Movable Type allows object types to be extended. This allows plugins and components to insert and associate additional pieces of data with a pre-existing data type. Furthermore, these additional pieces of data become a seamless extension of the original data element, allowing that object to be sorted by and filtered by the new data element quickly and easily.

Extending an object is done by declaring the extension within the registry. For example, to add a new "ratings" field to the core entry object for the purposes of allowing users to attach a zero to fives star rating to entry, one would do the following:

sub init_registry {
    my $plugin = shift;
    $plugin->registry({
        object_types => {
            'entry' => {
                rating => integer,
            },
        },
    });
}

Whenever a component or plugin declares an object type that already exists, then that object_type declaration acts as an extension to the pre-existing object type. Remember, if a plugin or component every changes an object type it declares, or changes the nature of an extension to a pre-existing object type, the schema_version associated with that plugin or component should be incremented to signal to Movable Type that some database maintenance may be required.