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In the earlier chapters, you always started by defining the domain model before creating the database and setting up your mapping. What if you already have a database in place, or even a mapping file No rule says that things have to be done in any particular order, so we ll present the different processes available and explain which projects they suit best. You ll find that once you ve created either a database, a domain model, or mapping files, NHibernate provides tools that can be used to generate the other representations. Figure 9.1 shows the input and output of tools used for NHiberPojo Code Generator (hbm2net) nate development. NHibernate Generally, you have to complete and Mapping Mapping Atributes Metadata customize the generated code, but the MyGeneration or CodeSmith tools can give you a valuable head start. SchemaExport Database (hbm2ddl) We ll review these tools and processes Schema in this section, starting with the topFigure 9.1 Development processes down approach.
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Writing real-world domain models
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Top down: generating the mapping and the database from entities
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The approach you ve been using in this book is commonly called top-down development. This is the most comfortable development style when you re starting a new project with no existing database to worry about. Looking at figure 9.1, the starting point is the Plain Old CLR Object (POCO). When using this approach, you first build your .NET domain model, typically as POCOs. If you ve used the NHibernate.Mapping.Attributes library to decorate your entities, you can use NHibernate to generate the mapping for you. Alternatively, you can manually write it using an XML editor, as demonstrated throughout this book. With your entities and mapping file in place, you can let NHibernate s hbm2ddl tool generate the database schema for you, using the mapping metadata. This tool is part of the NHibernate library. It isn t a graphical tool; you access the features from your own code by calling methods on the NHibernate.Tool.hbm2ddl.SchemaExport class. When you create your mapping with attributes or XML, you can add elements that help SchemaExport create a database schema to your liking. These are optional; without them, NHibernate will attempt to use sensible defaults when creating your databases schema. If you decide to include extra mapping metadata, having the ability to override naming strategies, data types, column sizes, and so on can be useful. Sometimes it s necessary, especially if you want your generated schema to follow house rules or your DBA s requirements.
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Using naming strategies was explained in section 3.4.7. This feature lets you change the way entities names are converted into tables names.
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We ll now look at how you can prepare the mapping metadata to control database schema generation.
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In this example, we ve marked up the mapping for the Item class with hbm2ddl-specific attributes and elements. These optional definitions integrate seamlessly with the other mapping elements, as you can see in listing 9.1.
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Listing 9.1 Additional elements in the Item mapping for SchemaExport
<class name="Item" table="ITEM"> <id name="Id" type="String"> <column name="ITEM_ID" sql-type="char(32)"/> <generator class="uuid.hex"/> </id> <property name="Name" type="String"> <column name="NAME" not-null="true" length="255" index="IDX_ITEMNAME"/> </property> <property name="Description"
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type="String" column="DESCRIPTION" length="4000"/> <property name="InitialPrice" type="MonetaryAmount"> <column name="INITIAL_PRICE" check="INITIAL_PRICE > 0"/> <column name="INITIAL_PRICE_CURRENCY"/> </property> <set name="Categories" table="CATEGORY_ITEM" cascade="none"> <key> <column="ITEM_ID" sql-type="char(32)"/> </key> <many-to-many class="Category"> <column="CATEGORY_ID" sql-type="char(32)/> </many-to-many> </set> ... </class>
hbm2ddl automatically generates an NVARCHAR typed column if a property (even the identifier property) is of mapping type String. You know the identifier generator uuid.hex always generates strings that are 32 characters long; you use a CHAR SQL type and set its size fixed at 32 characters B. The nested <column> element is required for this declaration because there is no attribute to specify the SQL data type on the <id> element. The column, not-null, and length attributes are also available on the <property> element; but because you want to create an additional index in the database, you again use a nested <column> element C. This index will speed your searches for items by name. If you reuse the same index name on other property mappings, you can create an index that includes multiple database columns. The value of this attribute is also used to name the index in the database catalog. For the description field, we chose the lazy approach, using the attributes on the <property> element instead of a <column> element. The DESCRIPTION column will be generated as VARCHAR(4000) D. The custom user-defined type MonetaryAmount requires two database columns to work with. You have to use the <column> element. The check attribute E triggers the creation of a check constraint; the value in that column must match the given arbitrary SQL expression. Note that there is also a check attribute for the <class> element, which is useful for multicolumn check constraints. A <column> element can also be used to declare the foreign key fields in an association mapping. Otherwise, the columns of your association table CATEGORY_ITEM would be NVARCHAR(32) instead of the more appropriate CHAR(32) type F. We ve grouped all attributes relevant for schema generation in table 9.1; some of them weren t included in the previous Item mapping example. After you ve reviewed (probably together with a DBA) your mapping files and added schema-related attributes, you can create the schema.
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