Provided by: libur-perl_0.440-1_all bug

NAME

       UR::Object::Type::Initializer - Class definition syntax

SYNOPSIS

         UR::Object::Type->define(
             class_name => 'Namespace::MyClass',
             id_by => 'my_class_id',
             has => ['prop_a', 'prop_b']
         );

         UR::Object::Type->define(
             class_name => 'Namespace::MyChildClass',
             is => 'Namespace::MyClass',
             has => [
                 'greeting' => { is => 'String', is_optional => 0,
                                 valid_values => ["hello","good morning","good evening"],
                                 default_value => 'hello' },
             ],
         );

         UR::Object::Type->define(
             class_name => 'Namespace::Helper',
             id_by => 'helper_id',
             has => [
                 'my_class_id' => { is => 'String', is_optional => 0 },
                 'my_class'    => { is => 'Namespace::MyClass', id_by => 'my_class_id' },
                 'my_class_a'  => { via => 'my_class', to => 'prop_a' },
             ],
             has_optional => [
                 'other_attribute' => { is => 'Integer' }
             ],
             data_source => 'Namespace::DataSource::DB',
             table_name  => 'HELPERS',
         );

         UR::Object::Type->define(
             class_name => 'Namespace::Users',
             id_by => ['uid'],
             has => [ 'login','passwd','gid','name','home','shell'],
             data_source => {
                 is => 'UR::DataSource::File',
                 file => '/etc/passwd',
                 column_order => ['login','passwd','uid','gid','name','home','shell',
                 skip_first_line => 0,
                 delimiter => ':'
             },
             id_generator => '-uuid',
         );

DESCRIPTION

       Defining a UR class is like drawing up a blueprint of what a particular kind of object
       will look like. That blueprint includes the properties these objects will have, what other
       classes the new class inherits from, and where the source data comes from such as a
       database or file.

   The Simplest Class
       The simplest class definition would look like this:

         use UR;
         class Thing {};

       You can create an instance of this class like this:

         my $thing_object = Thing->create();

       Instances of this class have no properties, no backing storage location, and no
       inheritance.

       Actually, none of those statements are fully true, but we'll come back to that later...

   A Little Background
       After using UR, or another class that inherits from UR::Namespace, the above "class"
       syntax above can be used to define a class.

       The equivalent, more plain-Perl way to define a class is like this:

         UR::Object::Type->define(
            class_name => 'Thing',
            # the remainder of the class definition would go here, if there were any
         );

       Classes become instances of another class called UR::Object::Type. It has a property
       called class_name that contains the package that instances of these objects are blessed
       into. Class properties are also instances of a class called UR::Object::Property, and
       those properties have properties (also UR::Object::Properties) that describe it, such as
       the type of data it holds, and its length. In fact, all the metadata about classes,
       properties, relationships, inheritance, data sources and contexts are available as
       instances of metadata classes. You can get information about any class currently available
       from the command line with the command

         ur show properties Thing

       with the caveat that "currently available" means you need to be under a namespace
       directory that contains the class you're describing.

   Making Something Useful
         class Vehicle {
             id_by => 'serial_number',
             has => ['color', 'weight'],
             has_optional => ['license_plate'],
         };

       Here we have a basic class definition for a thing we're calling a Vehicle.  It has 4
       properties: serial_number, color, weight and license_plate. Three of these properties are
       required, meaning that when you create one, you must give a value for those properties; it
       is similar to a 'NOT NULL' constraint on a database column. The serial_number property is
       an ID property of the class, meaning that no two instances (objects) of that class can
       exist with the same serial_number; it is similar to having a UNIQUE index on that column
       (or columns) in a database. Not all vehicles have license plates, so it is optional.

       After that, you've effectively created five object instances. One UR::Object::Type
       identified by its class_name being 'Vehicle', and four UR::Object::Property objects
       identified by the pairs of class_name and property_name. For these four properties,
       class_name is always 'Vehicle' and property name is one each of serial_number, color,
       weight and license_plate.

