return array
	 */
	private function get_registered_meta_keys( $object_subtype = '' ) {

		// Get the object type
		$object_type = $this->get_meta_type();

		// Return the keys
		return get_registered_meta_keys( $object_type, $object_subtype );
	}

	/**
	 * Maybe update meta values on item update/save.
	 *
	 * @since 1.0.0
	 *
	 * @param array $meta
	 */
	private function save_extra_item_meta( $item_id = 0, $meta = array() ) {

		// Bail if there is no bulk meta to save
		$item_id = $this->shape_item_id( $item_id );
		if ( empty( $item_id ) || empty( $meta ) ) {
			return;
		}

		// Bail if no meta table exists
		if ( false === $this->get_meta_table_name() ) {
			return;
		}

		// Only save registered keys
		$keys = $this->get_registered_meta_keys();
		$meta = array_intersect_key( $meta, $keys );

		// Bail if no registered meta keys
		if ( empty( $meta ) ) {
			return;
		}

		// Save or delete meta data
		foreach ( $meta as $key => $value ) {
			! empty( $value )
				? $this->update_item_meta( $item_id, $key, $value )
				: $this->delete_item_meta( $item_id, $key );
		}
	}

	/**
	 * Delete all meta data for an item.
	 *
	 * @since 1.0.0
	 *
	 * @param int $item_id
	 */
	private function delete_all_item_meta( $item_id = 0 ) {

		// Bail if no meta was returned
		$item_id = $this->shape_item_id( $item_id );
		if ( empty( $item_id ) ) {
			return;
		}

		// Get the meta table name
		$table = $this->get_meta_table_name();

		// Bail if no meta table exists
		if ( empty( $table ) ) {
			return;
		}

		// Guess the item ID column for the meta table
		$primary_id     = $this->get_primary_column_name();
		$item_id_column = $this->apply_prefix( "{$this->item_name}_{$primary_id}" );

		// Get meta IDs
		$query    = "SELECT meta_id FROM {$table} WHERE {$item_id_column} = %d";
		$prepared = $this->get_db()->prepare( $query, $item_id );
		$meta_ids = $this->get_db()->get_col( $prepared );

		// Bail if no meta IDs to delete
		if ( empty( $meta_ids ) ) {
			return;
		}

		// Get the meta type
		$meta_type = $this->get_meta_type();

		// Delete all meta data for this item ID
		foreach ( $meta_ids as $mid ) {
			delete_metadata_by_mid( $meta_type, $mid );
		}
	}

	/**
	 * Get the meta table for this query.
	 *
	 * Forked from WordPress\_get_meta_table() so it can be more accurately
	 * predicted in a future iteration and default to returning false.
	 *
	 * @since 1.0.0
	 *
	 * @return mixed Table name if exists, False if not
	 */
	private function get_meta_table_name() {

		// Get the meta-type
		$type       = $this->get_meta_type();

		// Append "meta" to end of meta-type
		$table_name = "{$type}meta";

		// Variable'ize the database interface, to use inside empty()
		$db         = $this->get_db();

		// If not empty, return table name
		if ( ! empty( $db->{$table_name} ) ) {
			return $table_name;
		}

		// Default return false
		return false;
	}

	/**
	 * Get the meta type for this query.
	 *
	 * This method exists to reduce some duplication for now. Future iterations
	 * will likely use Column::relationships to
	 *
	 * @since 1.1.0
	 *
	 * @return string
	 */
	private function get_meta_type() {
		return $this->apply_prefix( $this->item_name );
	}

	/** Cache *****************************************************************/

	/**
	 * Get cache key from query_vars and query_var_defaults.
	 *
	 * @since 1.0.0
	 *
	 * @return string
	 */
	private function get_cache_key( $group = '' ) {

		// Slice query vars
		$slice = wp_array_slice_assoc( $this->query_vars, array_keys( $this->query_var_defaults ) );

		// Unset `fields` so it does not effect the cache key
		unset( $slice['fields'] );

		// Setup key & last_changed
		$key          = md5( serialize( $slice ) );
		$last_changed = $this->get_last_changed_cache( $group );

		// Concatenate and return cache key
		return "get_{$this->item_name_plural}:{$key}:{$last_changed}";
	}

	/**
	 * Get the cache group, or fallback to the primary one.
	 *
	 * @since 1.0.0
	 *
	 * @param string $group
	 * @return string
	 */
	private function get_cache_group( $group = '' ) {

		// Get the primary column
		$primary = $this->get_primary_column_name();

		// Default return value
		$retval  = $this->cache_group;

		// Only allow non-primary groups
		if ( ! empty( $group ) && ( $group !== $primary ) ) {
			$retval = $group;
		}

