RDB DAS C++ - WorkingWithRelationships (TUSCANY) created by Adriano Crestani
http://cwiki.apache.org/confluence/pages/viewpage.action?pageId=69485
Content:
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h1. WorkingWithRelationships
As mentioned elsewhere in this guide, the RDB DAS provides a simple, implicit
mapping from SDO Types/Properties to Relational Tables/Columns. So, in the
absence of configuration to the otherwise, reading a table named CUSTOMER with
columns NAME and ADDRESS will result in a data graph containing SDO DataObjects
of Type CUSTOMER with properties NAME and ADDRESS.
This is great for reading entities from a single table but clients often need
to read data from related tables for example Customers and their related
Orders. The RDB DAS can work with related database tables and reads that span
tables produce graphs of related data objects. Continuing with our
Customer->Order example, a client can read a set of Customers and related
Orders by using a SELECT statement that includes a join as in the following
example:
{panel}
DASImpl\* das = (DASImpl*)
DASImpl::getFactory().createDAS(Config("CustomerOrderConfig.xml"),
getConnection());
// Read some customers and related orders
CommandPtr select = das->createCommand("SELECT * FROM CUSTOMER LEFT JOIN
ANORDER ON CUSTOMER.ID = ANORDER.CUSTOMER_ID WHERE CUSTOMER.ID = 1");
DataObjectPtr root = select->executeQuery();
DataObjectPtr customer = root->getDataObject("customer\[1\]");
DataObjectList& orders = customer->getList("orders");
{panel}
You can see that the client can work with the data graph as a set of related
objects. Once the application has a handle to a customer, it can reference that
customers related orders by accessing the customers "orders" property.
Notice that the example provides a config file as part of creating the DAS.
This file contains mapping information that defines to the DAS how the queried
tables are related. Here are the contents of the config file:
{noformat}
<Config
xsi:noNamespaceSchemaLocation="http:///org.apache.tuscany.das.rdb/config.xsd";>
<Table tableName="CUSTOMER">
<Column columnName="ID" primaryKey="true"/>
</Table>
<Table tableName="ANORDER">
<Column columnName="ID" primaryKey="true"/>
</Table>
<Relationship name="orders" primaryKeyTable="CUSTOMER"
foreignKeyTable="ANORDER" many="true">
<KeyPair primaryKeyColumn="ID" foreignKeyColumn="CUSTOMER_ID"/>
</Relationship>
</Config>
{noformat}
This config file specifies the primary keys of the related tables and then the
relationship between them. It also provides a convenient name for the
relationship. In short, this config specifies that the CUSTOMER table and
ANORDER table are related via ANORDERs column named "CUSTOMER_ID" which
references CUSTOMERS column named "ID". The name of the relationship is "orders"
As a side note, the config in this example is not required since the table
definitions follow a "convention" recognized by the DAS. See
[WorkingWithConventions\|RDB DAS C+\+ - WorkingWithConventions|RDB DAS C++ -
WorkingWithConventions].
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