item fields based on query_vars['fields'].
	 *
	 * @since 1.0.0
	 *
	 * @param array $items
	 * @return array
	 */
	private function get_item_fields( $items = array() ) {

		// Get the primary column name
		$primary = $this->get_primary_column_name();
		$fields  = $this->query_vars['fields'];

		// Strings need to be single columns
		if ( is_string( $fields ) ) {
			$field = sanitize_key( $fields );
			$items = ( 'ids' === $fields )
				? wp_list_pluck( $items, $primary )
				: wp_list_pluck( $items, $field, $primary );

		// Arrays could be anything
		} elseif ( is_array( $fields ) ) {
			$new_items = array();
			$fields    = array_flip( $fields );

			// Loop through items and pluck out the fields
			foreach ( $items as $item_id => $item ) {
				$new_items[ $item_id ] = (object) array_intersect_key( (array) $item, $fields );
			}

			// Set the items and unset the new items
			$items = $new_items;
			unset( $new_items );
		}

		// Return the item, possibly reduced
		return $items;
	}

	/**
	 * Shape an item ID from an object, array, or numeric value.
	 *
	 * @since 1.0.0
	 *
	 * @param  mixed $item
	 * @return int
	 */
	private function shape_item_id( $item = 0 ) {

		// Default return value
		$retval  = 0;

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

		// Numeric item ID
		if ( is_numeric( $item ) ) {
			$retval = $item;

		// Object item
		} elseif ( is_object( $item ) && isset( $item->{$primary} ) ) {
			$retval = $item->{$primary};

		// Array item
		} elseif ( is_array( $item ) && isset( $item[ $primary ] ) ) {
			$retval = $item[ $primary ];
		}

		// Return the item ID
		return absint( $retval );
	}

	/** Queries ***************************************************************/

	/**
	 * Get a single database row by the primary column ID, possibly from cache.
	 *
	 * Accepts an integer, object, or array, and attempts to get the ID from it,
	 * then attempts to retrieve that item fresh from the database or cache.
	 *
	 * @since 1.0.0
	 *
	 * @param int|array|object $item_id The ID of the item
	 * @return object|false False if empty/error, Object if successful
	 */
	public function get_item( $item_id = 0 ) {

		// Shape the item ID
		$item_id = $this->shape_item_id( $item_id );

		// Bail if no item to get by
		if ( empty( $item_id ) ) {
			return false;
		}

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

		// Get item by ID
		return $this->get_item_by( $primary, $item_id );
	}

	/**
	 * Get a single database row by any column and value, possibly from cache.
	 *
	 * Take care to only use this method on columns with unique values,
	 * preferably with a cache group for that column. See: get_item().
	 *
	 * @since 1.0.0
	 *
	 * @param string     $column_name  Name of database column
	 * @param int|string $column_value Value to query for
	 * @return object|false False if empty/error, Object if successful
	 */
	public function get_item_by( $column_name = '', $column_value = '' ) {

		// Default return value
		$retval = false;

		// Bail if no key or value
		if ( empty( $column_name ) || empty( $column_value ) ) {
			return $retval;
		}

		// Bail if name is not a string
		if ( ! is_string( $column_name ) ) {
			return $retval;
		}

		// Bail if value is not scalar (null values also not allowed)
		if ( ! is_scalar( $column_value ) ) {
			return $retval;
		}

		// Get the column names
		$columns = $this->get_column_names();

		// Bail if column does not exist
		if ( ! isset( $columns[ $column_name ] ) ) {
			return $retval;
		}

		// Cache groups
		$groups = $this->get_cache_groups();

		// Check cache
		if ( ! empty( $groups[ $column_name ] ) ) {
			$retval = $this->cache_get( $column_value, $groups[ $column_name ] );
		}

		// Item not cached
		if ( false === $retval ) {

			// Try to get item directly from DB
			$retval = $this->get_item_raw( $column_name, $column_value );

			// Bail on failure
			if ( ! $this->is_success( $retval ) ) {
				return false;
			}

