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/*
 * Copyright (C) 2012-2016 DuyHai DOAN
 *
 * Licensed 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 info.archinnov.achilles.annotations;

import java.lang.annotation.*;

/**

 * Annotation for computed column. A computed column is a column obtained by
 * applying a CQL function on other(s) column(s). A computed column can only be referenced
 * in SELECT/read statements, not UPDATE/INSERT/DELETE
 * 

 * Examples:

 * 

 * //Normal column name
 * {@literal @}Column(name = "first_name")
 * private String firstName;

 * //Write time of firstName
 * {@literal @}Column
 * {@literal @}Computed(function = "writetime", alias = "write_time", targetColumns = "first_name", cqlClass = Long.class)
 * private Long firstNameWriteTime;
 * 

 * In the above example, the field firstNameWriteTime is obtained by applying the function
 * writime on the column first_name.
 * The target CQL class is Long.
 * 

 * Please note that the targerFields should reference CQL column names, not Java field name. In the example
 * it should be "first_name" and not "firstName"
 * 


 * This annotation will generate the following SELECT statement at runtime:
 * 

 * 
 * SELECT first_name, writetime(first_name) AS write_time ....
 * 
 * 
 *
 * @see @Computed
 */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
@Documented
public @interface Computed {

    /**
     * Mandatory. The name of the CQL function to be used.
     */
    String function();

    /**
     * Mandatory. The alias of this computed column.
     * Warning: if you have multiple computed columns in the same entity, please take
     * care to choose different alias
     */
    String alias();

    /**
     * Mandatory. The target CQL columns on which the function will be applied.
     * You can passe in multiple columns. The ordering of columns does matter
     */
    String[] targetColumns();

    /**
     * Mandatory. To help Achilles determine the CQL type at compile time, you have to specify
     * the Cassandra Java data type produced by the function application. This type should be a CQL-compatible type.
     * 

     * Example: the writetime function will produce a CQL bigint so it's corresponding
     * Java type is Long
     */
    Class cqlClass();
}