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.