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    <h3>Editing: MetaToMetaTableMigrator.php</h3>
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/**
 * Generic Migration class to move any meta data associated to an entity, to a different meta table associated with a custom entity table.
 */

namespace Automattic\WooCommerce\Database\Migrations;

/**
 * Base class for implementing migrations from the standard WordPress meta table
 * to custom meta (key-value pairs) tables.
 *
 * @package Automattic\WooCommerce\Database\Migrations
 */
abstract class MetaToMetaTableMigrator extends TableMigrator {

	/**
	 * Schema config, see __construct for more details.
	 *
	 * @var array
	 */
	private $schema_config;

	/**
	 * Returns config for the migration.
	 *
	 * @return array Meta config, must be in following format:
	 * array(
	 *  'source'      =&gt; array(
	 *      'meta'          =&gt; array(
	 *          'table_name'        =&gt; source_meta_table_name,
	 *          'entity_id_column'  =&gt; entity_id column name in source meta table,
	 *          'meta_key_column'   =&gt; meta_key column',
	 *          'meta_value_column' =&gt; meta_value column',
	 *      ),
	 *      'entity' =&gt; array(
	 *          'table_name'       =&gt; entity table name for the meta table,
	 *          'source_id_column' =&gt; column name in entity table which maps to meta table,
	 *          'id_column'        =&gt; id column in entity table,
	 *      ),
	 *      'excluded_keys' =&gt; array of keys to exclude,
	 *  ),
	 *  'destination' =&gt; array(
	 *      'meta'   =&gt; array(
	 *          'table_name'        =&gt; destination meta table name,
	 *          'entity_id_column'  =&gt; entity_id column in meta table,
	 *          'meta_key_column'   =&gt; meta key column,
	 *          'meta_value_column' =&gt; meta_value column,
	 *          'entity_id_type'    =&gt; data type of entity id,
	 *          'meta_id_column'    =&gt; id column in meta table,
	 *      ),
	 *  ),
	 * )
	 */
	abstract protected function get_meta_config(): array;

	/**
	 * MetaToMetaTableMigrator constructor.
	 */
	public function __construct() {
		$this-&gt;schema_config = $this-&gt;get_meta_config();
	}

	/**
	 * Return data to be migrated for a batch of entities.
	 *
	 * @param array $entity_ids Ids of entities to migrate.
	 *
	 * @return array[] Data to be migrated. Would be of the form: array( 'data' =&gt; array( ... ), 'errors' =&gt; array( ... ) ).
	 */
	public function fetch_sanitized_migration_data( $entity_ids ) {
		$this-&gt;clear_errors();
		$to_migrate = $this-&gt;fetch_data_for_migration_for_ids( $entity_ids );
		if ( empty( $to_migrate ) ) {
			return array(
				'data'   =&gt; array(),
				'errors' =&gt; array(),
			);
		}

		$already_migrated = $this-&gt;get_already_migrated_records( array_keys( $to_migrate ) );

		return array(
			'data'   =&gt; $this-&gt;classify_update_insert_records( $to_migrate, $already_migrated ),
			'errors' =&gt; $this-&gt;get_errors(),
		);
	}

	/**
	 * Migrate a batch of entities from the posts table to the corresponding table.
	 *
	 * @param array $entity_ids Ids of entities ro migrate.
	 */
	protected function process_migration_batch_for_ids_core( array $entity_ids ): void {
		$sanitized_data = $this-&gt;fetch_sanitized_migration_data( $entity_ids );
		$this-&gt;process_migration_data( $sanitized_data );
	}

	/**
	 * Process migration data for a batch of entities.
	 *
	 * @param array $data Data to be migrated. Should be of the form: array( 'data' =&gt; array( ... ) ) as returned by the `fetch_sanitized_migration_data` method.
	 *
	 * @return array Array of errors and exception if any.
	 */
	public function process_migration_data( array $data ) {
		if ( isset( $data['data'] ) ) {
			$data = $data['data'];
		}
		$this-&gt;clear_errors();
		$exception = null;

		$to_insert = $data[0];
		$to_update = $data[1];

		try {
			if ( ! empty( $to_insert ) ) {
				$insert_queries       = $this-&gt;generate_insert_sql_for_batch( $to_insert );
				$processed_rows_count = $this-&gt;db_query( $insert_queries );
				$this-&gt;maybe_add_insert_or_update_error( 'insert', $processed_rows_count );
			}

			if ( ! empty( $to_update ) ) {
				$update_queries       = $this-&gt;generate_update_sql_for_batch( $to_update );
				$processed_rows_count = $this-&gt;db_query( $update_queries );
				$this-&gt;maybe_add_insert_or_update_error( 'update', $processed_rows_count );
			}
		} catch ( \Exception $e ) {
			$exception = $e;
		}

		return array(
			'errors'    =&gt; $this-&gt;get_errors(),
			'exception' =&gt; $exception,
		);
	}

