[castor-dev] how to find out if an object already exists in the database?

[Christian Schuhegger]

hello,
i would like to populate a database with objects from an xml file, e.g.:
<?xml version="1.0" encoding="UTF-8"?>
<root>
<PropertyType><name>VOLUME</name><description>Integer number of any kind 
of transactions.</description><id>3</id></PropertyType>
<PropertyType><name>PRICE</name><description>The price of a given equity 
in a given currency</description><id>2</id></PropertyType>
<PropertyType><name>DATETIME</name><description>The datetime at which an 
observation was made.</description><id>1</id></PropertyType>
</root>

i am therefore iterating over all child elements of <root> which may be 
of arbitrary type:
for(int i = 0; i < len; i++) {
	Node node = nl.item(i);
	if(node.getNodeType() != Node.ELEMENT_NODE)
		continue;
	System.out.println("node[" + i + "]: " + node.getNodeName());
	xmlobject = unmarshaller.unmarshal(node);

at this point i would like to find out if the object already exists in 
the database or if i have to create it. my initial attempt was doing it 
via an OQL query:

	String query = "SELECT obj FROM " + xmlobject.getClass().getName() + " 
obj WHERE obj = $1 ";
	System.out.println("query: " + query);
	try {
		oql = db.getOQLQuery(query);
		oql.bind(xmlobject);
		results = oql.execute();
		already_exists = results.hasMore();
	} finally {
		if(null != oql)
			oql.close();
	}

but with this query i get the following exception:
query: SELECT obj FROM net.sf.ojts.jdo.PropertyType obj WHERE obj = $1
org.exolab.castor.jdo.QueryException: The field obj was not found.
	at 
org.exolab.castor.jdo.oql.ParseTreeWalker.checkField(ParseTreeWalker.java:645)
	at 
org.exolab.castor.jdo.oql.ParseTreeWalker.checkWhereClause(ParseTreeWalker.java:578)
	at 
org.exolab.castor.jdo.oql.ParseTreeWalker.checkWhereClause(ParseTreeWalker.java:590)
	at 
org.exolab.castor.jdo.oql.ParseTreeWalker.checkWhereClause(ParseTreeWalker.java:590)
	at 
org.exolab.castor.jdo.oql.ParseTreeWalker.checkErrors(ParseTreeWalker.java:236)
	at 
org.exolab.castor.jdo.oql.ParseTreeWalker.<init>(ParseTreeWalker.java:137)
	at org.exolab.castor.jdo.engine.OQLQueryImpl.create(OQLQueryImpl.java:261)
	at 
org.exolab.castor.jdo.engine.DatabaseImpl.getOQLQuery(DatabaseImpl.java:439)
	at CastorReadXMLTest.main(CastorReadXMLTest.java:115)

i tried to debug the exception and i think the root cause of this 
problem is that castor only allows for fields in the WHERE clause, but 
not for the _fromClassAlias itself. look at the following code snipet 
from the class ParseTreeWalker (my modifications are marked with >):

class ParseTreeWalker:
  private JDOFieldDescriptor checkField(ParseTreeNode fieldTree)
        throws QueryException {
    //see if we've checked this field before.
    JDOFieldDescriptor field = 
(JDOFieldDescriptor)_fieldInfo.get(fieldTree);
    if ( field != null )
      return field;

    if ( fieldTree.getToken().getTokenType() == DOT ) {
        field = checkProjection( fieldTree, false, false );
    } else {
        field = getFieldDesc( fieldTree.getToken().getTokenValue(), 
_clsDesc );
        if (field != null)
            _fieldInfo.put(fieldTree, field);
>       if(_fromClassAlias.equals(fieldTree.getToken().getTokenValue()))
>        	return null;
    }
    if (field == null)
        throw new QueryException( "The field " + 
fieldTree.getToken().getTokenValue() + " was not found." );
    return field;

  }
i did not know what to return where i put a "return null;"?
my following attempt was to use the db.load() method:
        boolean already_exists = false;
        PropertyType pt = (PropertyType) xmlobject;
        Object loaded = null;
        try {
                loaded = db.load(xmlobject.getClass(), new Integer(pt.getId()));
        } catch (ObjectNotFoundException e) {
                // ignored
        }
        if(null != loaded)
                already_exists = true;
but this approach has the disadvantage that i first of all have to know 
which identifier is used to identify objects in the db, e.g. i have to 
know that the identifier is an Integer, and all of my objects would have 
to implement some kind of interface, e.g. HasIntegerId, in order to cast 
them to this type and then call the getId() method on them.

is there some other way to find out if a given object is already created 
in the database or not, without the need to know anything else about a 
given object, e.g. without the need to derive from a common interface 
and without the need to cast it?

thanks a lot for any infos!!
--
Christian Schuhegger
http://www.el-chef.de/

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