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)
Where $class_id is the Class identifier for_the newly created object, and
$storage is the Tangram::Storage object.
- •
- 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.