Question

I have an Abstract type called Product, and five “Types” that inherit from Product in a table per type hierarchy fashion as below:

I want to get all of the information for all of the Products, including a smattering of properties from the different objects that inherit from products to project them into a new class for use in an MVC web page. My linq query is below:

     //Return the required products
    var model = from p in Product.Products
                where p.archive == false && ((Prod_ID == 0) || (p.ID == Prod_ID))
                select new SearchViewModel 
                    {
                        ID = p.ID,
                        lend_name = p.Lender.lend_name,
                        pDes_rate = p.pDes_rate,
                        pDes_details = p.pDes_details,
                        pDes_totTerm = p.pDes_totTerm,
                        pDes_APR = p.pDes_APR,
                        pDes_revDesc = p.pDes_revDesc,
                        pMax_desc = p.pMax_desc,
                        dDipNeeded = p.dDipNeeded,
                        dAppNeeded = p.dAppNeeded,      

                        CalcFields = new DAL.SearchCalcFields
                        {
                            pDes_type = p.pDes_type,
                            pDes_rate = p.pDes_rate,
                            pTFi_fixedRate = p.pTFi_fixedRate 
                        }
                    }

The problem I have is accessing the p.pTFi_fixedRate, this is not returned with the Products collection of entities as it is in the super type of Fixed. How do I return the “super” type of Products (Fixed) properties using Linq and the Entity Framework. I actually need to return some fields from all the different supertypes (Disc, Track, etc) for use in calculations. Should I return these as separate Linq queries checking the type of “Product” that is returned?

Answer 1

This is a really good question. I’ve had a look in the Julie Lerman book and scouted around the internet and I can’t see an elegant answer.

If it were me I would create a data transfer object will all the properties of the types and then have a separate query for each type and then union them all up. I would insert blanks into the DTO properies where the properties aren’t relevant to that type. Then I would hope that the EF engine makes a reasonable stab at creating decent SQL.

Example

var results = (from p in context.Products.OfType<Disc>
        select new ProductDTO {basefield1 = p.val1, discField=p.val2, fixedField=""})
         .Union(
        from p in context.Products.OfType<Fixed>
        select new ProductDTO {basefield1 = p.val1, discField="", fixedField=p.val2});

But that can’t be the best answer can it. Is there any others?