To get to the solution we recommend, you need to understand the notion of a business key. in Visual C#

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To get to the solution we recommend, you need to understand the notion of a business key.
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USING BUSINESS KEY EQUALITY
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A business key is a property, or some combination of properties, that is unique for each instance with the same database identity. Essentially, it s the natural key you d use if you
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The persistence lifecycle
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weren t using a surrogate key. Unlike a natural primary key, it isn t an absolute requirement that the business key never change as long as it changes rarely, that s enough. We argue that every entity should have a business key, even if it includes all properties of the class (this would be appropriate for some immutable classes). The business key is what the user thinks of as uniquely identifying a particular record, whereas the surrogate key is what the application and database use. Business key equality means that the Equals() method compares only the properties that form the business key. This is a perfect solution that avoids all the problems described earlier. The only downside is that it requires extra thought to identify the correct business key in the first place. But this effort is required anyway; it s important to identify any unique keys if you want your database to help ensure data integrity via constraint checking. For the User class, username is a great candidate business key. It s never null, it s unique, and it changes rarely (if ever):
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public class User { //... public override bool Equals(object other) { if (object.ReferenceEquals(this,other)) return true; if ( !(other is User) ) return false; User that = (User) other; return this.Username == that.Username ); } public override int GetHashCode() { return Username.GetHashCode(); } }
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For some other classes, the business key may be more complex, consisting of a combination of properties. For example, candidate business keys for the Bid class are the item ID together with the bid amount, and the item ID together with the date and time of the bid. A good business key for the BillingDetails abstract class is the number together with the type (subclass) of billing details. Notice that it s almost never correct to override Equals() on a subclass and include another property in the comparison. It s tricky to satisfy the requirements that equality be both symmetric and transitive in this case; and, more important, the business key wouldn t correspond to any well-defined candidate natural key in the database (subclass properties may be mapped to a different table). You may have noticed that the Equals() and GetHashCode() methods always access the properties of the other object via the getter properties. This is important, because the object instance passed as other might be a proxy object, not the actual instance that holds the persistent state. This is one point where NHibernate isn t completely transparent, but it s a good practice to use properties instead of direct instance variable access anyway. Finally, take care when you re modifying the value of the business key properties; don t change the value while the domain object is in a set.
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So far, we ve talked about how the persistence manager behaves when working with instances that are transient, persistent, or detached. We ve also discussed issues of scope, and the importance of equality and identity. It s now time to take a closer look at the persistence manager and explore the NHibernate ISession API in greater detail. We come back to detached objects in more detail in the next chapter.
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Any transparent persistence tool like NHibernate will include some form of persistence manager API, which usually provides services for the following:
Performing basic CRUD operations Executing queries Controlling transactions Managing the transaction-level cache
The persistence manager can be exposed by several different interfaces (in the case of NHibernate, they include ISession, IQuery, ICriteria, and ITransaction). Under the covers, the implementations of these interfaces are coupled tightly. The central interface between the application and NHibernate is ISession; it s your starting point for all the operations just listed. For most of the rest of this book, we refer to the persistence manager and the session interchangeably; this is consistent with usage in the NHibernate community. How do you start using the session At the beginning of a unit of work, you create an instance of ISession using the application s ISessionFactory. The application may have multiple ISessionFactorys if it accesses multiple datasources. But you should never create a new ISessionFactory just to service a particular request creation of an ISessionFactory is extremely expensive. On the other hand, ISession creation is extremely inexpensive; the ISession doesn t even obtain an ADO.NET IDbConnection until a connection is required. After opening a new session, you use it to load and save objects. Note that this section explains some of the transitions shown earlier in figure 4.1.
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