			// Cache
			$this->update_item_cache( $retval );
		}

		// Reduce the item
		$retval = $this->reduce_item( 'select', $retval );

		// Return result
		return $this->shape_item( $retval );
	}

	/**
	 * Add an item to the database.
	 *
	 * @since 1.0.0
	 *
	 * @param array $data
	 * @return bool
	 */
	public function add_item( $data = array() ) {

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

		// If data includes primary column, check if item already exists
		if ( ! empty( $data[ $primary ] ) ) {

			// Shape the primary item ID
			$item_id = $this->shape_item_id( $data[ $primary ] );

			// Get item by ID (from database, not cache)
			$item    = $this->get_item_raw( $primary, $item_id );

			// Bail if item already exists
			if ( ! empty( $item ) ) {
				return false;
			}

			// Set data primary ID to newly shaped ID
			$data[ $primary ] = $item_id;
		}

		// Get default values for item (from columns)
		$item = $this->default_item();

		// Unset the primary key if not part of data array (auto-incremented)
		if ( empty( $data[ $primary ] ) ) {
			unset( $item[ $primary ] );
		}

		// Cut out non-keys for meta
		$columns = $this->get_column_names();
		$data    = array_merge( $item, $data );
		$meta    = array_diff_key( $data, $columns );
		$save    = array_intersect_key( $data, $columns );

		// Get the current time (maybe used by created/modified)
		$time = $this->get_current_time();

		// If date-created exists, but is empty or default, use the current time
		$created = $this->get_column_by( array( 'created' => true ) );
		if ( ! empty( $created ) && ( empty( $save[ $created->name ] ) || ( $save[ $created->name ] === $created->default ) ) ) {
			$save[ $created->name ] = $time;
		}

		// If date-modified exists, but is empty or default, use the current time
		$modified = $this->get_column_by( array( 'modified' => true ) );
		if ( ! empty( $modified ) && ( empty( $save[ $modified->name ] ) || ( $save[ $modified->name ] === $modified->default ) ) ) {
			$save[ $modified->name ] = $time;
		}

		// Try to add
		$table  = $this->get_table_name();
		$reduce = $this->reduce_item( 'insert', $save );
		$save   = $this->validate_item( $reduce );
		$result = ! empty( $save )
			? $this->get_db()->insert( $table, $save )
			: false;

		// Bail on failure
		if ( ! $this->is_success( $result ) ) {
			return false;
		}

		// Get the new item ID
		$item_id = $this->get_db()->insert_id;

		// Maybe save meta keys
		if ( ! empty( $meta ) ) {
			$this->save_extra_item_meta( $item_id, $meta );
		}

		// Use get item to prime caches
		$this->update_item_cache( $item_id );

		// Transition item data
		$this->transition_item( $save, $item_id );

		// Return result
		return $item_id;
	}

	/**
	 * Copy an item in the database to a new item.
	 *
	 * @since 1.1.0
	 *
	 * @param int $item_id
	 * @param array $data
	 * @return bool
	 */
	public function copy_item( $item_id = 0, $data = array() ) {

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

		// Get item by ID (from database, not cache)
		$item    = $this->get_item_raw( $primary, $item_id );

		// Bail if item does not exist
		if ( empty( $item ) ) {
			return false;
		}

		// Cast object to array
		$save = (array) $item;

		// Maybe merge data with original item
		if ( ! empty( $data ) && is_array( $data ) ) {
			$save = array_merge( $save, $data );
		}

		// Unset the primary key
		unset( $save[ $primary ] );

		// Return result
		return $this->add_item( $save );
	}

	/**
	 * Update an item in the database.
	 *
	 * @since 1.0.0
	 *
	 * @param int $item_id
	 * @param array $data
	 * @return bool
	 */
	public function update_item( $item_id = 0, $data = array() ) {

		// Bail if no item ID
		$item_id = $this->shape_item_id( $item_id );
		if ( empty( $item_id ) ) {
			return false;
		}

