The entity cannot be constructed in a LINQ to Enti

Question

There is an entity type called product that is generated by entity framework. I have writen this query

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

The code below throws the following error :

"The entity or complex type Shop.Product cannot be constructed in a LINQ to Entities query"

var products = productRepository.GetProducts(1).Tolist();

But when I use select p instead of select new Product { Name = p.Name}; it works correctly.

How can I preform a custom select section?

Answer 1

Here is one way to do this without declaring aditional class:

public List<Product> GetProducts(int categoryID)
{
    var query = from p in db.Products
            where p.CategoryID == categoryID
            select new { Name = p.Name };
    var products = query.ToList().Select(r => new Product
    {
        Name = r.Name;
    }).ToList();

    return products;
}

However, this is only to be used if you want to combine multiple entities in a single entity. The above functionality (simple product to product mapping) is done like this:

public List<Product> GetProducts(int categoryID)
{
    var query = from p in db.Products
            where p.CategoryID == categoryID
            select p;
    var products = query.ToList();

    return products;
}

Answer 2

If you are using Entity framework, then try removing property from DbContext which uses your complex model as Entity I had same problem when mapping multiple model into a viewmodel named Entity

public DbSet<Entity> Entities { get; set; }

Removing the entry from DbContext fixed my error.

Answer 3

You can project into anonymous type, and then from it to model type

public IEnumerable<Product> GetProducts(int categoryID)
{
    return (from p in Context.Set<Product>()
            where p.CategoryID == categoryID
            select new { Name = p.Name }).ToList()
           .Select(x => new Product { Name = x.Name });
}

Edit: I am going to be a bit more specific since this question got a lot of attention.

You cannot project into model type directly (EF restriction), so there is no way around this. The only way is to project into anonymous type (1st iteration), and then to model type (2nd iteration).

Please also be aware that when you partially load entities in this manner, they cannot be updated, so they should remain detached, as they are.

I never did completely understand why this is not possible, and the answers on this thread do not give strong reasons against it (mostly speaking about partially loaded data). It is correct that in partially loaded state entity cannot be updated, but then, this entity would be detached, so accidental attempts to save them would not be possible.

Consider method I used above: we still have a partially loaded model entity as a result. This entity is detached.

Consider this (wish-to-exist) possible code:

return (from p in Context.Set<Product>()
        where p.CategoryID == categoryID
        select new Product { Name = p.Name }).AsNoTracking().ToList();

This could also result in a list of detached entities, so we would not need to make two iterations. A compiler would be smart to see that AsNoTracking() has been used, which will result in detached entities, so it could allow us to do this. If, however, AsNoTracking() was omitted, it could throw the same exception as it is throwing now, to warn us that we need to be specific enough about the result we want.

Answer 4

you can add AsEnumerable to your collection like the follow :

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

Answer 5

There is another way that I found works, you have to build a class that derives from your Product class and use that. For instance:

public class PseudoProduct : Product { }

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

Not sure if this is "allowed", but it works.

Answer 6

You can solve this by using Data Transfer Objects (DTO's).

These are a bit like viewmodels where you put in the properties you need and you can map them manually in your controller or by using third-party solutions like AutoMapper.

With DTO's you can :

• Make data serialisable (Json)
• Get rid of circular references
• Reduce networktraffic by leaving properties you don't need (viewmodelwise)
• Use objectflattening

I've been learning this in school this year and it's a very useful tool.