Class::AutoDB - Almost automatic object persistence -- MySQL only for now
This class works closely with Class::AutoClass to provide almost transparent object persistence.
Person class with attributes 'name', 'sex', 'hobbies', and 'friends', where 'friends' is a list of Persons.
package Person;
use Class::AutoClass;
@ISA=qw(Class::AutoClass);
@AUTO_ATTRIBUTES=qw(name sex friends);
%AUTODB=
(-collection=>'Person',
-keys=>qq(name string, sex string, friends list(object)));
Class::AutoClass::declare(__PACKAGE__);
use Class::AutoDB; use Person; my $autodb=new Class::AutoDB(-database=>'test'); my $joe=new Person(-name=>'Joe',-sex=>'male'); my $mary=new Person(-name=>'Mary',-sex=>'female'); my $bill=new Person(-name=>'Bill',-sex=>'male'); # Set up friends lists $joe->friends([$mary,$bill]); $mary->friends([$joe,$bill]); $bill->friends([$joe,$mary]); $autodb->put_objects; # store objects in database
use Class::AutoDB;
use Person;
my $autodb=new Class::AutoDB(-database=>'test');
my $cursor=$autodb->find(-collection=>'Person',-name=>'Joe'); # run query
print "Number of Joe's in database: ",$cursor->count,"\n";
while (my $joe=$cursor->get_next) { # loop getting objects
# $joe is a Person object -- do what you want with it
my $friends=$joe->friends;
for my $friend (@$friends) {
my $friend_name=$friend->name;
print "Joe's friend is named $friend_name\n";
}
}
-- OR --
use Class::AutoDB; use Person; my $autodb=new Class::AutoDB(-database=>'test'); my $cursor=$autodb->find(-collection=>'Person',-name=>'Joe'); my @joes=$cursor->get;
-- OR --
use Class::AutoDB;
use Person;
my $autodb=new Class::AutoDB(-database=>'test');
my @joes=$autodb->get(-collection=>'Person',-name=>'Joe');
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Not yet implemented:
Delete existing objects
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use Class::AutoDB;
use Person;
my $autodb=new Class::AutoDB(-database=>'test');
my $cursor=$autodb->find(-collection=>'Person',-name=>'Joe');
while (my $joe=$cursor->get_next) {
$autodb->del($joe);
}
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This class implements a simple object persistence mechanism. It is designed to work with Class::AutoClass.
This is how you're supposed to imagine the system works. The section on Current Design explains how it really works at present.
Objects are stored in collections. Each collection has any number of search keys, which you can think of as attributes of an object or columns of a relational table. You can search for objects in the collection by specifying values of search keys. For example
my $cursor=$autodb->find(-collection=>'Person',-name=>'Joe');
finds all objects in the 'Person' collection whose 'name' key is 'Joe'. If you specify multiple search keys, the values are ANDed.
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Not yet implemented:
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The 'find' method also allows almost raw SQL
queries with the caveat that these are very closely tied to the
implementation and will not be portable if we ever change the
implementation.
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A collection can contain objects from many different classes. (This is Perl after all -- what else would you expect ??!!)
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Not yet implemented:
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To limit a search to objects of a specific class,
you can pass a 'class' parameter to find. In fact, you can search for
objects of a given class independent of the collection by specifying a
'class' parameter without a 'collection'.
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Not yet implemented:
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When you create an object, the system
automatically stores it in the database at an 'appropriate' time,
presently just before Perl destroys the in-memory copy of the object.
You can also store objects sooner. When you update an object, it gets
marked as such, and is likewise automatically updated in the database.
Again, you can do the update manually if you prefer.
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You can store specific objects in the database using the 'put' method on an object or store all objects using the 'put_objects' method on AutoDB.
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Not yet implemented:
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put_objects' consults the put_policy to
decide which objects to store.
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$object->put; # store one object
$autodb->put_objects; # store all objects
To use the mechanism, you define the %AUTODB variable in your AutoClass definition. See Class::AutoClass. If you do not set %AUTODB, or set it to undef or (), auto-persistence is turned off for your class.
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Not yet implemented
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In the simplest case, you can simply set
%AUTODB=(1);
This will cause your class to be persistent, using the default
collection name and without any search keys.
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More typically, you set %AUTODB to a HASH of the form
%AUTODB=(
-collection=>'Person',
-keys=>qq(name string, sex string, friends list(object)));
-collection is the name of the collection that will be used to store objects of your class (collection names must be <= 255 characters), and
-keys is a string that defines the search keys that will be defined for the class.
