NAME

UR::Manual::Metadata - Overview of the metadata classes in UR

SYNOPSIS

use MyNamespace;
my $class_meta = MyNamespace::SomeClass->__meta__;
my @property_metas = $class_meta->direct_property_metas();
my @parent_class_metas = $class_meta->ancestry_class_metas();

my $table_obj = UR::DataSource::RDBMS::Table->get(
                table_name => $class_meta->table_name,
            );
my @column_objs = $table_obj->columns();

DESCRIPTION

The UR system creates and uses several classes and objects to represent information about the many classes and objects in the system - metadata. For example, for each class, there is an object, called a class metadata object, to represent it. Each property in a class has metadata. So does the relationship between parent and child classes and relationships involved in delegated properties. Metadata about any database schemas your namespace knows about is also tracked and stored.

These classes define an API for introspection and reflection, a way for the system to change itself as it runs, and methods for tracking changes and applying those changes to files and databases.

APIs

The metadata API is divided into 5 primary parts:

Defining Classes

The mechanism for defining class structure, including their properties and relationships. It handles creating accessor/mutator methods for you. The syntax for defining classes is detailed in the UR::Object::Type::Initializer page.

Objects Representing Classes, Properties, Relationships, etc.

UR Classes aren't just conceptual entities like a package name, they have object instances to represent them. For every named class, you can get a UR::Object::Type instance with that class_name. Each property defined on that class has a UR::Object::Property with a matching class_name and property_name pair. Even those basic metadata classes have class, property and relationship metadata of their own.

Schema Objects

If you use the ur update classes command-line tool to manage the linkage between your database schema(s) and class structure (it's not necessary; you can also manage it by hand), then objects will also exist to represent the database entities. See also UR::DataSource::Meta

. tables UR::DataSource::RDBMS::Table

. columns UR::DataSource::RDBMS::TableColumn

. Foreign key constraints UR::DataSource::RDBMS::FkConstraint and UR::DataSource::RDBMS::FkConstraintColumn

. Primary key constraints UR::DataSource::RDBMS::PkConstraintColumn

. Unique constraints UR::DataSource::RDBMS::UniqueConstraintColumn

Namespaces, Contexts and Data Sources

Namespaces (UR::Namespace) collect and manage groups of related classes. Classes can be a member of one Namespace, and in practice will live in a subdirectory under the Namespace module's name.

Contexts (UR::Context) and Data Sources (UR::DataSource) provide a filtered view of the data that is reachable through the current Namespace.

Index, Change, Observer and other incidentals

And then there's everything else

UR::Object::Index objects are created by the system to handle get() requests for non-ID parameters.

UR::Change objects represent a change in the system during a software transaction, such as an object's property changind value or creating a new instance of something.

UR::Observer objects manage the change subscription system, where the application can be notified of changes through callbacks. See also "create_subscription" in UR::Object.