	/**
	 * Generate update SQL for given batch.
	 *
	 * @param array $batch List of data to generate update SQL for. Should be in same format as output of $this-&gt;fetch_data_for_migration_for_ids.
	 *
	 * @return string Query to update batch records.
	 */
	private function generate_update_sql_for_batch( array $batch ): string {
		global $wpdb;

		$table             = $this-&gt;schema_config['destination']['meta']['table_name'];
		$meta_id_column    = $this-&gt;schema_config['destination']['meta']['meta_id_column'];
		$meta_key_column   = $this-&gt;schema_config['destination']['meta']['meta_key_column'];
		$meta_value_column = $this-&gt;schema_config['destination']['meta']['meta_value_column'];
		$entity_id_column  = $this-&gt;schema_config['destination']['meta']['entity_id_column'];
		$columns           = array( $meta_id_column, $entity_id_column, $meta_key_column, $meta_value_column );
		$columns_sql       = implode( '`, `', $columns );

		$entity_id_column_placeholder = MigrationHelper::get_wpdb_placeholder_for_type( $this-&gt;schema_config['destination']['meta']['entity_id_type'] );
		$placeholder_string           = &quot;%d, $entity_id_column_placeholder, %s, %s&quot;;
		$values                       = array();
		foreach ( $batch as $entity_id =&gt; $rows ) {
			foreach ( $rows as $meta_key =&gt; $meta_details ) {

				// phpcs:disable WordPress.DB.PreparedSQL, WordPress.DB.PreparedSQLPlaceholders
				$values[] = $wpdb-&gt;prepare(
					&quot;( $placeholder_string )&quot;,
					array( $meta_details['id'], $entity_id, $meta_key, $meta_details['meta_value'] )
				);
				// phpcs:enable
			}
		}
		$value_sql = implode( ',', $values );

		$on_duplicate_key_clause = MigrationHelper::generate_on_duplicate_statement_clause( $columns );

		return &quot;INSERT INTO $table ( `$columns_sql` ) VALUES $value_sql $on_duplicate_key_clause&quot;;
	}

	/**
	 * Generate insert sql queries for batches.
	 *
	 * @param array $batch Data to generate queries for.
	 *
	 * @return string Insert SQL query.
	 */
	private function generate_insert_sql_for_batch( array $batch ): string {
		global $wpdb;

		$table             = $this-&gt;schema_config['destination']['meta']['table_name'];
		$meta_key_column   = $this-&gt;schema_config['destination']['meta']['meta_key_column'];
		$meta_value_column = $this-&gt;schema_config['destination']['meta']['meta_value_column'];
		$entity_id_column  = $this-&gt;schema_config['destination']['meta']['entity_id_column'];
		$column_sql        = &quot;(`$entity_id_column`, `$meta_key_column`, `$meta_value_column`)&quot;;

		$entity_id_column_placeholder = MigrationHelper::get_wpdb_placeholder_for_type( $this-&gt;schema_config['destination']['meta']['entity_id_type'] );
		$placeholder_string           = &quot;$entity_id_column_placeholder, %s, %s&quot;;
		$values                       = array();
		foreach ( $batch as $entity_id =&gt; $rows ) {
			foreach ( $rows as $meta_key =&gt; $meta_values ) {
				foreach ( $meta_values as $meta_value ) {
					$query_params = array(
						$entity_id,
						$meta_key,
						$meta_value,
					);
					// phpcs:ignore WordPress.DB.PreparedSQL.InterpolatedNotPrepared, WordPress.DB.PreparedSQLPlaceholders
					$value_sql = $wpdb-&gt;prepare( &quot;$placeholder_string&quot;, $query_params );
					$values[]  = $value_sql;
				}
			}
		}

		$values_sql = implode( '), (', $values );

		return &quot;INSERT IGNORE INTO $table $column_sql VALUES ($values_sql)&quot;;
	}

	/**
	 * Fetch data for migration.
	 *
	 * @param array $entity_ids Array of IDs to fetch data for.
	 *
	 * @return array[] Data, will of the form:
	 * array(
	 *   'id_1' =&gt; array( 'column1' =&gt; array( value1_1, value1_2...), 'column2' =&gt; array(value2_1, value2_2...), ...),
	 *   ...,
	 * )
	 */
	public function fetch_data_for_migration_for_ids( array $entity_ids ): array {
		if ( empty( $entity_ids ) ) {
			return array();
		}

