Bug 135490 Error occurs when Preview Results of Inner Join datasets
Description: Create a sample datasource and dataset, and then create a Joint dataset. Error occurs when preview results in the data set dialog. Steps to reproduce: 1.Create a sample datasource, and a dataset with sql statement "select * from CLASSICMODELS.CUSTOMERS". 2.Create a Joint dataset, the left and right dataset is the same. 3.Preview result in the dataset dialog, error pops up continuely.
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/impl/jointdataset/JoinConditionMatchUnit.java
* Get the value of current column.
	 * 
	 * @return
	 * @throws DataException
	 */
	public Object getColumnValue() throws DataException
	{
		if( this.isColumnReference )
			return ri.getCurrentResult( ).getFieldValue( this.columnName );
		else
		{
			Context cx = Context.enter( );
			try
			{
				Object leftValue = ScriptEvalUtil.evalExpr( this.expr,
						cx,
						this.scope,
						"JOINDATASET",
						0 );
				
				return leftValue;
			}	
			finally
			{
				Context.exit( );
			}
		}
	}
}