/**
 * The DatabaseSetPeer performs database queries on behalf of
 * PersistentSet sets.  Subclasses must provide methods specifying
 * the class name of persistent objects it should create from database
 * rows as well as the proper SELECT SQL for restoring objects from
 * the database.
 */
package imaginary.persist;

import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.Hashtable;

public abstract class DatabaseSetPeer implements PersistentSetPeer {
    /**
     * Subclasses must implement this method to provide the
     * DatabaseSetPeer with the name of a subclasses of Persistent
     * to instantiate for each row returned from the database.
     * @param h the Hashtable of values returned from the database query
     * @return the name of the class to instantiate for this row
     */
    public abstract String getPersistentClass(Hashtable h);

    /**
     * Subclasses must implement this method to provide the
     * DatabaseSetPeer with the proper SQL for performing a query.
     * The Hashtable may be null.
     * @param h a list of values with which to limit the query
     * @return the proper SELECT SQL for this set
     */
    public abstract String getSql(Hashtable h);

    /**
     * Implementation of the PersistentSetPeer method for restoring
     * a set without query criteria.
     * @param set the set being restored
     * @param trans the transaction to use for the restore
     * @exception imaginary.persist.PersistenceException An error
     * occurred restoring from the database.
     * @see imaginary.persist.PersistentSetPeer#restore
     */
    public void restore(PersistentSet set, Transaction trans)
    throws PersistenceException {
        restore(set, trans, null); 
    }

    /**
     * Implementation of the PersistentSetPeer method for restoring
     * based on specified query criteria
     * @param set the set being restored
     * @param trans the transaction to use for the restore
     * @param data the query criteria
     * @exception imaginary.persist.PersistenceException An error
     * occurred restoring from the database.
     * @see imaginary.persist.PersistentSetPeer#restore
     */
    public void restore(PersistentSet set, Transaction trans, Hashtable data)
    throws PersistenceException {
        ResultSetMetaData meta = null;
        Connection connection = null;
        Statement statement = null;
        ResultSet results = null;
        String sql = getSql(data); // Get the SQL from the subclass
        
        try {
            // Get the Connection from the DatabaseTransaction
            connection = ((DatabaseTransaction)trans).getConnection();
            // Create a Statement from the Connection
            statement = connection.createStatement();
            // Get the results
            results = statement.executeQuery(sql);
            // Get the meta data
            meta = results.getMetaData();
            // While there are rows in the result set
            while( results.next() ) {
                Hashtable h = new Hashtable(); // Store the results here
                String class_name;
                Persistent p;

                // For each column in the row
                for(int i=1; i<=meta.getColumnCount(); i++) {
                    Object ob = results.getObject(i); // Get the value
                    
                    h.put(meta.getColumnLabel(i), ob); // Stick it in Hashtable
                }
                // Get the name of the class to create from this row
                class_name = getPersistentClass(h);
                // Get a Persistent from the Persistent
                p = Persistent.getPersistent(trans, h, class_name);
                // Add the Persistent to the set
                set.addPersistent(p);
            }
            statement.close(); // closing statement closes results
            connection.commit(); // Release database locks
        }
        catch( SQLException e ) {
            if( statement != null ) {
                try { statement.close(); } // closes results too
                catch( SQLException e2 ) { }
            }
            throw new PersistenceException(e);
        }
    }
}