       Objects always have one property that is called 'id'. If you have only one property in the
       id_by section, then the 'id' property is effectively an alias for it. If you have several
       id_by properties, then the 'id' property becomes an amalgamation of the directly named id
       properties such that no two objects of that class will have the same 'id'. If there are no
       id_by properties given (including MyClass above that doesn't have _any_ properties), then
       an implicit 'id' property will get created.  Instances of that class will have an 'id'
       generated internally by an algorithm.  Finally, if the class has more than one ID
       property, none of them may be called 'id', since that name will be reserved for the
       amalgamated-value property.

       You can control how IDs get autogenerated with the class' id_generator metadata.  For
       classes that save their results in a relational database, it will get new IDs from a
       sequence (or some equivalent mechanism for databases that do not support sequences) based
       on the class' table name.  If you want to force the system to use some specific sequence,
       for example if many classes should use the same sequence generator, then put the name of
       this sequence in.

       If the id_generator begins with a dash (-), it indicates a method should be called to
       generate a new ID.  For example, if the name is "-uuid", then the system will call the
       internal method "$class_meta-"autogenerate_new_object_id_uuid>. nd will make object IDs as
       hex string UUIDs.  The default value is '-urinternal' which makes an ID string composed of
       the hostname, process ID, the time the program was started and an increasing integer.  If
       id_generator is a subroutine reference, then the sub will be called with the class
       metaobject and creation BoolExpr passed as parameters.

       You'll find that the parser for class definitions is pretty accepting about the kinds of
       data structures it will take. The first thing after class is used as a string to name the
       class. The second thing is a hashref containing key/value pairs. If the value part of the
       pair is a single string, as the id_by is in the Vehicle class definition, then one
       property is created. If the value portion is an arrayref, then each member of the array
       creates an additional property.

   Filling in the Details
       That same class definition can be made this way:

         class Vehicle {
             id_by => [
                 serial_number => { is => 'String', len => 25 },
             ],
             has => [
                 color => { is => 'String' },
                 weight => { is => 'Number' },
                 license_plate => { is => 'String', len => 8, is_optional => 1 },
             ],
         };

       Here we've more explicitly defined the class' properties by giving them a type.
       serial_number and license_number are given a maximum length, and license_number is
       declared as optional. Note that having a 'has_optional' section is the same as explicitly
       putting 'is_optional => 1' for all those properties. The same shortcut is supported for
       the other boolean properties of UR::Object::Property, such as is_transient, is_mutable,
       is_abstract, etc.

       The type system is pretty lax in that there's nothing stopping you from using the method
       for the property to assign a string into a property declared 'Number'.  Type, length,
       is_optional constraints are checked by calling "__errors__()" on the object, and
       indirectly when data is committed back to its data source.

   Inheritance
         class Car {
             is => 'Vehicle',
             has => [
                 passenger_count   => { is => 'Integer', default_value => 0 },
                 transmission_type => { is => 'String',
                                        valid_values => ['manual','automatic','cvt'] },
             ],
         };

         my $car = Car->create(color => 'blue',
                               serial_number => 'abc123',
                               transmission_type => 'manual');

       Here we define another class called Car. It inherits from Vehicle, meaning that all the
       properties that apply to Vehicle instances also apply to Car instances. In addition, Car
       instances have two new properties.  passenger_count has a default value of 0, and
       transmission_type is constrained to three possible values.