		// Return the group
		return $retval;
	}

	/**
	 * Get array of which database columns have uniquely cached groups.
	 *
	 * @since 1.0.0
	 *
	 * @return array
	 */
	private function get_cache_groups() {

		// Return value
		$cache_groups = array();

		// Get the cache groups
		$groups = $this->get_columns( array( 'cache_key' => true ), 'and', 'name' );

		if ( ! empty( $groups ) ) {

			// Get the primary column name
			$primary = $this->get_primary_column_name();

			// Setup return values
			foreach ( $groups as $name ) {
				if ( $primary !== $name ) {
					$cache_groups[ $name ] = "{$this->cache_group}-by-{$name}";
				} else {
					$cache_groups[ $name ] = $this->cache_group;
				}
			}
		}

		// Return cache groups array
		return $cache_groups;
	}

	/**
	 * Maybe prime item & item-meta caches by querying 1 time for all un-cached
	 * items.
	 *
	 * Accepts a single ID, or an array of IDs.
	 *
	 * The reason this accepts only IDs is because it gets called immediately
	 * after an item is inserted in the database, but before items have been
	 * "shaped" into proper objects, so object properties may not be set yet.
	 *
	 * @since 1.0.0
	 *
	 * @param array $item_ids
	 * @param bool  $force
	 *
	 * @return bool False if empty
	 */
	private function prime_item_caches( $item_ids = array(), $force = false ) {

		// Bail if no items to cache
		if ( empty( $item_ids ) ) {
			return false;
		}

		// Accepts single values, so cast to array
		$item_ids = (array) $item_ids;

		// Update item caches
		if ( ! empty( $force ) || ! empty( $this->query_vars['update_item_cache'] ) ) {

			// Look for non-cached IDs
			$ids = $this->get_non_cached_ids( $item_ids, $this->cache_group );

			// Bail if IDs are cached
			if ( empty( $ids ) ) {
				return false;
			}

			// Get query parts
			$table   = $this->get_table_name();
			$primary = $this->get_primary_column_name();

			// Query database
			$query   = "SELECT * FROM {$table} WHERE {$primary} IN (%s)";
			$ids     = implode( ',', array_map( 'absint', $ids ) );
			$prepare = sprintf( $query, $ids );
			$results = $this->get_db()->get_results( $prepare );

			// Update item caches
			$this->update_item_cache( $results );
		}

		// Update meta data caches
		if ( ! empty( $this->query_vars['update_meta_cache'] ) ) {
			$singular = rtrim( $this->table_name, 's' ); // sic
			update_meta_cache( $singular, $item_ids );
		}
	}

	/**
	 * Update the cache for an item. Does not update item-meta cache.
	 *
	 * Accepts a single object, or an array of objects.
	 *
	 * The reason this does not accept ID's is because this gets called
	 * after an item is already updated in the database, so we want to avoid
	 * querying for it again. It's just safer this way.
	 *
	 * @since 1.0.0
	 *
	 * @param array $items
	 */
	private function update_item_cache( $items = array() ) {

		// Maybe query for single item
		if ( is_numeric( $items ) ) {
			$primary = $this->get_primary_column_name();
			$items   = $this->get_item_raw( $primary, $items );
		}

		// Bail if no items to cache
		if ( empty( $items ) ) {
			return false;
		}

		// Make sure items are an array (without casting objects to arrays)
		if ( ! is_array( $items ) ) {
			$items = array( $items );
		}

		// Get the cache groups
		$groups = $this->get_cache_groups();

		// Loop through all items and cache them
		foreach ( $items as $item ) {

			// Skip if item is not an object
			if ( ! is_object( $item ) ) {
				continue;
			}

			// Loop through groups and set cache
			if ( ! empty( $groups ) ) {
				foreach ( $groups as $key => $group ) {
					$this->cache_set( $item->{$key}, $item, $group );
				}
			}
		}

		// Update last changed
		$this->update_last_changed_cache();
	}

	/**
	 * Clean the cache for an item. Does not clean item-meta.
	 *
	 * Accepts a single object, or an array of objects.
	 *
	 * The reason this does not accept ID's is because this gets called
	 * after an item is already deleted from the database, so it cannot be
	 * queried and may not exist in the cache. It's just safer this way.
	 *
	 * @since 1.0.0
	 *
	 * @param mixed $items Single object item, or Array of object items
	 *
	 * @return bool
	 */
	private function clean_item_cache( $items = array() ) {

		// Bail if no items to clean
		if ( empty( $items ) ) {
			return false;
		}

		// Make sure items are an array
		if ( ! is_array( $items ) ) {
			$items = array( $items );
		}

		// Get the cache groups
		$groups = $this->get_cache_groups();

		// Loop through all items and clean them
		foreach ( $items as $item ) {

			// Skip if item is not an object
			if ( ! is_object( $item ) ) {
				continue;
			}

