key
 that is a managed type with inheritance. This allows for
 construction of multi-node entity graphs that include related
 managed types.  Subclass subgraphs will include the specified
 attributes of superclass subgraphs.</div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>attribute</code> - attribute</dd>
<dd><code>type</code> - entity subclass</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>subgraph for the key attribute</dd>
<dt><span class="throwsLabel">Throws:</span