CODEWITHSUNDEEP
c#asp.net-mvclinqentity-frameworklinq-to-entities
Reza.Hoque
Reza.Hoque

Reputation: 2750

LINQ to Entities query error

I am encountered an error that I am not familier with. I tried to google with no success. I wrote the following query where I am having this error.

The entity or complex type 'MyWebProject.Models.UserDetail' cannot be constructed in a LINQ to Entities query.

The query:

UsersContext db = new UsersContext();
        var userdata = (from k in db.UserDetails
                        where k.UserId == WebSecurity.CurrentUserId
                        select new UserDetail()
                        {

                            FullName = k.FullName,
                            Email = k.Email,
                            About = k.About,
                            Link = k.Link,
                            UserSchool = new School()
                            {
                                SchoolId = k.UserSchool.SchoolId,
                                SchoolName = k.UserSchool.SchoolName
                            },
                            UserCourse = new Course()
                            {
                                CourseId=k.UserCourse.CourseId,
                                CourseName=k.UserCourse.CourseName
                            },
                            Country=k.Country
                        }).FirstOrDefault();

Class:

public class UserDetail
{
    public int Id { get; set; }
    public int UserId { get; set; }
    public string FullName { get; set; }
    public string Link { get; set; }
    public bool? Verified { get; set; }

    public string Email { get; set; }

    public string About { get; set; }
    public School UserSchool { get; set; }
    public Course UserCourse { get; set; }

    public string Country { get; set; }

}
public class School
{
    public int SchoolId { get; set; }
    public string SchoolName { get; set; }
    public string Country { get; set; }
}
public class Course
{

    public int CourseId { get; set; }
    public string CourseName { get; set; }
    public School School { get; set; }
}

Any idea what went wrong??

Upvotes: 2

Views: 769

Answers (4)

Jack
Jack

Reputation: 2650

A great answer from @Yakimych is given below.

You cannot (and should not be able to) project onto a mapped entity. You can, however, project onto an annonymous type or onto a DTO:

public class ProductDTO
{
    public string Name { get; set; }
    // Other field you may need from the Product entity
}

And your method will return a List of DTO's.

public List<ProductDTO> GetProducts(int categoryID)
{
    return (from p in db.Products
            where p.CategoryID == categoryID
            select new ProductDTO { Name = p.Name }).ToList();
}

Mapped entities in EF basically represent database tables. If you project onto a mapped entity, what you basically do is partially load an entity, which is not a valid state. EF won't have any clue how to e.g. handle an update of such an entity in the future (the default behaviour would be probably overwriting the non-loaded fields with nulls or whatever you'll have in your object). This would be a dangerous operation, since you would risk losing some of your data in the DB, therefore it is not allowed to partially load entities (or project onto mapped entities) in EF.

For more details please go to the following link: The entity cannot be constructed in a LINQ to Entities query

Upvotes: 0

AD.Net
AD.Net

Reputation: 13399

I think it's because your entity has the same name of the object you're trying to create. Try renaming the object you want to return back. If you want to return the same type as your entity try the eager loading with .Include("relationshipname") feature.

Upvotes: 0

Rick B
Rick B

Reputation: 1166

This should give you what you want. It will load your objects as part of the query (and not rely on lazy loading).

UsersContext db = new UsersContext();
var userdata = db.UserDetails.Include(x => x.UserSchool)
                             .Include(x => x.UserCourse)
                             .Include(x => x.Country)
                             .Where(x => x.UserId == WebSecurity.CurrentUserId)
                             .FirstOrDefault();

Upvotes: 3

Despertar
Despertar

Reputation: 22352

It looks like it is due to how you are creating the complex properties School and Course in the middle of the query. It would be better to select the User (remove the select transformation), then use navigation properties to access those objects instead of building them manually. The navigation are meant for this as long as you have the proper relations built with foreign keys.

UsersContext db = new UsersContext();

var userdata = (from k in db.UserDetails
    where k.UserId == WebSecurity.CurrentUserId})
    .FirstOrDefault();

// access navigation properties which will perform the joins on your behalf
// this also provides for lazy loading which would make it more effecient. (it wont load the school object until you need to access it)

userdata.School
userdata.Course

MSDN article about navigation properties: http://msdn.microsoft.com/en-us/library/vstudio/bb738520(v=vs.100).aspx

Upvotes: 4

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