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

		// Get item to update (from database, not cache)
		$item    = $this->get_item_raw( $primary, $item_id );

		// Bail if item does not exist to update
		if ( empty( $item ) ) {
			return false;
		}

		// Cast as an array for easier manipulation
		$item = (array) $item;

		// Unset the primary key from data to parse
		unset( $data[ $primary ] );

		// Splice new data into item, and cut out non-keys for meta
		$columns = $this->get_column_names();
		$data    = array_merge( $item, $data );
		$meta    = array_diff_key( $data, $columns );
		$save    = array_intersect_key( $data, $columns );

		// Maybe save meta keys
		if ( ! empty( $meta ) ) {
			$this->save_extra_item_meta( $item_id, $meta );
		}

		// Bail if no change
		if ( (array) $save === (array) $item ) {
			return true;
		}

		// Unset the primary key from data to save
		unset( $save[ $primary ] );

		// If date-modified is empty, use the current time
		$modified = $this->get_column_by( array( 'modified' => true ) );
		if ( ! empty( $modified ) ) {
			$save[ $modified->name ] = $this->get_current_time();
		}

		// Try to update
		$table  = $this->get_table_name();
		$reduce = $this->reduce_item( 'update', $save );
		$save   = $this->validate_item( $reduce );
		$where  = array( $primary => $item_id );
		$result = ! empty( $save )
			? $this->get_db()->update( $table, $save, $where )
			: false;

		// Bail on failure
		if ( ! $this->is_success( $result ) ) {
			return false;
		}

		// Use get item to prime caches
		$this->update_item_cache( $item_id );

		// Transition item data
		$this->transition_item( $save, $item );

		// Return result
		return $result;
	}

	/**
	 * Delete an item from the database.
	 *
	 * @since 1.0.0
	 *
	 * @param int $item_id
	 * @return bool
	 */
	public function delete_item( $item_id = 0 ) {

		// Bail if no item ID
		$item_id = $this->shape_item_id( $item_id );
		if ( empty( $item_id ) ) {
			return false;
		}

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

		// Get item (before it's deleted)
		$item = $this->get_item_raw( $primary, $item_id );

		// Bail if item does not exist to delete
		if ( empty( $item ) ) {
			return false;
		}

		// Attempt to reduce this item
		$item = $this->reduce_item( 'delete', $item );

		// Bail if item was reduced to nothing
		if ( empty( $item ) ) {
			return false;
		}

		// Try to delete
		$table  = $this->get_table_name();
		$where  = array( $primary => $item_id );
		$result = $this->get_db()->delete( $table, $where );

		// Bail on failure
		if ( ! $this->is_success( $result ) ) {
			return false;
		}

		// Clean caches on successful delete
		$this->delete_all_item_meta( $item_id );
		$this->clean_item_cache( $item );

		// Return result
		return $result;
	}

	/**
	 * Filter an item before it is inserted of updated in the database.
	 *
	 * This method is public to allow subclasses to perform JIT manipulation
	 * of the parameters passed into it.
	 *
	 * @since 1.0.0
	 *
	 * @param array $item
	 * @return array
	 */
	public function filter_item( $item = array() ) {
		return (array) apply_filters_ref_array( $this->apply_prefix( "filter_{$this->item_name}_item" ), array( $item, &$this ) );
	}

	/**
	 * Shape an item from the database into the type of object it always wanted
	 * to be when it grew up.
	 *
	 * @since 1.0.0
	 *
	 * @param mixed ID of item, or row from database
	 * @return mixed False on error, Object of single-object class type on success
	 */
	private function shape_item( $item = 0 ) {

		// Get the item from an ID
		if ( is_numeric( $item ) ) {
			$item = $this->get_item( $item );
		}

		// Return the item if it's already shaped
		if ( $item instanceof $this->item_shape ) {
			return $item;
		}

		// Shape the item as needed
		$item = ! empty( $this->item_shape )
			? new $this->item_shape( $item )
			: (object) $item;

