A 1-1 mapping means that you have an object with exactly one corresponding object. A good example of this would be a Product which can only be listed in one Category. Continuing our original example, here is our new Category object:

using System;
using Adapdev.Text;
using Elementary.Attributes;

namespace Elementary.Northwind.Samples.Attributes.ChildMappingOneToOneInferred
{
    [ElementaryClass]
	public class Category
	{
        [Persist(Column="categoryid")]
		public int Id;
        [Persist(Column="categoryname")]
		public string Name;
        [Persist]
		public string Description;
        [Persist]
		public byte[] Picture;

		public override string ToString() 
		{
			return StringUtil.ToString(this);
		}
	}
}

And our modified Product object:

using System;
using Adapdev.Text;
using Elementary.Attributes;

namespace Elementary.Northwind.Samples.Attributes.ChildMappingOneToOneInferred
{   
    [ElementaryClass]
	public class Product
	{
        [Persist]
		public int CategoryID = 0;
        [Persist]
		public int ProductID = 0;
        [Persist]
		public string ProductName = "";
        [Persist]
		public int SupplierID = 0;
        [HasOne]
		public Category Category = null;

		public override string ToString() 
		{
			return StringUtil.ToString(this);
		}
	}
}

This means that, when we load a Product, we can look at it's Category property to find out more about the category that it's in. For example:

Product p = new Product();
... // set values
Console.WriteLine(p.Category.Name); // outputs the category name

Lastly, let's see how we can access this child property. Create the following Test class:

using System;
using System.Collections.Generic;
using System.Transactions;
using Elementary;
using Elementary.Northwind.Samples;
using NUnit.Framework;

namespace Elementary.Northwind.Samples.Attributes.ChildMappingOneToOneGenericInferred
{
	/// <summary>
	/// Summary description for Test.
	/// </summary>
	[TestFixture]
	public class Test
	{
		[TestFixtureSetUp]
		public void SetUp()
		{
			Elementary.Clear();
			Elementary.LoadAssembly("Elementary.Northwind.Samples.dll");
		}
		
		[Test]
		public void GetObjectWithChildren()
		{
			IObjectMapper<Product> mapper = Elementary.GetIObjectMapper<Product>();
			
			// Retrieve product #1
			Product product = mapper.GetObjectWithChildren(1);
			
			Console.WriteLine(product);
		}
	}
}

When you run the test, you should see the following output:

Properties for: Product
	CategoryID(System.Int32): 1
	ProductID(System.Int32): 1
	ProductName(System.String): Chai
	SupplierID(System.Int32): 1
	Category(Elementary.Northwind.Samples.Attributes.ChildMappingOneToOneGeneric.Category): 
Properties for: Category
	Id(System.Int32): 1
	Name(System.String): Beverages
	Description(System.String): Soft drinks, coffees, teas, beers, and ales
	Picture(System.Byte[]): System.Byte[]

As you can see, Elementary not only retrieved the Product information, but also the information for all child properties (in this case, we've only defined the Category property).

You can still call the GetObject method, and it will ignore child properties.

		[Test]
		public void GetObject()
		{
			IObjectMapper<Product> mapper = Elementary.GetIObjectMapper<Product>();
			
			// Retrieve product #1
			Product product = mapper.GetObject(1);
			
			Console.WriteLine(product);
		}

Output:
Properties for: Product
	CategoryID(System.Int32): 1
	ProductID(System.Int32): 1
	ProductName(System.String): Chai
	SupplierID(System.Int32): 1
	Category(Elementary.Northwind.Samples.ChildMappingOneToOneGeneric.Category): 

You can also retrieve ALL Products and their child properties

		[Test]
		public void GetCollectionWithChildren()
		{
			IObjectMapper<Product> mapper = Elementary.GetIObjectMapper<Product>();

			List<Product> al = mapper.GetCollectionWithChildren();
			
			Assert.IsTrue(al.Count > 0, "No products loaded.");
			Assert.AreEqual(77, al.Count, "Not all products were loaded.");
			
			Console.WriteLine("Loaded {0} products.", al.Count);
			Console.WriteLine("\nFirst product:\n{0}", al[0]);
		}

Output:
Loaded 77 products.

First product:
Properties for: Product
	CategoryID(System.Int32): 1
	ProductID(System.Int32): 1
	ProductName(System.String): Chai
	SupplierID(System.Int32): 1
	Category(Elementary.Northwind.Samples.Attributes.ChildMappingOneToOneGeneric.Category): 
Properties for: Category
	Id(System.Int32): 1
	Name(System.String): Beverages
	Description(System.String): Soft drinks, coffees, teas, beers, and ales
	Picture(System.Byte[]): System.Byte[]

You can also save an object and its children:

		[Test]
		public void SaveWithChildren()
		{
			// Run in a trans so it doesn't commit
			using(TransactionScope trans = new TransactionScope())
			{
				IObjectMapper mapper = Elementary.GetIObjectMapper<Product>();
				Product e = new Product();
				e.ProductName = "Johns Sandwiches";
				
				Category cat = new Category();
				cat.Name = "J";
				cat.Description = "Some Description";

				e.Category = cat;

				mapper.SaveWithChildren(e);
				mapper.Delete(e);
			}
		}

SELECT CategoryId, CategoryName, Description, Picture
FROM Categories
WHERE Categories.CategoryId = @categoryId
' childColumn maps to Categories.CategoryId
' parentColumn maps to Products.CategoryId, which results in the value for @categoryId 

using System;
using Adapdev.Text;
using Elementary.Attributes;

namespace Elementary.Northwind.Samples.Attributes.ChildMappingOneToOneInferred
{   
    [ElementaryClass]
	public class Product
	{
        [Persist]
		public int CategoryID = 0;
        [Persist]
		public int ProductID = 0;
        [Persist]
		public string ProductName = "";
        [Persist]
		public int SupplierID = 0;
        [HasOne(ChildColumn="categoryid", ParentColumn="categoryid")]
		public Category Category = null;

		public override string ToString() 
		{
			return StringUtil.ToString(this);
		}
	}
}