Developer Safe Bidirectional Relationships with NHibernate

Question

I know how to map bidirectional relationships with NHibernate, but does anyone know an effective method to make them "safe for developers."

What I mean is that as soon as one end of the relationship is established, the other end must be set, for example:

[Test] public void TestBidirectionalRelationships_WhenAddingOptionToProduct() { var product = new Product(); var productOption = new ProductOption(); product.AddOption(productOption); Assert.That(product.Options.Contains(productOption)); Assert.That(productOption.Product, Is.EqualTo(product); } [Test] public void TestBidirectionalRelationships_WhenSettingProductOnOption() { /* Alternatively, productOption.Product would have no public setter */ var product = new Product(); var productOption = new ProductOption(); productOption.Product = product; Assert.That(product.Options.Contains(productOption)); Assert.That(productOption.Product, Is.EqualTo(product); } 

How to achieve this without making the code terribly complicated, as well as supporting NHibernate?

Answer 1

One side:

 class Product { public Product() { _Options = new List<ProductOption>(); } ICollection<ProductOption> _Options; public virtual IEnumerable<ProductOption> ProductOptions { get { return _Options.Select(x => x); } } public virtual AddOption(ProductOption option) { option.Product = this; } protected internal virtual AddOptionInternal(ProductOption option) { _Options.Add(option); } } 

Many sides:

 class ProductOption { Product _Product; public virtual Product Product { get { return _Product; } set { _Product = value; _Product.AddOptionInternal(this); } } } 

Mapping:

 <class name="Product"> ... <bag name="Options" access="field.pascalcase-underscore"> ... <class name="ProductOption"> ... <many-to-one name="Product" access="field.pascalcase-underscore"/> 

Removing options from the collection (on both sides) remains as an exercise :-)

Answer 2

Diego's solution is excellent, but I prefer this template:

One side:

 public class Product { private IList<ProductOption> _options; public Product() { _options = new List<ProductOption>(); } public virtual IEnumerable<ProductOption> ProductOptions { get { return _options; } } public virtual AddOption(ProductOption option) { // equality must be overridden for this to work // Check contains to break out of endless loop if (!_options.Contains(options)) { _options.Add(option); option.Product = this; } } } 

Many sides:

 public class ProductOption { Product _product; public virtual Product Product { get { return _product; } set { _product = value; _product.AddOption(this); } } } 

Answer 3

Check out this article specifically addressing this issue. The guidance here is very valuable in order to keep such relationships clean by encapsulating the collection operations in your domain model.