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								<H1><A name="RDBDASCPP-ConventionOverConfiguration-ConventionOverConfiguration"></A>ConventionOverConfiguration</H1>

<P>This concept reflects the notion that configuration (defining a bunch of stuff in a side file) can be avoided by following convention (adhering to prescribed patterns) and that this is generally a good thing. The RDB DAS has several reasonable conventions in place and using these can significantly reduce a developer's work-load.</P>

<P>As an example, the RDB DAS provides a straight-forward, one-to-one mapping of database tables to DataObject Types and from table columns to DataObject properties. If a user provides no mapping information to the contrary, a graph of DataObjects returned by the DAS will directly map Types/Properties to the Tables/Columns queried.</P>

<P>For example, consider a database with the following table:</P>
<DIV class="code panel" style="border-width: 1px;"><DIV class="codeContent panelContent">
<PRE class="code-java">TABLE CUSTOMER
     ID         INTEGER
     LASTNAME   VARCHAR(20)
     ADDRESS    VARCHAR(30)
</PRE>
</DIV></DIV>
<P>If an application uses the DAS to query this table with the following SQL statement:</P>
<DIV class="preformatted panel" style="border-width: 1px;"><DIV class="preformattedContent panelContent">
<PRE>SELECT * FROM CUSTOMER
</PRE>
</DIV></DIV>
<P>The DAS will return a graph of DataObjects each of Type CUSTOMER and each instance in will have three properties: (ID, LASTNAME, ADDRESS). Here is the example code. Note that no configuration data is provided at all.</P>
<DIV class="panel" style="border-width: 1px;"><DIV class="panelContent">
<P>DASImpl&#42; das = (DASImpl*) DASImpl::getFactory().createDAS(getConnection());<BR>
 CommandPtr readCustomers = das-&gt;createCommand(&quot;select * from CUSTOMER&quot;);<BR>
 DataObjectPtr root = readCustomers-&gt;executeQuery();</P>

<P>&nbsp;delete das;</P>
</DIV></DIV>
<P>Another piece of convention followed by the RDB DAS has to do with database table keys. When asked to &quot;apply changes&quot;, the RDB DAS scans the SDO ChangeSummery and generates the set of INSERT/UPDATE and DELETE statements needed to flush the graph changes to the database. To generate the correct statements the RDB DAS needs to know how a DataObject's properties map to a database table key. In typical Object/Relational mappings frameworks, this information is provided in a config file with something like this:</P>
<DIV class="preformatted panel" style="border-width: 1px;"><DIV class="preformattedContent panelContent">
<PRE>&lt;Table tableName=&quot;COMPANY&quot;&gt;
      &lt;Column columnName=&quot;ID&quot; primaryKey=&quot;true&quot;/&gt;
      .
      .
      .
   &lt;/Table&gt;
</PRE>
</DIV></DIV>
<P>However, the RDB DAS understands is that, in the absence of user provided cofiguration data, a DataObject property named &quot;ID&quot; maps to a key column named &quot;ID&quot; in the database. If a developer follows this convention (naming key columns &quot;ID&quot;) then this key-mapping information need not be specified in a configuration file. Convention Over Configuration strings are not case sensitives, so &quot;ID&quot;, &quot;Id&quot;, &quot;iD&quot; or&quot;id&quot; will be mapped as a primary key.</P>

<P>The RDB DAS also has convention in place for mapping DataObject relationships. If a database table (yyy) has a column named xxx_ID then the DAS will assume, in the absence of config data to the contrary, that there is a one-to-many relationship from table xxx to table yyy and that the key column for table xxx is named &quot;ID&quot;.</P>

<P>There is more &quot;convention over configuration&quot; coming. We are currently looking into support for optimistic concurrency control conventions and other areas. </P>
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