Provided by: libtangram-perl_2.12-2_all 
NAME
Tangram::Schema - describe a system of persistent classes
SYNOPSIS
use Tangram;
$schema = Tangram::Schema->new( $hashref );
Tangram::Relational->connect( $schema, ... );
# write SQL to FILE
$schema->deploy( \*FILE );
# write SQL to STDOUT
$schema->deploy();
DESCRIPTION
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.
note: No corresponding Perl classes are generated from calling "Tangram::Schema->new". If
you want that, and like the behaviour of the separately distributed "Class::Tangram"
module, then you should pass the $hashref in the above example to the
"Class::Tangram::Generator->new()" constructor. See Class::Tangram::Generator for more
information.
CLASS METHODS
new
$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. The available options are:
• default_null
"default_null" can be used to deal with those databases that don't support the
explicit 'NULL' specifier in column definitions. Defaults to 'NULL'. Note that this
does not get automatically appended to attributes that have a SQL type explicitly
declared.
• id
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 the rowid part of the object id.
It defaults to 'INTEGER'.
• oid_sequence
If set, this is the name of a sequence to use as the default OID generator, should a
particular class not define one.
Sequences are an alternate way of generating unique identifiers for rows. They are
more scalable when you have high concurrency, but most people won't notice that.
It is also possible to define an OID sequence on a per-class level; see below for
details.
Sequences are emulated on pretend databases like MySQL.
• make_id
This is a closure that is expected to return an unique ID. It is called like this:
$make_id->($class_id, $storage, $object)
Where $class_id is the Class identifier for_the newly created object, and $storage is
the Tangram::Storage object. The $object is the instance of the object to be
inserted.
• cid
Field "cid" contains the SQL type that is used to map the class id part of the object
id. It defaults to 'INTEGER'.
• cid_size
Field "cid_size" contains the number of decimal positions that the class id occupies
within the object id. It defaults to '4'. This does not affect the database, only the
in-memory representation of object IDs.
• oid
Historical spurious documentation bug. Documentation described the function that the
"id" option performs.
• table_type
Field "table_type" is a string that if set, will be appended to all CREATE TABLE
commands with TYPE=x. For instance, to use transactions with a MySQL database with
InnoDB enabled, set "table_type" to "InnoDB", and (re)deploy your database.
• dumper
This field sets the default mechanism by which arbitrary structures are serialised to
columns, in the absence of a portion of the Tangram schema covering their mapping.
The default value is "Storable" (see Storable), seeing as Storable is something like
the "native" serialisation mechanism for Perl. Currently, this setting only applies
to the idbif mapping type (see Tangram::Type::Dump::Any).
It would make more sense for the default to be "YAML", but unfortunately YAML doesn't
support dumping the entire range of native Perl data types, which sucks immensely.
The other fields are related to the SQL types that Tangram uses to store meta-information.
class registry
The class registry is an array containing one entry per persistent class. The array
contains a list of "key => value" pairs. 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 can have the following fields:
• abstract
• bases
• fields
• table
• table_type
• id
• oid_sequence
• make_id
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 "table_type" sets the type of the table, for instance, the storage back-end to the
RDBMS or storage format; it specifies on a per-table basis what the "table_type" attribute
of the schema defines. You almost certainly don't want to set this on a per-table basis.
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.
Fields "oid_sequence" and "make_id" are per-class versions of their schema-wide versions
documented above. These should be inherited by their subclasses, but currently (as of
2.07_06) aren't. To be safe, until this documentation is fixed, define them in all
subclasses.
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::Type::Array::FromMany and Tangram::Type::Array::FromOne).
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::Type::Scalar
* reference : see Tangram::Type::Ref::FromMany
* array : see Tangram::Type::Array::FromMany, Tangram::Type::Array::FromOne
* Set::Object : see Tangram::Type::Set::FromMany, Tangram::Type::Set::FromOne
INSTANCE METHODS
deploy
This method is deprecated. See Tangram::Relational.
retreat
This method is deprecated. See Tangram::Relational.