		$meta_query = $this-&gt;build_meta_table_query( $entity_ids );

		$meta_data_rows = $this-&gt;db_get_results( $meta_query );
		if ( ! is_array( $meta_data_rows ) || empty( $meta_data_rows ) ) {
			return array();
		}

		foreach ( $meta_data_rows as $migrate_row ) {
			if ( ! isset( $to_migrate[ $migrate_row-&gt;entity_id ] ) ) {
				$to_migrate[ $migrate_row-&gt;entity_id ] = array();
			}

			if ( ! isset( $to_migrate[ $migrate_row-&gt;entity_id ][ $migrate_row-&gt;meta_key ] ) ) {
				// phpcs:ignore WordPress.DB.SlowDBQuery.slow_db_query_meta_key
				$to_migrate[ $migrate_row-&gt;entity_id ][ $migrate_row-&gt;meta_key ] = array();
			}

			$to_migrate[ $migrate_row-&gt;entity_id ][ $migrate_row-&gt;meta_key ][] = $migrate_row-&gt;meta_value;
		}

		return $to_migrate;
	}

	/**
	 * Helper method to get already migrated records. Will be used to find prevent migration of already migrated records.
	 *
	 * @param array $entity_ids List of entity ids to check for.
	 *
	 * @return array Already migrated records.
	 */
	private function get_already_migrated_records( array $entity_ids ): array {
		global $wpdb;

		$destination_table_name        = $this-&gt;schema_config['destination']['meta']['table_name'];
		$destination_id_column         = $this-&gt;schema_config['destination']['meta']['meta_id_column'];
		$destination_entity_id_column  = $this-&gt;schema_config['destination']['meta']['entity_id_column'];
		$destination_meta_key_column   = $this-&gt;schema_config['destination']['meta']['meta_key_column'];
		$destination_meta_value_column = $this-&gt;schema_config['destination']['meta']['meta_value_column'];

		$entity_id_type_placeholder = MigrationHelper::get_wpdb_placeholder_for_type( $this-&gt;schema_config['destination']['meta']['entity_id_type'] );
		$entity_ids_placeholder     = implode( ',', array_fill( 0, count( $entity_ids ), $entity_id_type_placeholder ) );

		// phpcs:disable WordPress.DB.PreparedSQL.InterpolatedNotPrepared, WordPress.DB.PreparedSQLPlaceholders.UnfinishedPrepare
		$data_already_migrated = $this-&gt;db_get_results(
			$wpdb-&gt;prepare(
				&quot;
SELECT
	   $destination_id_column meta_id,
       $destination_entity_id_column entity_id,
       $destination_meta_key_column meta_key,
       $destination_meta_value_column meta_value
FROM $destination_table_name destination
WHERE destination.$destination_entity_id_column in ( $entity_ids_placeholder ) ORDER BY destination.$destination_entity_id_column
&quot;,
				$entity_ids
			)
		);
		// phpcs:enable

		$already_migrated = array();

		foreach ( $data_already_migrated as $migrate_row ) {
			if ( ! isset( $already_migrated[ $migrate_row-&gt;entity_id ] ) ) {
				$already_migrated[ $migrate_row-&gt;entity_id ] = array();
			}

			// phpcs:disable WordPress.DB.SlowDBQuery.slow_db_query_meta_key, WordPress.DB.SlowDBQuery.slow_db_query_meta_value
			if ( ! isset( $already_migrated[ $migrate_row-&gt;entity_id ][ $migrate_row-&gt;meta_key ] ) ) {
				$already_migrated[ $migrate_row-&gt;entity_id ][ $migrate_row-&gt;meta_key ] = array();
			}

			$already_migrated[ $migrate_row-&gt;entity_id ][ $migrate_row-&gt;meta_key ][] = array(
				'id'         =&gt; $migrate_row-&gt;meta_id,
				'meta_value' =&gt; $migrate_row-&gt;meta_value,
			);
			// phpcs:enable
		}

		return $already_migrated;
	}

	/**
	 * Classify each record on whether to migrate or update.
	 *
	 * @param array $to_migrate Records to migrate.
	 * @param array $already_migrated Records already migrated.
	 *
	 * @return array[] Returns two arrays, first for records to migrate, and second for records to upgrade.
	 */
	private function classify_update_insert_records( array $to_migrate, array $already_migrated ): array {
		$to_update = array();
		$to_insert = array();