		// Return the item object
		return $item;
	}

	/**
	 * Validate an item before it is updated in or added to the database.
	 *
	 * @since 1.0.0
	 *
	 * @param array $item
	 * @return array|false False on error, Array of validated values on success
	 */
	private function validate_item( $item = array() ) {

		// Bail if item is empty or not an array
		if ( empty( $item ) || ! is_array( $item ) ) {
			return $item;
		}

		// Loop through item attributes
		foreach ( $item as $key => $value ) {

			// Strip slashes from all strings
			/*if ( is_string( $value ) ) {
				$value = stripslashes( $value );// We removed this line at PR #3847 to solve if the content has backslash.
			}*/

			// Get the column
			$column = $this->get_column_by( array( 'name' => $key ) );

			// Null value is special for all item keys
			if ( is_null( $value ) ) {

				// Bail if null is not allowed
				if ( false === $column->allow_null ) {
					return false;
				}

			// Attempt to validate
			} elseif ( ! empty( $column->validate ) && is_callable( $column->validate ) ) {
				$validated = call_user_func( $column->validate, $value );

				// Bail if error
				if ( is_wp_error( $validated ) ) {
					return false;
				}

				// Update the value
				$item[ $key ] = $validated;

			/**
			 * Fallback to using the raw value.
			 *
			 * Note: This may change at a later date, so do not rely on this.
			 *       Please always validate all data.
			 */
			} else {
				$item[ $key ] = $value;
			}
		}

		// Return the validated item
		return $this->filter_item( $item );
	}

	/**
	 * Reduce an item down to the keys and values the current user has the
	 * appropriate capabilities to select|insert|update|delete.
	 *
	 * Note that internally, this method works with both arrays and objects of
	 * any type, and also resets the key values. It looks weird, but is
	 * currently by design to protect the integrity of the return value.
	 *
	 * @since 1.0.0
	 *
	 * @param string $method select|insert|update|delete
	 * @param mixed  $item   Object|Array of keys/values to reduce
	 *
	 * @return mixed Object|Array without keys the current user does not have caps for
	 */
	private function reduce_item( $method = 'update', $item = array() ) {

		// Bail if item is empty
		if ( empty( $item ) ) {
			return $item;
		}

		// Loop through item attributes
		foreach ( $item as $key => $value ) {

			// Get capabilities for this column
			$caps = $this->get_column_field( array( 'name' => $key ), 'caps' );

			// Unset if not explicitly allowed
			if ( empty( $caps[ $method ] ) || ! current_user_can( $caps[ $method ] ) ) {
				if ( is_array( $item ) ) {
					unset( $item[ $key ] );
				} elseif ( is_object( $item ) ) {
					$item->{$key} = null;
				}

			// Set if explicitly allowed
			} elseif ( is_array( $item ) ) {
				$item[ $key ] = $value;
			} elseif ( is_object( $item ) ) {
				$item->{$key} = $value;
			}
		}

		// Return the reduced item
		return $item;
	}

	/**
	 * Return an item comprised of all default values.
	 *
	 * This is used by `add_item()` to populate known default values, to ensure
	 * new item data is always what we expect it to be.
	 *
	 * @since 1.0.0
	 *
	 * @return array
	 */
	private function default_item() {

		// Default return value
		$retval   = array();

		// Get the column names and their defaults
		$names    = $this->get_columns( array(), 'and', 'name'    );
		$defaults = $this->get_columns( array(), 'and', 'default' );

		// Put together an item using default values
		foreach ( $names as $key => $name ) {
			$retval[ $name ] = $defaults[ $key ];
		}

		// Return
		return $retval;
	}

	/**
	 * Transition an item when adding or updating.
	 *
	 * This method takes the data being saved, looks for any columns that are
	 * known to transition between values, and fires actions on them.
	 *
	 * @since 1.0.0
	 *
	 * @param array $item
	 * @return array
	 */
	private function transition_item( $new_data = array(), $old_data = array() ) {

