Polymorphism joins against all sub types when querying

[nfplee]

My application was previously structured like so:

public abstract class Category : Entity {
    public virtual Category? Parent { get; set; }
    ...
}

public abstract class ContentCategory : Category { ... }
public class BlogPostCategory : ContentCategory { ... }
public class PageCategory : ContentCategory { ... }

public abstract class ContentItem : Entity {
    public virtual ContentCategory Category { get; set; }
    ...
}

public BlogPost : ContentItem { ... }
public Page : ContentItem { ... }

Where each type (apart from Entity) is mapped to a table in the database.

However if I tried retrieving the BlogPost or Page and fetching the Category, like so:

await session.Query<BlogBlog>().Fetch(p => p.Category).ToListAsync();

I noticed that the SQL does a join against the PageCategory aswell as the BlogPostCategory. I understand this is because the Category property is of type ContentCategory and therefore it doesn't understand that it should be a BlogPostCategory. I decided to take the Category property out of the ContentItem type and move it to under the BlogPost and Page types, like so:

public BlogPost : ContentItem {
    public virtual BlogPostCategory Category { get; set; }
    ...
}

public Page : ContentItem {
    public virtual PageCategory Category { get; set; }
    ...
}

This fixed the issue. However if I now say:

await session.Query<BlogBlogCategory>().Fetch(c => c.Parent).ToListAsync();

The same problem exists as the Parent property is of type Category and not BlogPostCategory. I don't like the idea of moving the Parent property out of the Category type as I did to solve the previous issue. I also can't see how I can solve this with generics because if I make the Category type generic (to pass the Parent type) then I won't be able to map the Category type. Finally after reading up on this I tried adding Polymorphism(PolymorphismType.Explicit) to my Category mapping but this didn't seem to change anything.

I'd really appreciate any advice.

[bahusoid]

Mappings?

[nfplee]

Anything type which inherits from Entity is mapped as a class mapping and all other types use a joined subclass mapping. Here's a couple of examples:

public class ContentItemMapping : ClassMapping<ContentItem> {
    public ContentItemMapping() {
        Table("ContentItems");
        Id(x => x.Id, m => m.Generator(NHibernate.Mapping.ByCode.Generators.Identity));
        ...
    }
}

public class BlogPostMapping : JoinedSubclassMapping<BlogPost> {
    public BlogPostMapping() {
        Table("BlogPosts");
        Key(k => k.Column("Id"));
        ManyToOne(x => x.Category);
        ...
    }
}

[bahusoid]

I would avoid subclass mapping if possible.

And regarding your issues. You can keep ContentItem.Category in base class as ContentCategory. But instead of mapping it in ContentItemMapping - map it in concrete subclass mappings with specifying concrete type inside ManyToOne call.

public class ContentItemMapping : ClassMapping<ContentItem> {
    public ContentItemMapping() {
        Table("ContentItems");
        Id(x => x.Id, m => m.Generator(NHibernate.Mapping.ByCode.Generators.Identity));
        // ManyToOne(x => x.Category); <-- Remove Category mapping
        ...
    }
}

public class BlogPostMapping : JoinedSubclassMapping<BlogPost> {
    public BlogPostMapping() {
        Table("BlogPosts");
        Key(k => k.Column("Id"));
        ManyToOne(x => x.Category, m => m.Class(typeof(BlogPostCategory ))); <-- specify concrete type via Class method
        ...
    }

public abstract class ContentItem : Entity {
    public virtual ContentCategory Category { get; set; }
    ...
}

Same applies for Parent property.

[nfplee]

Thanks, I never even thought of not mapping it in the base class. I think I'll keep my ContentItem as is, since I think making the category optional against the content types is a good thing. However this should point me in the right direction for the Parent property.