UR::Manual::Metadata - Overview of the metadata classes in UR
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();
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.
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
property_namepair. Parent-child relationships have UR::Object::Inheritance instances, and has-a relationships have UR::Object::Reference and UR::Object::Reference::Property instances. Even those basic metadata classes have class, property and relationship objects of their own.
- Schema Objects
If you use the
ur update classescommand-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.
- 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.