SecondaryMultiKeyCreator (Oracle - Berkeley DB Java API)
Date: 2013-09-09

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## com.sleepycat.db
Interface SecondaryMultiKeyCreator

    public interface SecondaryMultiKeyCreator

The interface implemented for extracting multi-valued secondary keys from primary records.

The key creator object is specified by calling `SecondaryConfig.setMultiKeyCreator`. The secondary database configuration is specified when calling `Environment.openSecondaryDatabase`.

For example:

        class MyMultiKeyCreator implements SecondaryMultiKeyCreator {
        public void createSecondaryKeys(SecondaryDatabase secondary,
                                            DatabaseEntry key,
                                            DatabaseEntry data,
                                            Set results)
                throws DatabaseException {
                //
                // DO HERE: Extract the secondary keys from the primary key and
                // data.  For each key extracted, create a DatabaseEntry and add it
                // to the results set.
                //
            }
        }
        ...
        SecondaryConfig secConfig = new SecondaryConfig();
        secConfig.setMultiKeyCreator(new MyMultiKeyCreator());
        // Now pass secConfig to Environment.openSecondaryDatabase

Use this interface when any number of secondary keys may be present in a single primary record, in other words, for many-to-many and one-to-many relationships. When only zero or one secondary key is present (for many-to-one and one-to-one relationships) you may use the `SecondaryKeyCreator` interface instead. The table below summarizes how to create all four variations of relationships.

Relationship | Interface | Duplicates | Example
--- | --- | --- | ---
One-to-one | `SecondaryKeyCreator` | No | A person record with a unique social security number key.
Many-to-one | `SecondaryKeyCreator` | Yes | A person record with a non-unique employer key.
One-to-many | `SecondaryMultiKeyCreator` | No | A person record with multiple unique email address keys.
Many-to-many | `SecondaryMultiKeyCreator` | Yes | A person record with multiple non-unique organization keys.

To configure a database for duplicates. pass true to `DatabaseConfig.setSortedDuplicates(boolean)`.

Note that `SecondaryMultiKeyCreator` may also be used for single key secondaries (many-to-one and one-to-one); in this case, at most a single key is added to the results set. `SecondaryMultiKeyCreator` is only slightly less efficient than `SecondaryKeyCreator` in that two or three temporary sets must be created to hold the results.

See Also:
  `SecondaryConfig`

Method Summary
`void` | `createSecondaryKeys(SecondaryDatabase secondary, DatabaseEntry key, DatabaseEntry data, Set results)` Creates a secondary key entry, given a primary key and data entry.

Method Detail

### createSecondaryKeys

    void createSecondaryKeys(SecondaryDatabase secondary,
                             DatabaseEntry key,
                             DatabaseEntry data,
                             Set results)
                             throws DatabaseException

  Creates a secondary key entry, given a primary key and data entry.

  A secondary key may be derived from the primary key, primary data, or a combination of the primary key and data. Zero or more secondary keys may be derived from the primary record and returned in the results parameter. To ensure the integrity of a secondary database the key creator method must always return the same results for a given set of input parameters.

  Parameters:
    `secondary` - the database to which the secondary key will be added. This parameter is passed for informational purposes but is not commonly used.

    `key` - the primary key entry. This parameter must not be modified by this method.

    `data` - the primary data entry. This parameter must not be modified by this method.

    `results` - the set to contain the the secondary key DatabaseEntry objects created by this method.

  Throws:
    `DatabaseException` - if an error occurs attempting to create the secondary key.

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