openjpa-jdbc/src/main/java/org/apache/openjpa/jdbc/kernel/exps/GetColumn.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.    
 */
package org.apache.openjpa.jdbc.kernel.exps;

import org.apache.openjpa.jdbc.kernel.JDBCStore;
import org.apache.openjpa.jdbc.meta.ClassMapping;
import org.apache.openjpa.jdbc.sql.SQLBuffer;
import org.apache.openjpa.kernel.StoreContext;
import org.apache.openjpa.lib.util.Localizer;
import org.apache.openjpa.util.UnsupportedException;
import org.apache.openjpa.util.UserException;

/**
 * Returns the SQL alias of the named column for use in a query. Note that
 * to retrieve columns in the table of the candidate object, you must
 * explicitly qualify the extension with <code>this</code>, as demonstrated
 * in the second example below.
 *  Examples:<br />
 * <code> "company.address.ext:getColumn (\"ID\") == 5"<br />
 * "this.ext:getColumn (\"CLS\") == \"org.apache.openjpa.example.Person\""
 * </code>
 *
 * @nojavadoc
 */
public class GetColumn
    implements JDBCFilterListener {

    public static final String TAG = "getColumn";

    private static final Localizer _loc = Localizer.forPackage
        (GetColumn.class);

    public String getTag() {
        return TAG;
    }

    public boolean expectsArguments() {
        return true;
    }

    public boolean expectsTarget() {
        return true;
    }

    public Object evaluate(Object target, Class targetClass, Object[] args,
        Class[] argClasses, Object candidate, StoreContext ctx) {
        throw new UnsupportedException(_loc.get("no-in-mem", TAG));
    }

    public void appendTo(SQLBuffer buf, FilterValue target,
        FilterValue[] args, ClassMapping type, JDBCStore store) {
        if (!args[0].isConstant())
            throw new UserException(_loc.get("const-only", TAG));
        if (!target.isPath())
            throw new UserException(_loc.get("path-only", TAG));

        // alias the column using the target's context, so that the
        // correct relation path to the target is used
        ClassMapping mapping = target.getClassMapping();
        String colName = args[0].getValue().toString();
        buf.append(target.getColumnAlias(colName, mapping.getTable()));
    }

    public Class getType(Class targetClass, Class[] argClasses) {
        return Object.class;
    }
}