		foreach ( $to_migrate as $entity_id =&gt; $rows ) {
			foreach ( $rows as $meta_key =&gt; $meta_values ) {
				// If there is no corresponding record in the destination table then insert.
				// If there is single value in both already migrated and current then update.
				// If there are multiple values in either already_migrated records or in to_migrate_records, then insert instead of updating.
				if ( ! isset( $already_migrated[ $entity_id ][ $meta_key ] ) ) {
					if ( ! isset( $to_insert[ $entity_id ] ) ) {
						$to_insert[ $entity_id ] = array();
					}
					$to_insert[ $entity_id ][ $meta_key ] = $meta_values;
				} else {
					if ( 1 === count( $meta_values ) &amp;&amp; 1 === count( $already_migrated[ $entity_id ][ $meta_key ] ) ) {
						if ( $meta_values[0] === $already_migrated[ $entity_id ][ $meta_key ][0]['meta_value'] ) {
							continue;
						}
						if ( ! isset( $to_update[ $entity_id ] ) ) {
							$to_update[ $entity_id ] = array();
						}
						$to_update[ $entity_id ][ $meta_key ] = array(
							'id'         =&gt; $already_migrated[ $entity_id ][ $meta_key ][0]['id'],
							// phpcs:ignore WordPress.DB.SlowDBQuery.slow_db_query_meta_value
							'meta_value' =&gt; $meta_values[0],
						);
						continue;
					}

					// There are multiple meta entries, let's find the unique entries and insert.
					$unique_meta_values = array_diff( $meta_values, array_column( $already_migrated[ $entity_id ][ $meta_key ], 'meta_value' ) );
					if ( 0 === count( $unique_meta_values ) ) {
						continue;
					}
					if ( ! isset( $to_insert[ $entity_id ] ) ) {
						$to_insert[ $entity_id ] = array();
					}
					$to_insert[ $entity_id ][ $meta_key ] = $unique_meta_values;
				}
			}
		}

		return array( $to_insert, $to_update );
	}

	/**
	 * Helper method to build query used to fetch data from source meta table.
	 *
	 * @param array $entity_ids List of entity IDs to build meta query for.
	 *
	 * @return string Query that can be used to fetch data.
	 */
	private function build_meta_table_query( array $entity_ids ): string {
		global $wpdb;
		$source_meta_table        = $this-&gt;schema_config['source']['meta']['table_name'];
		$source_meta_key_column   = $this-&gt;schema_config['source']['meta']['meta_key_column'];
		$source_meta_value_column = $this-&gt;schema_config['source']['meta']['meta_value_column'];
		$source_entity_id_column  = $this-&gt;schema_config['source']['meta']['entity_id_column'];
		$order_by                 = &quot;source.$source_entity_id_column ASC&quot;;

		$where_clause = &quot;source.`$source_entity_id_column` IN (&quot; . implode( ', ', array_fill( 0, count( $entity_ids ), '%d' ) ) . ')';

		$entity_table                  = $this-&gt;schema_config['source']['entity']['table_name'];
		$entity_id_column              = $this-&gt;schema_config['source']['entity']['id_column'];
		$entity_meta_id_mapping_column = $this-&gt;schema_config['source']['entity']['source_id_column'];

		if ( isset( $this-&gt;schema_config['source']['excluded_keys'] ) &amp;&amp; is_array( $this-&gt;schema_config['source']['excluded_keys'] ) ) {
			$key_placeholder = implode( ',', array_fill( 0, count( $this-&gt;schema_config['source']['excluded_keys'] ), '%s' ) );
			// phpcs:ignore WordPress.DB.PreparedSQL.InterpolatedNotPrepared, WordPress.DB.PreparedSQLPlaceholders.UnfinishedPrepare -- $source_meta_key_column is escaped for backticks, $key_placeholder is hardcoded.
			$exclude_clause = $wpdb-&gt;prepare( &quot;source.$source_meta_key_column NOT IN ( $key_placeholder )&quot;, $this-&gt;schema_config['source']['excluded_keys'] );
			$where_clause   = &quot;$where_clause AND $exclude_clause&quot;;
		}

		// phpcs:disable WordPress.DB.PreparedSQL.InterpolatedNotPrepared, WordPress.DB.PreparedSQLPlaceholders.UnfinishedPrepare
		return $wpdb-&gt;prepare(
			&quot;
SELECT
	source.`$source_entity_id_column` as source_entity_id,
	entity.`$entity_id_column` as entity_id,
	source.`$source_meta_key_column` as meta_key,
	source.`$source_meta_value_column` as meta_value
FROM `$source_meta_table` source
JOIN `$entity_table` entity ON entity.`$entity_meta_id_mapping_column` = source.`$source_entity_id_column`
WHERE $where_clause ORDER BY $order_by
&quot;,
			$entity_ids
		);
		// phpcs:enable
	}
}
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