			// Loop through groups and delete cache
			if ( ! empty( $groups ) ) {
				foreach ( $groups as $key => $group ) {
					$this->cache_delete( $item->{$key}, $group );
				}
			}
		}

		// Update last changed
		$this->update_last_changed_cache();
	}

	/**
	 * Update the last_changed key for the cache group.
	 *
	 * @since 1.0.0
	 *
	 * @return string The last time a cache group was changed.
	 */
	private function update_last_changed_cache( $group = '' ) {

		// Fallback to microtime
		if ( empty( $this->last_changed ) ) {
			$this->set_last_changed();
		}

		// Set the last changed time for this cache group
		$this->cache_set( 'last_changed', $this->last_changed, $group );

		// Return the last changed time
		return $this->last_changed;
	}

	/**
	 * Get the last_changed key for a cache group.
	 *
	 * @since 1.0.0
	 *
	 * @param string $group Cache group. Defaults to $this->cache_group
	 *
	 * @return string The last time a cache group was changed.
	 */
	private function get_last_changed_cache( $group = '' ) {

		// Get the last changed cache value
		$last_changed = $this->cache_get( 'last_changed', $group );

		// Maybe update the last changed value
		if ( false === $last_changed ) {
			$last_changed = $this->update_last_changed_cache( $group );
		}

		// Return the last changed value for the cache group
		return $last_changed;
	}

	/**
	 * Get array of non-cached item IDs.
	 *
	 * @since 1.0.0
	 *
	 * @param array  $item_ids Array of item IDs
	 * @param string $group    Cache group. Defaults to $this->cache_group
	 *
	 * @return array
	 */
	private function get_non_cached_ids( $item_ids = array(), $group = '' ) {
		$retval = array();

		// Bail if no item IDs
		if ( empty( $item_ids ) ) {
			return $retval;
		}

		// Loop through item IDs
		foreach ( $item_ids as $id ) {
			$id = $this->shape_item_id( $id );

			if ( false === $this->cache_get( $id, $group ) ) {
				$retval[] = $id;
			}
		}

		// Return array of IDs
		return $retval;
	}

	/**
	 * Add a cache value for a key and group.
	 *
	 * @since 1.0.0
	 *
	 * @param string $key    Cache key.
	 * @param mixed  $value  Cache value.
	 * @param string $group  Cache group. Defaults to $this->cache_group
	 * @param int    $expire Expiration.
	 */
	private function cache_add( $key = '', $value = '', $group = '', $expire = 0 ) {

		// Bail if cache invalidation is suspended
		if ( wp_suspend_cache_addition() ) {
			return;
		}

		// Bail if no cache key
		if ( empty( $key ) ) {
			return;
		}

		// Get the cache group
		$group = $this->get_cache_group( $group );

		// Add to the cache
		wp_cache_add( $key, $value, $group, $expire );
	}

	/**
	 * Get a cache value for a key and group.
	 *
	 * @since 1.0.0
	 *
	 * @param string  $key   Cache key.
	 * @param string  $group Cache group. Defaults to $this->cache_group
	 * @param bool    $force
	 */
	private function cache_get( $key = '', $group = '', $force = false ) {

		// Bail if no cache key
		if ( empty( $key ) ) {
			return;
		}

		// Get the cache group
		$group = $this->get_cache_group( $group );

		// Return from the cache
		return wp_cache_get( $key, $group, $force );
	}

	/**
	 * Set a cache value for a key and group.
	 *
	 * @since 1.0.0
	 *
	 * @param string $key    Cache key.
	 * @param mixed  $value  Cache value.
	 * @param string $group  Cache group. Defaults to $this->cache_group
	 * @param int    $expire Expiration.
	 */
	private function cache_set( $key = '', $value = '', $group = '', $expire = 0 ) {

		// Bail if cache invalidation is suspended
		if ( wp_suspend_cache_addition() ) {
			return;
		}

		// Bail if no cache key
		if ( empty( $key ) ) {
			return;
		}

		// Get the cache group
		$group = $this->get_cache_group( $group );

		// Update the cache
		wp_cache_set( $key, $value, $group, $expire );
	}

	/**
	 * Delete a cache key for a group.
	 *
	 * @since 1.0.0
	 *
	 * @global bool $_wp_suspend_cache_invalidation
	 *
	 * @param string $key   Cache key.
	 * @param string $group Cache group. Defaults to $this->cache_group
	 */
	private function cache_delete( $key = '', $group = '' ) {
		global $_wp_suspend_cache_invalidation;

		// Bail if cache invalidation is suspended
		if ( ! empty( $_wp_suspend_cache_invalidation ) ) {
			return;
		}

		// Bail if no cache key
		if ( empty( $key ) ) {
			return;
		}

		// Get the cache group
		$group = $this->get_cache_group( $group );

