C# – NHibernate POCO / Basics

cmappingnhibernatepoco

I've formerly used L2S and am looking at using NHib along with the Sharp Architecture on a project. I've started prototyping and come up against the first issue which i have no idea how to google for.

Given a POCO with some simple properties, and one reference property (Category – class not shown here):

public class Post
{
  public Post()
  {
    this.DateCreated = DateTime.Now;
  }

  public virtual string Title {get;set;}
  public virtual DateCreated {get;set;}
  public virtual Category {get;set;}
}

This is handy if i want to say produce a summary of posts in tabular format – if i want a column named "Category" and i want to show the category title i can simply use Post.Category.Title.

However a user creates a new post (they select a category from a drop down list or similar). They click submit, and i new up a Post object. However before i can persist the new Post, i have to retrieve an instance of the Category (by id) to assign to the Category property?

How to get the best of both worlds? If i update the Post POCO, making the Category an int, I've made it simpler to create new instances. But harder for some rendering code which will now have to resolve the category name given the Id?

I feel i'm missing some basic concept here?

I know with Linq to Sql given a schema where the Post table had an integer foriegn key column named CategoryId, would generate me both the underlying table column (CategoryId) and and EntitySet which contained the foreign key row.

How to acheive similar in NHibernate? How is this generally managed?

Thanks

Best Answer

Associated entities should be mapped as entities - e.g. Category, not int CategoryId.

When you're creating a new Post, if you don't have an actual Category instance (and why not? if you're picking categories from a list you've already loaded them) but only have its id you can use a Load() method to get a persistent Category instance for given id without actually hitting a database:

post.Category = (Category) sesssion.Load(typeof(Category), categoryId);

You can also use the generic version:

post.Category = session.Load<Category>(categoryId);

Related Solutions

‘POCO’ definition

"Plain Old C# Object"

Just a normal class, no attributes describing infrastructure concerns or other responsibilities that your domain objects shouldn't have.

EDIT - as other answers have stated, it is technically "Plain Old CLR Object" but I, like David Arno comments, prefer "Plain Old Class Object" to avoid ties to specific languages or technologies.

TO CLARIFY: In other words, they don’t derive from some special base class, nor do they return any special types for their properties.

See below for an example of each.

Example of a POCO:

public class Person
{
    public string Name { get; set; }

    public int Age { get; set; }
}

Example of something that isn’t a POCO:

public class PersonComponent : System.ComponentModel.Component
{
    [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
    public string Name { get; set; }

    public int Age { get; set; }
}

The example above both inherits from a special class to give it additional behavior as well as uses a custom attribute to change behavior… the same properties exist on both classes, but one is not just a plain old object anymore.

Sql – How do i populate a POCO (child) IList property from a Linq2Sql query

If your LINQ to SQL classes have a relationship between them, then an "Answers" property should have been generated on the "Question" LINQ to SQL class. So you should be able to simply do this:

return from q in _db.Questions
       select new Question
       {
           Title = q.Title,
           Answers = q.Answers.ToList(),
       }

You may be able to omit the call to ToList() - I'm not sure what type LINQ to SQL uses for generated related rows (I believe it's IQueryable<T>).

To force the Answers property to be populated up front, rather than firing off another query for each question, you can use the DataLoadOptions class. Essentially you tell LINQ to SQL to load answers any time you query questions (using the LoadWith method - see the MSDN documentation).

Edit

You're right - since your Question.Answers property is a list of your own POCO, assigning it with q.Answers isn't the way to go. This might work better:

return from q in _db.Questions
       select new Question
       {
           Title = q.Title,
           Answers = (from a in q.Answers
                     select new Answer { ... }).ToList(),
       }

Best Answer

Related Solutions

‘POCO’ definition

Sql – How do i populate a POCO (child) IList property from a Linq2Sql query

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