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THE PROBLEM OF INHERITANCE AND POLYMORPHISM
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Object-oriented languages support the notion of inheritance, but relational databases typically don t. Let s say that the auction application can have many kinds of items. You could create subclasses like Furniture and Book, each with specific information. How can you persist this hierarchy of entities in a relational database A Bid can refer to any subclass of Item. It should be possible to run polymorphic queries, such as retrieving all bids on books. In section 3.8, we discuss how ORM solutions like NHibernate solve the problem of persisting a class hierarchy to a database table or tables.
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THE PROBLEM OF IDENTITY
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The identity of a database row is commonly expressed as the primary key value. As you ll see in section 3.5, .NET object identity isn t naturally equivalent to the primary key value. With relational databases, it s recommended that you use a surrogate key a primary key column with no meaning to the user. But .NET objects have an intrinsic identity, which is based either on their memory location or on a user-defined convention (by using the implementation of the Equals() method). Given this problem, how can you represent associations Let s look at that next.
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PROBLEMS RELATING TO ASSOCIATIONS
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In an object model, associations represent the relationships between objects. For instance, a bid has a relationship with an item. This association is created using object references. In the relational world, an association is represented by a foreign key column, with copies of key values in several tables. There are subtle differences between the two representations. Object references are inherently directional: the association is from one object to the other. If an association between objects should be navigable in both directions, you must define the association twice, once in each of the associated classes. On the other hand, foreign-key associations aren t by nature directional. Navigation has no meaning for a relational data model, because you can create arbitrary data associations with table joins and projection. We discuss association mappings in detail in chapters 3 and 6. If you think about the DataSet in all these problems, you ll realize how rigid its structure is. The information in a DataSet is presented exactly as in the database. To navigate from one row to another, you must manually resolve their relationship by using a foreign key to find the referred row in the related table. Let s move from the representation of the entities to how you can manipulate them efficiently.
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Units of work and conversations
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When users work on applications, they perform distinct unitary operations. These operations can be referred to as conversations (or business transactions or application
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Why do we need NHibernate
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transactions). For example, placing a bid on an item is a conversation. Seasoned programmers know how hard it can be to make sure that many related operations performed by the user are treated as if they were a single bigger business transaction (a unit). You ll learn in this section that NHibernate makes this easier to achieve. Let s take another example to illustrate this concept. Popular media players allow you to rate the songs you hear and later sort them based on your rating. This means your ratings are persisted. When you open a list of songs, you listen and rate them one by one. When should persistence take place The first solution that may come to mind is to persist the rating when the user enters it. This technique is inefficient: the user may change the rating many times, and the persistence will be done separately for each song. (But this approach is safest if you expect the application to crash at any moment.) Instead, you can let the user rate all the songs and then persist the ratings when the user closes the list. The process of rating these songs is a conversation. Let s see how it works and what its benefits are.
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