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Defining a Relational Database Table
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The Book class declaration has to have an associated relational database table. The associated table could be a view, but that is a choice for the developer. In the Book example, the table is called BOOKS, and is defined as follows: CREATE TABLE "BOOKS" { "ISBN" VARCHAR(20) NOT NULL, "TITLE" VARCHAR(120) NOT NULL, "AUTHOR" VARCHAR(50) NOT NULL, PRIMARY KEY( "ISBN") }; The table BOOKS has three columns, which are all varchar, or variable length strings, that can t contain null values. The primary key index is ISBN, and it s expected to be unique. When using NHibernate, it isn t necessary for a table to declare a primary key.
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CHAPTER 8 DATA PERSISTENCE
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Defining the Class Type Mapping Configuration File
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NHibernate encompasses the notion of attributes, but they are defined in configuration files. For the Book class, NHibernate, when resolving how to persist Book, will search for the configuration file Book.hbm.xml or the configuration file that is bound as a resource in an assembly. In most cases, the configuration file is added to the assembly as an embedded resource, and there can be multiple embedded configuration files. Book.hbm.xml is defined as follows: <hibernate-mapping xmlns="urn:nhibernate-mapping-2.0"> <class name="Chap08.ObjectRelational.Book,Chap08.ObjectRelational" table="Books"> <id name="ISBN"> <generator class="assigned" /> </id> <property name="Title"/> <property name="Author"/> </class> </hibernate-mapping> The XML tag hibernate-mapping specifies the root-level NHibernate parsing point that contains a number of subelements defining the mapping between the class and relational database. The XML tag class represents the .NET class that will be mapped. The attribute name represents the name of the class to load, which follows .NET type naming conventions. From the example, this means specifying at a minimum the class identifier and assembly. Not specifying the assembly will cause NHibernate to generate a class not found exception. The attribute table specifies the table that contains the data for the class Book. The table can be a view, but remember, whether or not data can be added or updated depends on the relational database implementation of view. Contained within the XML tag class are various child tags used to cross-reference the data members of class Book and the table Books. In a relational database, most tables have a primary key, but it isn t a necessity. When defining a class in NHibernate, there has to be a primary key, and the simplest is to use the XML tag id. The id tag has a single attribute, name, which references the name of the property that uniquely identifies an object instance. The case of the value for the name attribute has to match the case of the referenced property. If the case doesn t match, then an error is generated. When NHibernate transfers the state from the property to the relational table, the default is to search for a column with the same name as the property. If the column is named differently from the property, then, a hidden attribute called column can be used. The attribute column is the name of the column in the table that is used by NHibernate to transfer to and from the property. Contained within the id XML tag is a generator XML tag. The generator tag defines how the property and column value will be generated. Consider it as follows. You are adding a record to a database. To be able to distinguish one record from another, there usually needs to be a primary key. The problem with the primary key is how to generate a unique value. The simplest is to count from zero and update the counter for each record. In the example, the XML tag generator has assigned the attribute class to assigned, meaning that the primary key value for the property Book.ISBN will be assigned by the program. The remaining child XML tag s property represents the individual mapping of the class properties to relational database columns. Like the id XML tag, the attribute name represents
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