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Object-oriented languages like C# make it easy to define new types by writing new classes. Indeed, this is a fundamental part of the definition of object orientation. If you were limited to the predefined built-in NHibernate mapping types when declaring properties of persistent classes, you d lose much of C# s expressiveness. Furthermore, your domain model implementation would be tightly coupled to the physical data model, because new type conversions would be impossible. In order to avoid that, NHibernate provides a powerful feature called custom mapping types. NHibernate provides two user-friendly interfaces that applications can use when defining new mapping types. The first is NHibernate.UserTypes.IUserType. IUserType is suitable for most simple cases and even for some more complex problems. Let s use it in a simple scenario. The Bid class defines an Amount property, and the Item class defines an InitialPrice property; both are monetary values. So far, you ve only used a System.Double to represent the value, mapped with Double to a single DbType.Double column. Suppose you want to support multiple currencies in the auction application and that you have to refactor the existing domain model for this change. One way to implement this change is to add new properties to Bid and Item: AmountCurrency and InitialPriceCurrency. You can then map these new properties to additional VARCHAR columns with the built-in String mapping type. If you have currency stored in 100 places, this is a lot of changes. We hope you never use this approach!
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Instead, you should create a MonetaryAmount class that encapsulates both currency and amount. This is a class of the domain model and doesn t have any dependency on NHibernate interfaces:
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[Serializable] public class MonetaryAmount { private readonly double value; private readonly string currency;
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public MonetaryAmount(double value, string currency) { this.value = value; this.currency = currency; } public double Value { get { return value; } } public string Currency { get { return currency; } } public override bool Equals(object obj) { ... } public override int GetHashCode() { ... } }
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You also make life simpler by making MonetaryAmount an immutable class, meaning it can t be changed after it s instantiated. You have to implement Equals() and GetHashCode() to complete the class but there is nothing special to consider here aside from the facts that they must be consistent, and GetHashCode() should return mostly unique numbers. You ll use this new MonetaryAmount to replace the Double, as defined on the InitialPrice property for Item. You ll benefit from using this new class in other places, such as the Bid.Amount. The next challenge is in mapping the new MonetaryAmount properties to the database. Suppose you re working with a legacy database that contains all monetary amounts in USD. The new class means the application code is no longer restricted to a single currency, but it will take time for the database team to make the changes. Until this happens, you d like to store just the Amount property of MonetaryAmount to the database. Because you can t store the currency yet, you ll convert all Amounts to USD before you save them and from USD when you load them. The first step in handling this is to tell NHibernate how to handle the MonetaryAmount type. To do so, you create a MonetaryAmountUserType class that implements the NHibernate interface IUserType. The custom mapping type is shown in listing 6.1.
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Listing 6.1 Custom mapping type for monetary amounts in USD
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using System; using System.Data; using NHibernate.UserTypes; public class MonetaryAmountUserType : IUserType { private static readonly NHibernate.SqlTypes.SqlType[] SQL_TYPES = { NHibernateUtil.Double.SqlType }; public NHibernate.SqlTypes.SqlType[] SqlTypes { get { return SQL_TYPES; } } public Type ReturnedType { get { return typeof(MonetaryAmount); } } public new bool Equals( object x, object y ) { if ( object.ReferenceEquals(x,y) ) return true; if (x == null || y == null) return false; return x.Equals(y); } public object DeepCopy(object value) { return value; } public bool IsMutable { get { return false; } }
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