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.