The 'keys' string consists of attribute, data type pairs. Each attribute is generally an attribute defined in the AutoClass @AUTO_ATTRIBUTES or @OTHER_ATTRIBUTES variables. Technically, it's the name of a method that can be called with no arguments. The value of an attribute must be a scalar, an object reference, or an ARRAY (or list) of such values.
The data type can be 'string', 'integer', 'float', 'object', [not yet implemented: any legal MySQL column type], or the phrase list(<data type>), eg, 'list(integer)'. These are translated into MySQL types as follows:
---------------------------------- | AutoDB type | MySQL type | ---------------------------------- | string | longtext | | integer | int | | float | double | | object | bigint | | | (unsigned) | ----------------------------------
The 'keys' parameter can also be an array of attribute names, eg,
-keys=>[qw(name sex)]
in which case the data type of each attribute is assumed to be 'string'. This works in many cases even if the data is really numeric.
The types 'object' and 'list(object)' only work on objects whose persistence is managed by our Persistence mechanisms.
The 'collection' value may also be an array of collection names (and may be called 'collections') in which case the object is stored in all the collections.
A subclass need not define %AUTODB, but may instead rely on the value set by its super-classes. If the subclass does define %AUTODB, its values are 'added' to those of its super-classes. Thus, if the suclass uses a different collection than its super-class, the object is stored in both. It is an error for a subclass to define the type of a search key differently than its super-classes. It is also an error for a subclass to inherit a search key from multiple super-classes with different types We hope this situation is rare!
Technically, %AUTODB is a parameter list for the register method of Class::AutoDB. See that method for more details. Some commonly used paramters are
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Not yet implemented
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-transients: an array of attributes that should not be stored. This is
useful for objects that contain computed values or other information of
a transient nature.
-auto_gets: an array of attributes that should be automatically
retrieved when this object is retrieved. These should be attributes
that refer to other auto-persistent objects. This useful in cases where
there are attributes that are used so often that it makes sense to
retrieve them as soon as possible.
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After setting up your classes to use AutoDB, here's how you use the mechanism.
The first step is to connect your program to the database. This is accomplished via the 'new' method.
Then you typically retrieve some number of "top level" objects explcitly by running a query. This is accomplished via the 'find' and 'get' methods (there are several flavors of 'get'). Then, you operate on objects as usual. If you touch an object that has not yet been retrieved from the database, the system will automatically get it for you.
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Not yet implemented
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You can also manually retrieve objects at
any time by running 'get'.
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Not yet implemented
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You can create new objects as usual and they will
be automatically written to the database when you're done with them.
More precisely, Class::AutoDB::Object::DESTROY writes the object to the
database when Perl determines that the in-memory representation of the
object is no longer needed. This is guaranteed to happen when the
program terminates if not before. You can also manually write objects
to the database earlier if you so desire by running the 'put' method on
them. If you override DESTROY, make sure you call
Class::AutoDB::Object::DESTROY in your method.
You can modify objects as usual and the system will take care of
writing the updates to the database, just as it does for new objects.
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You can store specific objects in the database using the 'put' method on an object or store all objects using the 'put_objects' method on AutoDB.
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Not yet implemented
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'put_objects' consults the put_policy to
decide which objects to store.
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$object->put; # store one object
$autodb->put_objects; # store all objects
The examples in the SYNOPSIS use variables named $autodb, $cursor, $joe, and @joes among others. These names reflect the various stages of data access that arise.
The first step is to connect to the database. This is accomplished by 'new'.
Next a query is sent to the database and executed. This is typically accomplished by invoking 'find' on an AutoDB object. The resulting object (called $cursor in the SYNOPSIS) is called a 'cursor' object. A cursor object's main purpose is to enable data access. (DBI afficionados will recogonize that it's possible to 'prepare' a query before executing it. This is done under the covers here.)
Finally data is retrieved. This is typically accomplished by invoking 'get_next' or 'get' on a cursor object. Data can be retrieved one object at a time (via 'get_next') or all at once (via 'get'). As a convenience, you can do the 'find' and 'get' in one step, by invoking 'get' on an AutoDB object.
The query executed by 'find' can either be a simple key based search, or [not yet implemented: an almost raw, arbitrarily complex SQL query]. The former is specified by providing key=>value pairs as was done in the SYNOPSIS, eg,
$cursor=$autodb->find(-collection=>'Person',-name=>'Joe',-sex=>'male');
The key=>value pairs are ANDed as one would expect. The above query retrieves all Persons whose name is Joe and sex is male.
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Not yet implemented
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'find' can also be invoked on a cursor object.
The effect is to AND the new query with the old. This only works with
queries expressed as key=E<gt>value pairs, and not raw SQL, since
conjoining raw SQL is a bear.
