; set; } = new List<ProjectOwnerJoint>();
}

public class ApplicationDbContext : DbContext
{
    public DbSet<IotaProject> IotaProjects { get; set; }
    public DbSet<User> Users { get; set; }
    public DbSet<ProjectOwnerJoint> ProjectOwnerJoint { get; set; }
}

public class IotaProjectDisplayDto
{
    public int Id { get; set; }
    public string ProjectName { get; set; }
    public DateTime Create { get; set; }

    public UserMinDto Owner { get; set; }
    public int Count { get; set; }

    public IEnumerable<UserMinDto> Reviewers { get; set; }
}

public class UserMinDto
{
    public int Id { get; set; }
    public string FullName { get; set; }
    public string ShortName { get; set; }
}

Following LINQ is the problem, the LINQ purpose is to convert IotaProject to IotaProjectDisplayDto, and key part is that Owners property of IotaProject is ICollection and Owner property in IotaProjectDisplayDto is just one single element UserMinDto , so I only need to get the first element of IotaProject's Owners and that's FirstOrDefault() comes.

IEnumerable<IotaProjectDisplayDto> results = _db.IotaProjects.Select(x => new IotaProjectDisplayDto
            {
                Id = x.Id,
                ProjectName = x.ProjectName,
                Create = x.Create,
                Owner = x.Owners.Select(y => y.User).Select(z => new UserMinDto { Id = z.Id, FullName = z.FullName, ShortName = z.ShortName }).FirstOrDefault()
            });
            return results;

it throws run-time exception

Expression of type 'System.Collections.Generic.List`1[ToolHub.Shared.iota.UserMinDto]' cannot be used for parameter 
of type 'System.Linq.IQueryable`1[ToolHub.Shared.iota.UserMinDto]' 
of method 'ToolHub.Shared.iota.UserMinDto FirstOrDefault[UserMinDto](System.Linq.IQueryable`1[ToolHub.Shared.iota.UserMinDto])' (Parameter 'arg0')

I'm guessing it's probably related to deferred execution, but after read some posts, I still can't resolve it.

Any tips would be appreciated.

Right now, the only way I can get this work is I change type of Owner property in IotaProjectDisplayDto into IEnumrable, which will no longer need FirstOrDefault() to immediate execution. And later on, I manually get the first element in the client to display.

Answer 1

This issue happened in Microsoft.EntityFrameworkCore.SqlServer 3.0.0-preview7.19362.6

I end up downgrade to EF core stable 2.2.6 as Ivan suggested in comment, and everything works fine.