   More class properties
       Besides property definitions, there are other things that can be specified in a class
       definition.

       is  Used to name the parent class(es). Single inheritance can be specified by just listing
           the name of the parent class as a string. Multiple inheritance is specified by an
           arrayref containing the parent class names.  If no 'is' is listed, then the class will
           inherit from 'UR::Entity'

       doc A single string to list some short, useful documentation about the class.

       data_source
           A string to list the data source ID. For classes with no data_source, the only objects
           get() can return are those that had previously been instantiated with create() or
           define() earlier in the program, and they do not get saved anywhere during a commit().
           They do, however, exist in the object cache during the program's execution.

           data_source can also be a hashref to define a data source in line with the class
           definition.  See below for more information about "Inline Data Sources".

       table_name
           When the class' data source is some kind of database, "table_name" Specifies the name
           of the table where this class' data is stored to.

       select_hint
           Some relational databases use hints as a way of telling the query optimizer to behave
           differently than usual.  These hints are specified inside comments like this:
               /* the hint */ If the class is the primary class of a query, and it has a hint,
           then the hint will appear after the word 'select' in the SQL.

       join_hint
           If the class is part of a query where it is joined, then its hint will be added to the
           hints already part of the query.  The primary table's hint will be first, followed by
           the joined class' hints in the order they are joined.  All the hints are separated by
           a single space.

       is_abstract
           A flag indicating that no instances of this class may be instantiated, instead it is
           used as a parent of other classes.

       sub_classification_method_name
           Holds the name of a method that is called whenever new instances of the class are
           loaded from a data source. This method will be called with two arguments: the name of
           the class the get() was called on, and the object instance being loaded. The method
           should return the complete name of a subclass the object should be blessed into.

       sub_classification_property_name
           Works like 'sub_classification_method_name', except that the value of the property is
           directly used to subclass the loaded object.

   Properties properties
       "ur show properties UR::Object::Property" will print out an exhaustive list of all the
       properties of a Class Property. A class' properties are declared in the 'id_by' or one of
       the 'has' sections.  Some of the more important ones:

       class_name
           The name of the class this property belongs to.

       property_name
           The name of the property. 'property_name' and 'class_name' do not actually appear in
           the hashref that defines the property inside a class definition, though they are
           properties of UR::Object::Property instances.

       is  Specifies the data type of this property. Basic types include 'String', 'Integer',
           'Float'.  Relationships between classes are defined by having the name of another
           class here. See the Relationships section of UR::Manual::Cookbook for more
           information.

           Object properties do not normally hold Perl references to other objects, but you may
           use 'ARRAY' or 'HASH' here to indicate that the object will store the reference
           directly.  Note that these properties are not usually saveable to outside data
           sources.

       data_type
           A synonym for 'is'

       len Specifies the maximum length of the data, usually in bytes.

       doc A space for useful documentation about the property

       default_value
           The value a property will have if it is not specified when the object is created.  If
           used on a property that is 'via' another property (see the Indirect Properties section
           below), it can trigger creation of a referent object.

       calculated_default
           A name of an instance method or a code ref to be used to resolve a default value
           during object creation.  The instance method will be called immediately after UR
           creates the initial entity so the method will have access to other parameters used
           during creation.

           Specifying "calculated_default > 1" is equivalent to,

             calculated_default => '__default_' . $prop_name . '__'

           and is meant to establish a naming convention without requiring it.

       is_mutable
           A flag indicating that this property can be changed. It is the default state of a
           property.  Set this to 0 in the property definition if the property is not changeable
           after the object is created.

       is_constant
           A flag indicating that the value of this property may not be changed after the object
           is created. It is a synonym for having is mutable = 0

       is_many
           Indicates that this returns a list of values.  Usually used with reverse_as
           properties.

       is_optional
           Indicates that this property can hold the value undef.

       Calculated Properties

       is_calculated
           A flag indicating that the value of this property is determined from a function.

       calculate_from
           A listref of other property names used by the calculation

       calculate
           A specification for how the property is to be calculated in Perl.

           ·     if the value is a coderef, it will be called when that property is accessed, and
                 the first argument will be the object instance being acted on.

           ·     the value may be a string containing Perl code that is eval-ed when the accessor
                 is called. The Perl code can refer to $self, which will hold the correct object
                 instance during execution of that code. Any properties listed in the
                 'calculate_from' list will also be initialized

           ·     The special value 'sum' means that the values of all the properties in the
                 calculate_from list are added together and returned

           Any property can be effectively turned into a calculated property by defining a method
           with the same name as the property.