		// Delete the cache
		wp_cache_delete( $key, $group );
	}

	/**
	 * Fetch raw results directly from the database.
	 *
	 * @since 1.0.0
	 *
	 * @param array  $cols       Columns for `SELECT`.
	 * @param array  $where_cols Where clauses. Each key-value pair in the array
	 *                           represents a column and a comparison.
	 * @param int    $limit      Optional. LIMIT value. Default 25.
	 * @param null   $offset     Optional. OFFSET value. Default null.
	 * @param string $output     Optional. Any of ARRAY_A | ARRAY_N | OBJECT | OBJECT_K constants.
	 *                           Default OBJECT.
	 *                           With one of the first three, return an array of
	 *                           rows indexed from 0 by SQL result row number.
	 *                           Each row is an associative array (column => value, ...),
	 *                           a numerically indexed array (0 => value, ...),
	 *                           or an object. ( ->column = value ), respectively.
	 *                           With OBJECT_K, return an associative array of
	 *                           row objects keyed by the value of each row's
	 *                           first column's value.
	 *
	 * @return array|object|null Database query results.
	 */
	public function get_results( $cols = array(), $where_cols = array(), $limit = 25, $offset = null, $output = OBJECT ) {

		// Bail if no columns have been passed
		if ( empty( $cols ) ) {
			return null;
		}

		// Fetch all the columns for the table being queried
		$column_names = $this->get_column_names();

		// Ensure valid column names have been passed for the `SELECT` clause
		foreach ( $cols as $index => $column ) {
			if ( ! array_key_exists( $column, $column_names ) ) {
				unset( $cols[ $index ] );
			}
		}

		// Columns to retrieve
		$columns = implode( ',', $cols );

		// Get the table name
		$table = $this->get_table_name();

		// Setup base query
		$query = implode( ' ', array(
			"SELECT",
			$columns,
			"FROM {$table} {$this->table_alias}",
			"WHERE 1=1"
		) );

		// Ensure valid columns have been passed for the `WHERE` clause
		if ( ! empty( $where_cols ) ) {

			// Get keys from where columns
			$columns = array_keys( $where_cols );

			// Loop through columns and unset any invalid names
			foreach ( $columns as $index => $column ) {
				if ( ! array_key_exists( $column, $column_names ) ) {
					unset( $where_cols[ $index ] );
				}
			}

			// Parse WHERE clauses
			foreach ( $where_cols as $column => $compare ) {

				// Basic WHERE clause
				if ( ! is_array( $compare ) ) {
					$pattern   = $this->get_column_field( array( 'name' => $column ), 'pattern', '%s' );
					$statement = " AND {$this->table_alias}.{$column} = {$pattern} ";
					$query    .= $this->get_db()->prepare( $statement, $compare );

				// More complex WHERE clause
				} else {
					$value = isset( $compare['value'] )
						? $compare['value']
						: false;

					// Skip if a value was not provided
					if ( false === $value ) {
						continue;
					}

					// Default compare clause to equals
					$compare_clause = isset( $compare['compare_query'] )
						? trim( strtoupper( $compare['compare_query'] ) )
						: '=';

					// Array (unprepared)
					if ( is_array( $compare['value'] ) ) {

						// Default to IN if clause not specified
						if ( ! in_array( $compare_clause, array( 'IN', 'NOT IN', 'BETWEEN' ), true ) ) {
							$compare_clause = 'IN';
						}

						// Parse & escape for IN and NOT IN
						if ( 'IN' === $compare_clause || 'NOT IN' === $compare_clause ) {
							$value = "('" . implode( "','", $this->get_db()->_escape( $compare['value'] ) ) . "')";

						// Parse & escape for BETWEEN
						} elseif ( is_array( $value ) && 2 === count( $value ) && 'BETWEEN' === $compare_clause ) {
							$_this = $this->get_db()->_escape( $value[0] );
							$_that = $this->get_db()->_escape( $value[1] );
							$value = " {$_this} AND {$_that} ";
						}
					}

					// Add WHERE clause
					$query .= " AND {$this->table_alias}.{$column} {$compare_clause} {$value} ";
				}
			}
		}

		// Maybe set an offset
		if ( ! empty( $offset ) ) {
			$values = explode( ',', $offset );
			$values = array_filter( $values, 'intval' );
			$offset = implode( ',', $values );
			$query .= " OFFSET {$offset} ";
		}

		// Maybe set a limit
		if ( ! empty( $limit ) && ( $limit > 0 ) ) {
			$limit  = intval( $limit );
			$query .= " LIMIT {$limit} ";
		}

		// Execute query
		$results = $this->get_db()->get_results( $query, $output );

		// Return results
		return $results;
	}
}