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Not yet implemented
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The raw form is specifed by providing a SQL query
(as a string) that lacks the SELECT phrase, eg,
$cursor=$autodb-E<gt>find(qq(FROM Person WHERE name="Joe" AND
sex="male"));
To use this form, you have to understand the relational database schema
generated by AutoDB. This is not portable across implementations, It's
main value is to write complex queries that cannot be represented in
the key=E<gt>value form. For example
$cursor=$autodb->find(qq(FROM Person p, Person friend, Person_friends friends
WHERE p.name="Joe" AND
(friend.name="Mary" OR friend.name="Bill") AND
friends.person=p));
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Before you can use AutoDB, you have to create a MySQL database that will hold your data. We do not provide a means to do this here, since you may want to put your AutoDB data in a database that holds other data as well. The database can be empty or not. AutoDB creates all the tables it needs -- you need not (and should not create) these yourself.
Important note: Hereafter, the term 'database' refers to the tables created by AutoDB. Phrases like 'create the database' or 'initialize the database' refer to these tables only, and not the entire MySQL database that contains the AutoDB database.
Methods are provided to create or drop the entire database (meaning, of course, the AutoDB database, not the MySQL database) or individual collections.
AutoDB maintains a registry that describes the collections stored in the database. Registration of collections is usually handled by AutoClass behind the scenes as class definitions are encountered. The system consults the registry when running queries, when writing objects to the database, and when modifying the database schema.
When 'new' connects to the database, it reads the registry saved from the last time AutoDB ran. It merges this with an in-memory registry that generally reflects the currently loaded classes. 'new' merges the registries, and stores the result for next time if the new registry is different from the old.
Caveat: The present implementation assumes MySQL. It is unclear how deeply this assumption affects the design.
Every object is stored as a BLOB constructed by Data::Dumper. The database contains a single 'object table' for the entire database whose schema is
create table _AutoDB (
oid bigint unsigned not null,
primary key (oid),
object longblob
);
The oid is a unique object identifier assigned by the system. An oid is a permanent, immutable identifier for the object.
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Not yet implemented
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The name of this table can be chosen when the
database is created. '_AutoDB' is the default.
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For each collection, there is one table we call the base table that holds scalar search keys, and one table per list-valued search keys. The name of the base table is the same as the name of the collection; there is no way to change this at present. For our Person example, the base table would be
create table Person (
oid bigint unsigned not null, --- foreign key pointing to _AutoDB
primary key (oid), --- also primary key here
name longtext,
sex longtext
);
If a Person has a significant_other (also a Person), the table would look like this:
create table Person (
oid bigint unsigned not null, --- foreign key pointing to _AutoDB
primary key (oid), --- also primary key here
name longtext,
sex longtext
significant_other bigint unsigned --- foreign key pointing to _AutoDB
);
The data types specified in the 'keys' parameter are used to define the data types of these columns. They are also used to ensure correct quoting of values bound into SQL queries. It is safe to use 'string' as the data type even if the data is numeric unless you intend to run 'raw' SQL queries against the database and want to do numeric comparisons.
For each list valued search key, eg, 'friends' in our example, we need another table which (no surprise) is a classic link table. The name is constructed by concatenating the collection name and key name, with a '_' in between.
create table Person_friends (
oid bigint unsigned not null, --- foreign key pointing to _AutoDB
friends bigint unsigned --- foreign key pointing to _AutoDB
--- (will be a Person)
);
A small detail: since the whole purpose of these tables is to enable querying, indexes are created for each column by default (indexes can be turned off by specifiying index=>0 to the AutoDB constructor).
When the system stores an object, it converts any object references contained therein into the oids for those objects. In other words, objects in the database refer to each other using oids rather than the in-memory object references used by Perl. There is no loss of information since an oid is a permanent, immutable identifier for the object.
When an object is retrieved from the database, the system does NOT immediately process the oids it contains. Instead, the system waits until the program tries to access the referenced object at which point it automatically retrieves the object from the database. Options are provided to retrieve oids earlier.
If a program retrieves the same oid multiple times, the system short circuits the database access and ensures that only one copy of the object exists in memory at any point in time. If this weren't the case, a program could end up with two objects representing Joe, and an update to one would not be visible in the other. If both copies were later stored, one update would be lost. The core of the solution is to maintain an in-memory hash of all fetched objects (keyed by oid). The software consults this hash when asked to retrieve an object; if the object is already in memory, it is returned without even going to the database.
see http://search.cpan.org/~ccavnor/Class-AutoDB-0.091/docs/AutoDB.html#bugs_and_wishlist
see http://search.cpan.org/~ccavnor/Class-AutoDB-0.091/docs/AutoDB.html#methods_and_functions
