Question

This is related to my previous question: Coalesce fields in a .net MVC 4 model without getting "Only initializers, entity members, and entity navigation properties are supported" from LINQ

After adding a static LINQ expression to my model I am able to use that expression in method-style LINQ statements when selecting from an enumerable of that model class directly (as in the selected answer to the above question, and the example below). Now I must try to use that expression from a related model and am once again exceeding my knowledge of LINQ and EF.

The simplified models involved are:

//The model class for Items for sale in the system
public class Item
{
    [Key, Column(Order = 0)]
    public String CompanyId { get; set; }

    [Key, Column(Order = 1)]
    public String ItemId { get; set; }

    public Int32 CategoryId { get; set; }

    [ForeignKey("CategoryId")]
    public virtual GlobalCategory GlobalCategory { get; set; }

    [ForeignKey("CompanyId, CategoryId")]
    public virtual CompanyCategory CompanyCategory { get; set; }

    //LINQ Expression to coalesce a company specific category alias
    //with the default global category when no alias exists
    public static Expression<Func<Item, String>> linqCategoryName
    {
        get
        {
            return i => (i.CompanyCategory.CategoryAlias == null || 
                         i.CompanyCategory.CategoryAlias == "") ?
                         i.GlobalCategory.CategoryName :
                         i.CompanyCategory.CategoryAlias;
        }
    }
}

and

//The model class for Item Relationships, used to relate two items 
//(a "Parent Item" and "Related Item") such as one is a supply for to the other
public class ItemRelationship
{
    [Key]
    public Int32 RelationshipId { get; set; }

    public Int32 RelationshipTypeId { get; set; }

    [ForeignKey("RelationshipTypeId")]
    public virtual RelationshipType RelationshipType { get; set; }

    public String CompanyId{ get; set; }

    public String ItemId{ get; set; }

    [ForeignKey("CompanyId, ItemId")]
    public virtual Item ParentItem { get; set; }

    public String RelatedCompanyId{ get; set; }

    public String RelatedItemId{ get; set; }

    [ForeignKey("RelatedCompanyId, RelatedItemId")]
    public virtual Item RelatedItem { get; set; }
}

The LINQ Expression member of the Item class can be used successfully with this kind of LINQ statement:

var categories = usscodb.Items.Where(i => i.CompanyId = siteCompanyId)
                               .OrderBy(Item.linqCategoryName)
                               .Select(Item.linqCategoryName)
                               .Distinct();

However, I am unsure how to use the static Item.linqCategoryName in a situation such as this:

var categories = db.ItemRelationships
                .Where(ir => ir.RelationshipType.RelationshipTypeName == "Supply"
                    && ir.ParentItem.Active
                    && ir.RelatedItem.Active
                    && ir.ParentItem.CompanyId == siteCompanyId
                    && ir.RelatedItem.CompanyId == siteCompanyId)

                //the linqCategoryName member is not available for use this way
                .Select(ir.ParentItem.linqCategoryName)
                .Distinct();

I think I’m close here. I’ve tried explicitly joining the Item Relationship to Item using the Parent Item’s keys and then attempting to use the static LINQ expression, but I think since there are two Items present in the Item Relationship LINQ can’t determine which I want. I’m not sure how to explicitly tell LINQ from which Item I want to use the static method.

Answer 1

Try this:

.Select(ir => ir.ParentItem)
.Select(Item.linqCategoryName)