qr code scaner java app Mapping collections and entity associations in Java

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Mapping collections and entity associations
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A surrogate primary key allows duplicate bag elements.
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In this case, the primary key is the generated ITEM_IMAGE_ID, as you can see in figure 6.2. Note that the native generator for primary keys isn t supported for <idbag> mappings; you have to name a concrete strategy. This usually isn t a problem, because real-world applications often use a customized identifier generator anyway. You can also isolate your identifier generation strategy with placeholders; see chapter 3, section 3.3.4.3, Using placeholders. Also note that the ITEM_IMAGE_ID column isn t exposed to the application in any way. Hibernate manages it internally. A more likely scenario is one in which you wish to preserve the order in which images are attached to the Item. There are a number of good ways to do this; one way is to use a real list, instead of a bag.
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Mapping a list
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First, let s update the Item class:
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private List images = new ArrayList(); ... public List getImages() { return this.images; } public void setImages(List images) { this.images = images; }
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A <list> mapping requires the addition of an index column to the collection table. The index column defines the position of the element in the collection. Thus, Hibernate is able to preserve the ordering of the collection elements. Map the collection as a <list>:
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<list name="images" table="ITEM_IMAGE"> <key column="ITEM_ID"/> <list-index column="POSITION"/>
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Sets, bags, lists, and maps of value types
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<element type="string" column="FILENAME" not-null="true"/> </list>
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(There is also an index element in the XML DTD, for compatibility with Hibernate 2.x. The new list-index is recommended; it s less confusing and does the same thing.) The primary key of the collection table is a composite of ITEM_ID and POSITION. Notice that duplicate elements (FILENAME) are now allowed, which is consistent with the semantics of a list, see figure 6.3.
The collection table preserves the position of each element.
The index of the persistent list starts at zero. You could change this, for example, with <list-index base="1".../> in your mapping. Note that Hibernate adds null elements to your Java list if the index numbers in the database aren t continuous. Alternatively, you could map a Java array instead of a list. Hibernate supports this; an array mapping is virtually identical to the previous example, except with different element and attribute names (<array> and <array-index>). However, for reasons explained earlier, Hibernate applications rarely use arrays. Now, suppose that the images for an item have user-supplied names in addition to the filename. One way to model this in Java is a map, with names as keys and filenames as values of the map.
Mapping a map
Again, make a small change to the Java class:
private Map images = new HashMap(); ... public Map getImages() { return this.images; } public void setImages(Map images) { this.images = images; }
Mapping a <map> (pardon us) is similar to mapping a list.
Mapping collections and entity associations
Tables for a map, using strings as indexes and elements
<map name="images" table="ITEM_IMAGE"> <key column="ITEM_ID"/> <map-key column="IMAGENAME" type="string"/> <element type="string" column="FILENAME" not-null="true"/> </map>
The primary key of the collection table is a composite of ITEM_ID and IMAGENAME. The IMAGENAME column holds the keys of the map. Again, duplicate elements are allowed; see figure 6.4 for a graphical view of the tables. This map is unordered. What if you want to always sort your map by the name of the image
Sorted and ordered collections
In a startling abuse of the English language, the words sorted and ordered mean different things when it comes to Hibernate persistent collections. A sorted collection is sorted in memory using a Java comparator. An ordered collection is ordered at the database level using an SQL query with an order by clause. Let s make the map of images a sorted map. First, you need to change the initialization of the Java property to a java.util.TreeMap and switch to the java.util.SortedMap interface:
private SortedMap images = new TreeMap(); ... public SortedMap getImages() { return this.images; } public void setImages(SortedMap images) { this.images = images; }
Hibernate handles this collection accordingly, if you map it as sorted:
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