Basic property and class mappings in C#

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Basic property and class mappings
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In this section, you ll learn a number of features and tips that will help you write better mappings. NHibernate can guess some information to make your mappings shorter. It s also possible to configure it to access your entities in a specific way. Let s start with a deeper review of the mapping of simple properties.
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Property mapping overview
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A typical NHibernate property mapping defines a property name, a database column name, and the name of an NHibernate type. It maps a .NET property to a table column.
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Basic property and class mappings
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The basic declaration provides many variations and optional settings; for example, it s often possible to omit the type name. For example, if Description is a property of (.NET) type String, NHibernate uses the NHibernate type String by default (we discuss the NHibernate type system in chapter 7). NHibernate uses reflection to determine the .NET type of the property. Thus, the following mappings are equivalent, as long as they re on the property Description:
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[Property(Name="Description", Column="DESCRIPTION", Type="String")] [Property(Column="DESCRIPTION")] public string Description { ... }
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These mapping can be written using XML. The following mappings are equivalent:
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<property name="Description" column="DESCRIPTION" type="String"/> <property name="Description" column="DESCRIPTION"/>
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As you already know, you can omit the column name if it s the same as the property name, ignoring case. (This is one of the sensible defaults we mentioned earlier.) In some cases, you may need to tell NHibernate more about the database column than just its name. For this, you can use the <column> element instead of the column attribute. The <column> element provides more flexibility; it has more optional attributes and may appear more than once. The following two property mappings are equivalent:
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[Property] [Column(1, Name="DESCRIPTION")] public string Description { ... }
Using XML, you can write
<property name="Description" type="String"> <column name="DESCRIPTION"/> </property>
Because .NET attributes aren t ordered, you sometimes need to specify their position. Here, [Column] comes after [Property], so its position is 1; the position of [Property] is 0 (the default value). NHibernate.Mapping.Attributes mimics XML mapping, so if you can write one, you can deduce how to write the other. The main difference is that you don t need to specify names with attribute mappings because NHibernate.Mapping.Attributes can guess them based on where the attribute mappings are in the code. The exception is [Id]; you must specify the identifier s name when it has one, because it s optional. The <property> element (and especially the <column> element) also defines certain attributes that apply mainly to automatic database-schema generation. If you aren t using the hbm2ddl tool (see section 10.1.1) to automatically generate the database schema, you can safely omit these. But it s still preferable to include at least the not-null attribute, because NHibernate can then report illegal null property values without going to the database:
<property name="InitialPrice" column="INITIAL_PRICE" not-null="true"/>
Writing and mapping classes
Detection of illegal null values is mainly useful for providing sensible exceptions at development time. It isn t intended for true data validation, which is outside the scope of NHibernate. Some properties don t map to a column. In particular, a derived property takes its value from a SQL expression.
Using derived properties
The value of a derived property is calculated at runtime by evaluating an expression. You define the expression using the formula attribute. For example, for a ShoppingCart class, you might map a TotalIncludingTax property. Because it s a formula, there is no column to store that value in the database:
<property name="TotalIncludingTax" formula="TOTAL + TAX_RATE * TOTAL" type="Double"/>
The given SQL formula is evaluated every time the entity is retrieved from the database. So the database does the calculation rather than the .NET object. The property doesn t have a column attribute (or sub-element) and never appears in a SQL INSERT or UPDATE, only in SELECTs. Formulas may refer to columns of the database table, call SQL functions, and include SQL subselects. This example, mapping a derived property of Item, uses a correlated subselect to calculate the average amount of all bids for an item:
<property name="AverageBidAmount" formula="( select AVG(b.AMOUNT) from BID b where b.ITEM_ID = ITEM_ID )" type="Double"/>
Notice that unqualified column names (in this case, those not preceded by b). refer to table columns of the class to which the derived property belongs. As we mentioned earlier, NHibernate doesn t require properties on entities if you define a new property-access strategy. The next section explains the various strategies and when you should use them in your mapping.
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