如何在LINQ Join中将Factory方法用作resultSelector

Question

I would like to use a method from my ModelFactory class to create an instance of CommentWithUserDetails instead of using an Object Initializer. Can this be done?

ExceptionMessage

LINQ to Entities does not recognize the method 'WebApi.Models.CommentWithUserDetails Create(WebApi.Models.Comment, WebApi.Models.ApplicationUser)' method, and this method cannot be translated into a store expression.

---

public IEnumerable<CommentWithUserDetails> GetAllPostComments(int postId)
    {
        var commentsWithUserDetails = _context.Comments.Join(_context.Users,
            c => c.UserId,
            u => u.Id,
            (comment, user) => _modelFactory.Create(comment, user));

        return commentsWithUserDetails;
    }

---

public class ModelFactory
{
    public CommentWithUserDetails Create(Comment comment, ApplicationUser user)
    {
        return new CommentWithUserDetails
        {
            Id = comment.Id,
            PostId = comment.PostId,
            Body = comment.Body,
            Name = user.Name
        };
    }
}

Answer 1

Rather than having a method as a selector, you'll need to have an expression, but you can of course write a method (or property) that returns the expression that you want, so that you can use it in multiple places:

public class ModelFactory
{
    public Expression<Func<Comment, ApplicationUser, CommentWithUserDetails>> Create()
    {
        return (comment, user) => new CommentWithUserDetails
        {
            Id = comment.Id,
            PostId = comment.PostId,
            Body = comment.Body,
            Name = user.Name
        };
    }
}

You can then pass in ModelFactory.Create into the result selector for the Join .

Answer 2

Yes, but. By using a factory method you will only be able to use linq to objects. Query Providers will not know what to do with your factory method. (That is what the exception you are getting is saying.)

If you do the join first then call .AsEnumerable() then use a .Select(...) with your factory this will work. You will lose composability.

public IEnumerable<CommentWithUserDetails> GetAllPostComments(int postId)
{
    var commentsWithUserDetails = _context.Comments.Join(_context.Users,
        c => c.UserId,
        u => u.Id,
        (comment, user) => new { User = user, Comment = comment})
        .AsEnumerable()
        .Select(i=>_modelFactory.Create(i.Comment, i.User))
        ;

    return commentsWithUserDetails;
}