Map One to One Relationship at Run-Time

Question

i'm trying to upgrade an old CMS to use NHibernate and can't deter from the original database structure much. Here is the bit which is causing an issue. Say i have the following 2 tables:

Articles: 
- Id (PK, Identity)
- Title 
- Content 

Meta: 
- ArticleId (PK, FK to Articles)
- Description 
- Keywords 

I have created the following classes:

public class Article { 
  public virtual int Id { get; set; } 
  public virtual string Title { get; set; } 
  public virtual string Content { get; set; } 
} 

public class Meta : IComponent { 
  public virtual string Description { get; set; } 
  public virtual string Keywords { get; set; } 
}

public interface IComponent {
}

Usually the Meta would normally be mapped as a component (or a one to one relationship) property on the Article class. However in the application i'm building an admin can enable/disable the components that apply to articles. Also i'd like them to extend the application to add their own components without touching the Article class.

For them reasons i can't add a property against the Article class. Now ideally in my code i'd like to be able to say:

var articles = session.Query<Article>()
    .Fetch(a = a.Component<Meta>())
    .Where(a => a.Component<Meta>().Keywords.Contains("Some Word"))
    .ToList();

// This wouldn't generate an extra SQL statement
var keywords = articles[0].Component<Meta>().Keywords;

Which would generate the following SQL (or similar):

SELECT * FROM Articles INNER JOIN Meta ON Articles.Id = Meta.ArticleId WHERE Meta.Keywords LIKE '%Some Word%'

Is it possible to map the Component method so that it does an inner join to get the Meta. The concept seems pretty simple but i don't have a clue where begin. I'd really appreciate the help.

Thanks

Answer 1

In your NHibernate mapping file, you can specify the fetch type. The spec for the one-to-one xml is located in the NHibernate Documentation. Notice that number 5 has an option of Join and Select and it defaults to select.

Answer 2

Given this:

public class Article
{
    public virtual int ArticleId { get; set; }
    public virtual string Title { get; set; }
    public virtual string Content { get; set; }
}


public class Meta : IComponent
{
    public virtual Article Article { get; set; }

    public virtual int MetaId { get; set; }
    public virtual string Description { get; set; }
    public virtual string Keywords { get; set; }
}

AFAIK, you cannot Fetch something that isn't part of an entity. So from your example, it's not possible to fetch Meta from the Article entity.

So if you want to fetch the other info of an Article, you just have to join Article to them, then project the complete data in your Linq, example:

var articles =
        from a in s.Query<Article>()
        join m in s.Query<Meta>() on a equals m.Article
        where m.Keywords.Contains("Some Word")
        select new { a, m };

foreach(var x in articles)
    Console.WriteLine("{0} {1}", x.a.Title, x.m.Description);

Resulting query:

select *
from [Article] article0_, [Meta] meta1_ 
where meta1_.ArticleId = article0_.ArticleId 
    and meta1_.Keywords like '%Some Word%'

Another approach, start from Meta, then fetch the Article; on query, this will join the Article immediately, i.e. no lazy loading:

var artB =
        from m in s.Query<Meta>().Fetch(x => x.Article)
        where m.Keywords.Contains("Some Word")
        select m;

foreach (var x in artB)
    Console.WriteLine("{0} {1}", x.Article.Title, x.Description);

Resulting query:

select *
from [Meta] meta0_ 
left outer join [Article] article1_ on meta0_.ArticleId = article1_.ArticleId 
where meta0_.Keywords like '%Some Word%'

To keep an Article having only one Meta, put a Unique on Meta's reference:

create table Article
(
ArticleId int identity(1,1) not null primary key,
Title varchar(100) not null,
Content varchar(100) not null
);

create table Meta
(
    -- this prevents an Article having two Meta
ArticleId int not null references Article(ArticleId) unique, 

MetaId int identity(1,1) not null primary key,

Description varchar(100) not null,
Keywords varchar(100) not null
);

insert into Article(Title,Content) values('Great','Yeah')

insert into Meta(ArticleId, Description, Keywords) values(1,'Oh','Some Word');

Answer 3

Would the following solution be of interest?

You can make a protected mapping to your components and access these from this public generic method.

This way you can choose to eager/lazy load your components.

public class Article
{
    protected virtual ICollection<IComponent> Components { get; set; }

    public virtual T Component<T>() where T : IComponent
    {
        return Components.FirstOrDefault(c=>c.Type==typeof(T));
    }
}