The entity cannot be constructed in a LINQ to Entities query

Question

There is an entity type called Product that is generated by entity framework. I have written this query

public IQueryable GetProdu         


        

Answer 1

It won't let you map back onto Product since that is your table you are querying. You need an anonymous function, then you can add it to a ViewModel, and add each ViewModel to a List<MyViewModel> and return these. It's a slight digression, but I include caveats about handling nullable dates because these are a pain in the behind to deal with, just in case you have any. This is how I handled it.

Hopefully you have a ProductViewModel:

public class ProductViewModel
{
    [Key]
    public string ID { get; set; }
    public string Name { get; set; }
}

I have dependency injection/repository framework where I call a function to grab my data. Using your post as an example, in your Controller function call, it would look like this:

int categoryID = 1;
var prods = repository.GetProducts(categoryID);

In the repository class:

public IEnumerable<ProductViewModel> GetProducts(int categoryID)
{
   List<ProductViewModel> lstPVM = new List<ProductViewModel>();

   var anonymousObjResult = from p in db.Products
                            where p.CategoryID == categoryID 
                            select new
                            {
                                CatID = p.CategoryID,
                                Name = p.Name
                            };

        // NOTE: If you have any dates that are nullable and null, you'll need to
        // take care of that:  ClosedDate = (DateTime?)p.ClosedDate ?? DateTime.Now

        // If you want a particular date, you have to define a DateTime variable,
        // assign your value to it, then replace DateTime.Now with that variable. You
        // cannot call a DateTime.Parse there, unfortunately. 
        // Using 
        //    new Date("1","1","1800"); 
        // works, though. (I add a particular date so I can edit it out later.)

        // I do this foreach below so I can return a List<ProductViewModel>. 
        // You could do: return anonymousObjResult.ToList(); here
        // but it's not as clean and is an anonymous type instead of defined
        // by a ViewModel where you can control the individual field types

        foreach (var a in anonymousObjResult)
        {                
            ProductViewModel pvm = new ProductViewModel();
            pvm.ID = a.CatID;  
            pvm.Name = a.Name;
            lstPVM.Add(rvm);
        }

        // Obviously you will just have ONE item there, but I built it 
        // like this so you could bring back the whole table, if you wanted
        // to remove your Where clause, above.

        return lstPVM;
    }

Back in the controller, you do:

 List<ProductViewModel> lstProd = new List<ProductViewModel>();

 if (prods != null) 
 {
    // For setting the dates back to nulls, I'm looking for this value:
    // DateTime stdDate = DateTime.Parse("01/01/1800");

    foreach (var a in prods)
    {
        ProductViewModel o_prod = new ReportViewModel();
        o_prod.ID = a.ID;
        o_prod.Name = a.Name;
       // o_prod.ClosedDate = a.ClosedDate == stdDate ? null : a.ClosedDate;
        lstProd.Add(o_prod);
    }
}
return View(lstProd);  // use this in your View as:   @model IEnumerable<ProductViewModel>

Answer 2

Another simple way :)

public IQueryable<Product> GetProducts(int categoryID)
{
    var productList = db.Products
        .Where(p => p.CategoryID == categoryID)
        .Select(item => 
            new Product
            {
                Name = item.Name
            })
        .ToList()
        .AsQueryable(); // actually it's not useful after "ToList()" :D

    return productList;
}

Answer 3

You can use this and it should be working --> You must use toList before making the new list using select:

db.Products
    .where(x=>x.CategoryID == categoryID).ToList()
    .select(x=>new Product { Name = p.Name}).ToList(); 

Answer 4

In response to the other question which was marked as duplicate (see here) I figured out a quick and easy solution based on the answer of Soren:

data.Tasks.AddRange(
    data.Task.AsEnumerable().Select(t => new Task{
        creator_id   = t.ID,
        start_date   = t.Incident.DateOpened,
        end_date     = t.Incident.DateCLosed,
        product_code = t.Incident.ProductCode
        // so on...
    })
);
data.SaveChanges();

Note: This solution only works if you have a navigation property (foreign key) on the Task class (here called 'Incident'). If you don't have that, you can just use one of the other posted solutions with "AsQueryable()".

Answer 5

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.

Answer 6

only add AsEnumerable() :

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