		// Look for transition columns
		$columns = $this->get_columns( array( 'transition' => true ), 'and', 'name' );

		// Bail if no columns to transition
		if ( empty( $columns ) ) {
			return;
		}

		// Get the item ID
		$item_id = $this->shape_item_id( $old_data );

		// Bail if item ID cannot be retrieved
		if ( empty( $item_id ) ) {
			return;
		}

		// If no old value(s), it's new
		if ( ! is_array( $old_data ) ) {
			$old_data = $new_data;

			// Set all old values to "new"
			foreach ( $old_data as $key => $value ) {
				$value            = 'new';
				$old_data[ $key ] = $value;
			}
		}

		// Compare
		$keys = array_flip( $columns );
		$new  = array_intersect_key( $new_data, $keys );
		$old  = array_intersect_key( $old_data, $keys );

		// Get the difference
		$diff = array_diff( $new, $old );

		// Bail if nothing is changing
		if ( empty( $diff ) ) {
			return;
		}

		// Do the actions
		foreach ( $diff as $key => $value ) {
			$old_value  = $old_data[ $key ];
			$new_value  = $new_data[ $key ];
			$key_action = $this->apply_prefix( "transition_{$this->item_name}_{$key}" );

			/**
			 * Fires after an object value has transitioned.
			 *
			 * @since 1.0.0
			 *
			 * @param mixed $old_value The value being transitioned FROM.
			 * @param mixed $new_value The value being transitioned TO.
			 * @param int   $item_id   The ID of the item that is transitioning.
			 */
			do_action( $key_action, $old_value, $new_value, $item_id );
		}
	}

	/** Meta ******************************************************************/

	/**
	 * Add meta data to an item.
	 *
	 * @since 1.0.0
	 *
	 * @param int    $item_id
	 * @param string $meta_key
	 * @param string $meta_value
	 * @param string $unique
	 * @return int|false The meta ID on success, false on failure.
	 */
	protected function add_item_meta( $item_id = 0, $meta_key = '', $meta_value = '', $unique = false ) {

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

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

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

		// Return results of adding meta data
		return add_metadata( $meta_type, $item_id, $meta_key, $meta_value, $unique );
	}

	/**
	 * Get meta data for an item.
	 *
	 * @since 1.0.0
	 *
	 * @param int     $item_id
	 * @param string  $meta_key
	 * @param bool    $single
	 * @return mixed Single metadata value, or array of values
	 */
	protected function get_item_meta( $item_id = 0, $meta_key = '', $single = false ) {

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

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

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

		// Return results of getting meta data
		return get_metadata( $meta_type, $item_id, $meta_key, $single );
	}

	/**
	 * Update meta data for an item.
	 *
	 * @since 1.0.0
	 *
	 * @param int    $item_id
	 * @param string $meta_key
	 * @param string $meta_value
	 * @param string $prev_value
	 * @return bool True on successful update, false on failure.
	 */
	protected function update_item_meta( $item_id = 0, $meta_key = '', $meta_value = '', $prev_value = '' ) {

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

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

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

		// Return results of updating meta data
		return update_metadata( $meta_type, $item_id, $meta_key, $meta_value, $prev_value );
	}

	/**
	 * Delete meta data for an item.
	 *
	 * @since 1.0.0
	 *
	 * @param int    $item_id
	 * @param string $meta_key
	 * @param string $meta_value
	 * @param string $delete_all
	 * @return bool True on successful delete, false on failure.
	 */
	protected function delete_item_meta( $item_id = 0, $meta_key = '', $meta_value = '', $delete_all = false ) {

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

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

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

		// Return results of deleting meta data
		return delete_metadata( $meta_type, $item_id, $meta_key, $meta_value, $delete_all );
	}

	/**
	 * Get registered meta data keys.
	 *
	 * @since 1.0.0
	 *
	 * @param string $object_subtype The sub-type of meta keys
	 *
	 * @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;
	}
}