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Jean-Louis Leroy > Tangram-2.04 > Tangram::Schema



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Source   Latest Release: Tangram-2.07


Tangram::Schema - describe a system of persistent classes


   use Tangram;

   $schema = Tangram::Schema->new( $hashref );

   Tangram::Relational->connect( $schema, ... );

   # write SQL to FILE
   $schema->deploy( \*FILE );

   # write SQL to STDOUT


A Schema contains all the information about the persistent aspects of a system of classes. That information is used to perform the mapping between OO constructs and a relational database.

Schema objects are initialized from a nested data structure called a schema hash. The general structure of the schema hash is described here.

The resulting Schema object becomes the owner of the schema hash passed to new(). The hash may not be modified afterwards, and no assumptions can be made regarding its content.



   $schema = Tangram::Schema->new( $hash );

Returns a new Schema object.

The newly created Schema object becomes the owner of the hash, which can no longer be modified nor reused by client code.

The schema hash describes the persistent aspects of a system of classes. It is a multilevel data structure.

  1. The first level of the hash contains information that is relevant to the system as a whole.
  2. The second level contains information on a per-class basis.
  3. The third level contains information about the individual fields in a class. That information depends on the type of the field and is not documented here; see "field hash" for a list of predefined persistent types.

Global properties

The first level of the schema hash describes aspects that are global to a system of persistent classes. It has the following aspect:

      classes =>
        Identity =>
            table => 'IdentityState',
         abstract => 1

         NaturalPerson =>
            bases => [ qw( Identity ) ],

         LegalPerson =>
            bases => [ qw( Identity ) ],

      make_object => sub { ... },
      set_id => sub { ... }
      get_id => sub { ... }
      normalize => sub { ... },

      control => '...'

      sql => { ... },

classes is an array called the "class registry". It contains a description of each persistent class.

make_object contains a reference to a closure that returns a new object of a given class. This field is optional: by default, Tangram calls class method new().

set_id and get_id are used together to associate an object ID with a persistent object. By default, Tangram converts a reference to an object to a unique integer value by evaluating the expression 0 + $obj. The result is used as a key in a hash contained in the Storage object. The values in that hash are the object IDs.

If any of your classes use overloading, this approach will not work and you will need to supply your own get/set_id methods.

control is the name of a table that has a single row, containing the major and minor version numbers of the Tangram that created the storage, and the highest allocated object id. It defaults to 'Tangram'.

Optional field normalize contains a subroutine that's called to transform classnames and fieldnames into table and column names. The function is called with two arguments; the name to be transformed, and a 'type' argument (currently one of 'tablename' or 'fieldname'). The return value should be the transformed string.

Note that it is expected that the normalize sub will return identical strings with identical arguments, that normalize(normalize($string, $flag), $flag) eq normalize($string, $flag)

Optional field sql contains a hash that can be used to customize some of the SQL generated by Tangram. That hash has the following fields:

All the fields are optional.

default_null can be used to deal with those databases that don't support the explicit 'NULL' specifier in column definitions. Defaults to 'NULL'.

The other fields are related to the SQL types that Tangram uses to store meta-information.

Object ids encode the type of the object. Tangram assigns a class id to each persistent concrete class within a Schema. When an object is inserted, Tangram allocates a unique integer from a class-specific allocator, then appends the class id to it. Thus the object id for a NaturalPerson may look like 270005, where 0005 is the class id.

Field id contains the SQL type that is used to map an entire object id. It defaults to 'INTEGER'.

Field cid contains the SQL type that is used to map a class id. It defaults to 'INTEGER'.

Field cid_size contains the number of decimal positions that the class id occupies within the object id. It defaults to '4'.

Field oid contains the SQL type that is used to store the class-specific object counters that are used in the id generation process. It defaults to 'INTEGER'.

class registry

The class registry is a hash containing one entry per persistent class. The key is the class name, the value is a reference to a hash called the class hash. It contains information on how to map the class.

The class hash has the following fields:

Field abstract contains a boolean that should be true if the class is abstract. If this field is not present, the class is considered to be concrete.

Field bases contains a reference to an array of base classes.

Field fields contains a reference to the "field hash".

Field table sets the name of the table that Tangram should use to store the state of objects pertaining to this class. This field is optional: it defaults to the class name. If the class name is not an acceptable SQL table identifier, you will need to set this field.

Field id contains an integer identifier for this class. That identifier must be unique within the same schema. If this field is not present, Tangram sets it to the last class id plus one.

field hash

Each persistent type is identified by a 'typetag', e.g. int, string or array.

All the persistent fields of a given type are grouped together inside the field hash, where the typetag is used as a key. The individual fields are specified in an array or a hash, whose layout is type-dependant. For example:

   fields =>
      string   => [ qw( firstName name ) ],
      int      => [ qw( age ) ],
      ref      => { partner => { null => 1 } },
      array    => { children => 'NaturalPerson' },

The typetag not only specifies the type of a field, but also the way in which it should be mapped to SQL constructs. Sometimes the same Perl type lends itself to more than one mapping, for example there are at least two plausible ways of mapping a Perl array (see Tangram::Array and Tangram::IntrArray).

Tangram's persistent type system is extensible, allowing you to mount your own types and make them persistent. All you have to do is to register your type and provide mapping code. See Tangram::Type.

Tangram comes with built-in support for the following types:

* string, int, real: see Tangram::Scalar

* reference : see Tangram::Ref

* array : see Tangram::Array, Tangram::IntrArray

* Set::Object : see Tangram::Set, Tangram::IntrSet



This method is deprecated. See Tangram::Relational.


This method is deprecated. See Tangram::Relational.

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