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1.1.1 Identity
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One of the more significant areas of the impedance mismatch is in regard to identity. Java objects exist independently of the values they contain, which is to say that
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objectA == objectB;
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So, objects can either be identical or equal. If two objects are identical, they re the same object. If the objects are equal, they contain the same values. These different notions of identity don t exist in relational models. Rows in a relational database are only identified by the values they contain. How can you identify objects with their relational counterparts A common way to overcome this problem in relational models is to introduce a unique identifier column, typically called a sequence or identifier. The relational identifier is also represented in the object model and becomes a surrogate for identity checking.
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Understanding object persistence
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Although the object identity problem can be resolved, a more significant mismatch occurs when your object model uses inheritance.
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1.1.2 Inheritance
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A core feature of object-oriented languages is the ability to have one object inherit from one or many parent objects. Figure 1.1 illustrates an object hierarchy used in our example application. Relational databases don t support the notion of inheritance, so persisting a rich object hierarchy to a relational schema can be complex. Since inheritance is difficult to translate to a relational model, why can t you just design your object model without hierarchies Object-oriented languages were developed to model real-world problems. Inheritance is vital because it allows you to create a precise model of the problem while allowing shared properties and behavior to cascade down the hierarchy. You shouldn t be forced into sacrificing this feature because the relational model doesn t support it.
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1.1.3 Associations
The last portion of the impedance mismatch we ll look at is the differences in associations between object and relational models. Associations are probably one of the easiest portions of the mismatch to overcome since both models support this notion. The relational model understands only one type of association: a foreign key reference to a
Event id:Long name:String startDate:Date duration:int location:Location
Location id:Long name:String
Figure 1.1 Simple object hierarchy
Why Hibernate
One-to-one and one-to-many associations are . direct relationships between two tables
events id .....
speakers id event_id .....
Figure 1.2 One-to-one and one-to-many associations in a relational model
primary key stored in another table. Compare that to the associations available in an object model: one-to-one, one-to-many and many-to-many. Converting an object-based one-to-one association to a relational schema is simple: Reference the primary key of the associated objects. If you have a one-to-many association, you can repeat this process for each row on the many side. The only problem is that the database doesn t understand the one-to-many association it only knows that a foreign key refers to a primary key. Let s look at a diagram of one-toone and one-to-many associations in a relational schema, shown in figure 1.2. Mapping a many-to-many association is typically done with a join table. The join table contains two foreign key columns referencing two tables, allowing multiple entries for each side of the association. Figure 1.3 illustrates a many-to-many join table.
1.1.4 Object/relational mapping
We ve touched on a few problems illustrating the impedance mismatch, but we haven t covered all areas of the mismatch. Our goal was simply
Many-to-many associations use a join table.
events id .....
event _ speakers
event _ id speaker _ id .....
speakers id .....
Figure 1.3 Many-to-many associations in a relational model
Understanding object persistence
to explain what most developers realize intrinsically: Merging objects with a relational schema isn t easy. Object/relational mapping was developed to solve these problems. Object/relational mapping (ORM) is the process of persisting objects in a relational database. ORM bridges the gap between object and relational schemas, allowing your application to persist objects directly without requiring you to convert objects to and from a relational format. There are many types of ORM solutions, offering varying levels of mapping support. Some ORM frameworks require that persistent objects inherit from a base class or perform post-processing of bytecode. Hibernate, on the other hand, requires a small amount of metadata for each persistent object. Hibernate is a noninvasive ORM service. It doesn t require bytecode processing or a base persistent class. Hibernate operates independently of application architecture, allowing it to be used in various applications. It provides full object/relational mapping, meaning that it supports all the available object-oriented features that relational databases lack. Developers accustomed to using the standard JDBC API may wonder why a tool like Hibernate is needed. After all, JDBC provides a simple and complete interface to relational databases. Using JDBC directly is ideal for basic applications, since you can quickly persist objects with well-understood code. However, JDBC can get out of hand with larger applications or when requirements change. If an object changes, the code that persists the object must be changed and tested, as well as all the SQL used to manage the object s state. Using Hibernate for application persistence helps avoid the drawbacks of raw JDBC. In section 1.2, we demonstrate the shortcomings of these techniques; then, in section 1.3, we explain how Hibernate allows you to bypass them. Of course, object/relational mapping isn t a silver bullet for persistence problems. There are instances where ORM, and therefore Hibernate, isn t the best solution.
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