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.