In addition to the usual object-oriented reasons for using inheritance, this inheritance model provides a very important kind of querying functionality. Given the above model, one can now perform a query like:
Get all the clothing items that have a shipping weight > 2 pounds.
Most important, the items returned will be a combination of coat items and shorts items. Without SQL repository support for inheritance, this query would have to be run against each subtype in turn. At first blush this may not seem significant, but envision a store with perhaps ten subtypes like this. Running this query against all of the subtypes would be painful. By building the inheritance knowledge into the SQL repository, we enable it to optimize the actual SQL queries needed.
Here is what the code for this query would look like:
// get hold of the repository
Repository gsa = ...;
// get the view to use for querying "clothing" type items
RepositoryView clothingView = gsa.getView("clothing");
// get a query builder
QueryBuilder qb = clothingView.getQueryBuilder();
// build the query
QueryExpression weightLimit = qb.createConstantQueryExpression(new Integer(2));
QueryExpression itemWeight = qb.createPropertyQueryExpression("shippingWeight");
Query q = qb.createComparisonQuery(itemWeight,
weightLimit,
QueryBuilder.GREATER_THAN);
// run the query
RepositoryItem[] items = clothingView.executeQuery(q);
// separate the coats and shorts and do whatever with them
for (int i=0; i<items.length; i++)
{
RepositoryItem item = items[i];
// all clothing items have a name and a description
logDebug("clothing: " + item.getPropertyValue("name") +
' ' + item.getPropertyValue("description"));
// the ItemDescriptor defines the "type" of an item
RepositoryItemDescriptor desc = item.getItemDescriptor();
// now we do different things, depending on the
// type of clothing item we have
if (desc.getItemDescriptorName().equals("coat")
{
// coats have a property called "season"
logDebug("\tcoat, season = " + item.getPropertyValue("season"));
// do coat-related things
myCoatProcessor(item);
}
else
{
// shorts have a property called "pleated"
logDebug("\tcoat, season = " + item.getPropertyValue("pleated"));
// do shorts-related things
myShortsProcessor(item);
}
}In this example, we used the name of the item descriptor to determine the item type. You can also look at the value of the type property declared in your template. In our example, we’d define the enumerated properties with the useCodeForValue attribute set to true and then the query would go something like:
...
RepositoryItem item = items[i];
Integer itemTypeCode = (Integer)item.getPropertyValue("type");
if (itemTypeCode.intValue() == 0)
{
... coats ...
}
else
{
... shorts ...
}Which technique to use is up to you and may be largely a matter of style. The item descriptor approach uses the actual name like coat or shorts. The type attribute approach uses the type code stored in the clothing table: typically something like 0 or 1, as in this case.
