Entity Framework - Inheritance

Question

I am just starting with Entity Framework and have the following problem:

I have a Site entity which contains a navigation property Paragraphs.

I have multiple entities (i.e. ImageParagraph, LinkListParagraph) that should be inherited form Paragraph.

I would like to query for a Site object and access its Paragraphs and work with them depending on their concrete type (i.e. ImageParagraph, LinkListParagraph).

It would work with a Table Per Hierarchy approach (with conditions), but then i would end up in a very dirty solution. Querying for the concrete type based on the Paragraph ID would work, but I hope there is a way better solution.

I want to query for a Site and show some Site specific data and the data in the Paragraphs (ImageParagraph, LinkListParagraph). I do not know how to set up the mapping in a way that it is possible to retrieve ImageParagraph, LinkListParagraph objects directly over the Paragraphs navigation property.

How would you solve this problem?

ER-diagram: http://img443.imageshack.us/img443/46/69863714ks0.jpg

alt text http://img443.imageshack.us/img443/46/69863714ks0.jpg

Marc Gravell · Accepted Answer · 2009-02-12 09:59:38Z

2

If you want to work with them based on their concrete type - that sounds like a case for polymorphism... can you (in a partial class) add a few methods?

partial class Paragraph {
    public abstract void Foo();
}
partial class ImageParagraph {
    public override void Foo() {/*code*/}
}
partial class LinkListParagraph {
    public override void Foo() {/*code*/}
}

Otherwise, if you want to filter the set, you can use the OfType extension method - i.e.

foreach(var imgPara in obj.Paragraphs.OfType<ImageParagraph>())
{ ... }

Perhaps you could add properties for the above too (in the parent object) - i.e.

partial class Site {
    public IQueryable<ImageParagraph> ImageParagraphs
    { get {return Paragraphs.OfType<ImageParagraph>(); }}

    public IQueryable<LinkListParagraph> LinkListParagraphs
    { get {return Paragraphs.OfType<LinkListParagraph>(); }}
}

answered Feb 12, 2009 at 9:59

Marc Gravell

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1 Comment

chbu Over a year ago

Thanks.My question was unclear.I want to do: var site =context.Site.Include("Paragraphs").Where(c => c.SiteID == 1).First(); foreach(var paragraph in site.Paragraphs) { do something }; My problem is that I do not know the mapping in the EDM.(like with Table per Hierarchy with conditions)

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