<map name="Images" lazy="true" table="ITEM_IMAGE" sort="natural"> in C#.NET

Painting Code 39 Full ASCII in C#.NET <map name="Images" lazy="true" table="ITEM_IMAGE" sort="natural">

<key column="ITEM_ID"/> <index column="IMAGE_NAME" type="string"/> <element type="String" column="FILENAME" not-null="true"/> </map>

By specifying sort="natural", you tell NHibernate to use a SortedMap, sorting the image names according to the CompareTo() method of System.String. If you want some other sorted order for example, reverse alphabetical order you can specify the name of a class that implements System.Collections.IComparer in the sort attribute. Here s an example:

<map name="Images" lazy="true" table="ITEM_IMAGE" sort="NHibernate.Auction.ReverseStringComparer, NHibernate.Auction"> <key column="ITEM_ID"/> <index column="IMAGE_NAME" type="string"/> <element type="String" column="FILENAME" not-null="true"/> </map>

NHibernate sorted maps use System.Collections.SortedList in their implementa-

tion. A sorted set, which behaves like Iesi.Collections.SortedSet, is mapped in a similar way:

<set name="Images" lazy="true" table="ITEM_IMAGE" sort="natural"> <key column="ITEM_ID"/> <element type="String" column="FILENAME" not-null="true"/> </set>

Bags can t be sorted, and there is no SortedBag, unfortunately. Nor may lists be sorted, because the order of list elements is defined by the list index. Alternatively, you may choose to use an ordered map, using the sorting capabilities of the database instead of in-memory sorting:

<map name="Images" lazy="true" table="ITEM_IMAGE" order-by="IMAGE_NAME asc"> <key column="ITEM_ID"/> <index column="IMAGE_NAME" type="String"/> <element type="String" column="FILENAME" not-null="true"/> </map>

The expression in the order-by attribute is a fragment of a SQL order by clause. In this case, you order by the IMAGE_NAME column, in ascending order. You can even write SQL function calls in the order-by attribute:

<key column="ITEM_ID"/> <index column="IMAGE_NAME" type="String"/> <element type="String" column="FILENAME" not-null="true"/> </map>

Notice that you can order by any column of the collection table. Both sets and bags accept the order-by attribute; but again, lists don t. This example uses a bag:

<idbag name="Images" lazy="true" table="ITEM_IMAGE" order-by="ITEM_IMAGE_ID desc"> <collection-id type="Int32" column="ITEM_IMAGE_ID"> <generator class="sequence"/> </collection-id> <key column="ITEM_ID"/> <element type="String" column="FILENAME" not-null="true"/> </idbag>

Under the covers, NHibernate uses an Iesi.Collections.ListSet and a System. Collections.Specialized.ListDictionary to implement ordered sets and maps; use this functionality carefully because it doesn t perform well with large numbers of elements. In a real system, it s likely that you ll need to keep more than just the image name and filename; you l probably need to create an Image class to store this extra information. Of course, you could map Image as an entity class; but because we ve already concluded that this isn t absolutely necessary, let s see how much further you can get without an Image entity, which would require an association mapping and more complex lifecycle handling. In chapter 3, you saw that NHibernate lets you map user-defined classes as components, which are considered to be value types. This is still true, even when component instances are collection elements. Let s now look at how you can map collections of components.

The Image class defines the properties Name, Filename, SizeX, and SizeY. It has a single association, with its parent Item class, as shown in figure 6.7. As you can see from the aggregation association style depicted by a black diamond, Image is a component of Item, and Item is Name : string the entity that is responsible for the lifecycle Description : string of Image. References to images aren t InitialPrice : double Name : string ReservePrice : double Filename : string shared, so our first choice is an NHibernate StartDate : DateTime SizeX : int EndDate : DateTime SizeY : int component mapping. The multiplicity of the Created : DateTime association further declares this association as many-valued that is, zero or more Images Figure 6.7 Collection of for the same Item. Image components in Item

First, you implement the Image class. This is a POCO, with nothing special to consider. As you know from chapter 4, component classes don t have an identifier property. But you must implement Equals() and GetHashCode() to compare the Name, Filename, SizeX, and SizeY properties. This allows NHibernate s dirty checking to function correctly. Strictly speaking, implementing Equals() and GetHashCode() isn t required for all component classes; but we recommend it for any component class because the implementation is fairly easy, and better safe than sorry is a good motto. The Item class hasn t changed, but the objects in the collection are now Images instead of Strings. Let s map this to the database.