       Database-backed properties

       column_name
           For classes whose data is stored in a database table (meaning the class has a
           data_source), the column_name holds the name of the database column in its table. In
           the default case, the column_name is the same as the 'property_name'.

       calc_sql
           Specifies that this property is calculated, and its value is a string containing SQL
           code inserted into that property's "column" in the SELECT clause

   Relation Properties
       Some properties are not used to hold actual data, but instead describe some kind of
       relationship between two classes.  For example:

         class Person {
             id_by => 'person_id',
             has => ['name'],
         };
         class Thing {
             id_by => 'thing_id',
             has => [
                 owner => { is => 'Person', id_by => 'owner_id' },
             ],
         };
         $person = Person->create(person_id => 1, name => 'Bob');
         $thing = Thing->create(thing_id => 2, owner_id => 1);

       Here, Thing has a property called "owner".  It implicitly defines a property called
       "owner_id".  "owner" becomes a read-only property that returns an object of type Person by
       using the object's value for the "owner_id" property, and looking up a Person object where
       its ID matches.  In the above case, "$thing->owner" will return the same object that
       $person contains.

       Indirect properties can also link classes with multiple ID properties.

         class City {
             id_by => ['name', 'state']
         };
         class Location {
             has => [
                city    => { is => 'String' },
                state   => { is => 'String' },
                cityobj => { is => 'City',
                             id_by => ['city', 'state' ] },
             ],
         };

       Note that the order the properties are linked must match in the relationship property's
       "id_by" and the related class's "id_by"

   Reverse Relationships
       When one class has a relation property to another, the target class can also define the
       converse relationship.  In this case, OtherClass is the same as the first "Relation
       Properties" example where the relationship from OtherClass to MyClass, but we also define
       the relationship in the other direction, from MyClass to OtherClass.

       Many Things can point back to the same Person.

         class Person {
             id_by => 'person_id',
             has => ['name'],
             has_many => [
                 things => { is => 'Thing', reverse_as => 'owner' },
             ]
         };
         class Thing {
             id_by => 'thing_id',
             has => [
                 owner => { is => 'Person', id_by => 'owner_id' },
             ],
         };

       Note that the value for "reverse_as" needs to be the name of the relation property in the
       related class that would point back to "me".  Yes, it's a bit obtuse, but it's the best we
       have for now.

   Indirect Properties
       When the property of a related object has meaning to another object, that relationship can
       be defined through an indirect property.  Things already have owners, but it is also
       useful to know a Thing's owner's name.

         class Thing {
             id_by => 'thing_id',
             has => [
                 owner => { is => 'Person', id_by => 'owner_id' },
                 owner_name => { via => 'owner', to => 'name', default_value => 'No one' },
             ],
         };
         $name = $thing->owner_name();
         $name eq $person->name;  # evaluates to true

       The values of indirect properties are not stored in the object.  When the property's
       method is called, it looks up the related object through the accessor named in "via", and
       on that result, returns whatever the method named in "to" returns.

       If one of these Thing objects is created by calling Thing->create(), and no value is
       specified for owner_id, owner or owner_name, then the system will find a Person object
       where its 'name' is 'No one' and assign the Thing's owner_id to point to that Person.  If
       no matching Person is found, it will first create one with the name 'No one'.

   Alias Properties
       Sometimes it's useful to have a property that is an alias for another property, perhaps as
       a refactoring tool or to make the API clearer.  The is accomilished by defining an
       indirect property where the 'via' is __self__.

         class Thing {
             id_by => 'thing_id',
             has => [
                 owner => { is => 'Person', id_by => 'owner_id' },
                 titleholder => { via => '__self__', to => 'owner' },
             ]
         };

       In this case, 'titleholder' is an alias for the 'owner' property.  titleholder can be
       called as a method any place owner is a valid method call.  BoolExprs may refer to
       titleholder, but any such references will be rewrittn to 'owner' when they are normalized.

   Subclassing Members of an Abstract Class
       In some cases, objects may be loaded using a parent class, but all the objects are binned
       into some other subclass.

         class Widget {
             has => [
                 manufacturer => { is => 'String',
                                   valid_values => ['CoolCo','Vectornox'] },
             ],
             is_abstract => 1,
             sub_classification_method_name => 'subclasser',
         };
         sub Widget::subclasser {
             my($class,$pending_object) = @_;
             my $subclass = 'Widget::' . $pending_object->manufacturer;
             return $subclass;
         }

         class Widget::CoolCo {
             is => 'Widget',
             has => 'serial_number',
         };
         class Widget::Vextornox {
             is => 'Widget',
             has => 'series_string',
         }

         my $cool_widget = Widget->create(manufacturer => 'CoolCo');
         $cool_widget->isa('Widget::CoolCo'); # evaluates to true
         $cool_widget->serial_number(12345);  # works
         $cool_widget->series_srting();       # dies

       In the class definition for the parent class, Widget, it is marked as being an abstract
       class, and the sub_classification_method_name specifies the name of a method to call
       whenever a new Widget object is created or loaded.  That method is passed the pre-
       subclassed object and must return the fully qualified subclass name the object really
       belongs in.  All the objects returned to the caller will be blessed into the appropriate
       subclass.

       Alternatively, a property can be designated to hold the fully qualified subclass name.

         class Widget {
             has => [
                 subclass_name => { is => 'String',
                                    valid_values => ['Widget::CoolCo',
                                                     'Widget::Vectornox'] },
             ],
             is_abstract => 1,
             subclassify_by => 'subclass_name',
         }

         my $cool_widget = Widget->create(subclass_name => 'Widget::CoolCo');
         $cool_widget = Widget::CoolCo->create();  # subclass_name is automatically "Widget::CoolCo"

       These subclass names will be saved to the data source if the class has a data source.
       Also, when objects of the base class are retrieved with get(), the results will be
       automatically put in the appropriate child class.

   Inline Data Sources
       If the data_source of a class definition is a hashref instead of a simple string, that
       defines an in-line data source.  The only required item in that hashref is "is", which
       declares what class this data source will be created from, such as
       "UR::DataSource::Oracle" or "UR::DataSource::File".  From there, each type of data source
       will have its own requirements for what is allowed in an inline definition.

       For UR::DataSource::RDBMS-derived data sources, it accepts these keys corresponding to the
       properties of the same name:

         server, user, auth, owner

       For UR::DataSource::File data sources:

         server, file_list, column_order, sort_order, skip_first_line,
         delimiter, record_separator

       In addition, file is a synonym for server.

       For UR::DataSource::FileMux data sources:

         column_order, sort_order, skip_first_line, delimiter,
         record_separator, required_for_get, constant_values, file_resolver

       In addition, resolve_path_with can replace "file_resolver" and accepts several formats:

       subref
           A reference to a subroutine.  In this case, "resolve_path_with" is a synonym for
           "file_resolver".

       [ $subref, param1, param2, ..., paramn ]
           The subref will be called to resolve the path.  Its arguments will be taken from the
           values in the rule from properties mentioned.

       [ $format, param1, param2, ..., paramn ]
           $format will be interpreted as an sprintf() format.  The placeholders in the format
           will be filled in from the values in the rule from properties mentioned.

       Finally, "base_path" and "resolve_path_with" can be used together.  In this case,
       resolve_path_with is a listref of property names, base_path is a string specifying the
       first part of the pathname.  The final path is created by joining the base_path and all
       the property's values together with '/', as in
         join('/', $base_path, param1, param2, ..., paramn )

SEE ALSO

       UR::Object::Type, UR::Object::Property, UR::Manual::Cookbook