Provided by: libdbix-oo-perl_0.0.9-6_all bug

NAME

       DBIx::OO - Database to Perl objects abstraction

SYNOPSIS

           package MyDB;
           use base 'DBIx::OO';

           # We need to overwrite get_dbh since it's an abstract function.
           # The way you connect to the DB is really your job; this function
           # should return the database handle.  The default get_dbh() croaks.

           my $dbh;
           sub get_dbh {
               $dbh = DBI->connect_cached('dbi:mysql:test', 'user', 'passwd')
                 if !defined $dbh;
               return $dbh;
           }

           package MyDB::Users;
           use base 'MyDB';

           __PACKAGE__->table('Users');
           __PACKAGE__->columns(P => [ 'id' ],
                                E => [qw/ first_name last_name email /]);
           __PACKAGE__->has_many(pages => 'MyDB::Pages', 'user');

           package MyDB::Pages;
           use base 'MyDB';

           __PACKAGE__->table('Pages');
           __PACKAGE__->columns(P => [ 'id' ],
                                E => [qw/ title content user /]);
           __PACKAGE__->has_a(user => 'MyDB::Users');

           package main;

           my $u = MyDB::Users->create({ id          => 'userid',
                                         first_name  => 'Q',
                                         last_name   => 'W' });

           my $foo = MyDB::Users->retrieve('userid');
           my @p = @{ $foo->fk_pages };
           print "User: ", $foo->first_name, " ", $foo->last_name, " pages:\n";
           foreach (@p) {
               print $_->title, "\n";
           }

           $foo->first_name('John');
           $foo->last_name('Doe');
       # or
           $foo->set(first_name => 'John', last_name => 'Doe');
           $foo->update;

IMPORTANT NOTE

       This code is tested only with MySQL.  That's what I use.  I don't have too much time to test/fix it for
       other DBMS-es (it shouldn't be too difficult though), but for now this is it...  Volunteers are welcome.

DESCRIPTION

       This module has been inspired by the wonderful Class::DBI.  It is a database-to-Perl-Objects abstraction
       layer, allowing you to interact with a database using common Perl syntax.

   Why another Class::DBI "clone"?
       1.  I  had  the  feeling  that  Class::DBI  is  no  longer maintained.  This doesn't seem to be the case,
           because:

       2.  My code was broken multiple times by Class::DBI upgrades.

       3.  Class::DBI doesn't quote table or field names, making it impossible  to  use  a  column  named,  say,
           'group' with MySQL.

       4.  I wanted to know very well what happens "under the hood".

       5.  I  hoped  my  module  would be faster than CDBI.  I'm not sure this is the case, but it certainly has
           less features. :-)

       6.  There's more than one way to do it.

       All in all, I now use it in production code so this thing is here to stay.

   Features
       retrieve, search, create, update, delete
           As Class::DBI, we have functions to retrieve an object by the primary key, search a table and  create
           multiple objects at once, create a new object, update an existing object.

       manage fields with convenient accessors
           Same  like  Class::DBI,  we provide accessors for each declared column in a table.  Usually accessors
           will have the same name as the column name, but note that there are cases when we can't do that, such
           as "can", "get", "set", etc. -- because DBIx::OO or parent objects already define these functions and
           have a different meaning.

           When it is not possible to use the column name, it is prefixed with "col_" -- so if you have a  table
           with a column named "can", its accessor will be named "col_can".

       has_a, has_many, has_mapping
           We support a few types of table relationships.  They provide a few nice features, though overally are
           not  as  flexible  as  Class::DBI's.   The  syntax  is  quite  different  too,  be  sure to check the
           documentation of these functions.

       JOIN-s
           has_a also creates a search function that allows you to retrieve data from both tables using  a  JOIN
           construct.   This  can  drastically  reduce  the  number  of  SQL queries required to fetch a list of
           objects.

   Missing features:
       NO caching of any kind
           DBIx::OO does not cache objects.  This means that you can have the same DB record  in  multiple  Perl
           objects.  Sometimes this can put you in trouble (not if you're careful though).

           At some point I might want to implement object uniqueness like Class::DBI, but not for now.

       NO triggers
           Triggers are nice, but can cause considerable performance problems when misused.

           UPDATE: The only trigger that currently exists is before_set(), check its documentation.

       A lot others
           Constraints,  integrity  maintenance,  etc.  By contrast Class::DBI has a lot of nice features, but I
           think the performance price we pay for them is just too big.  I hope this module to stay small and be
           fast.

QUICK START

       You need to subclass DBIx::OO in order to provide  an  implementation  to  the  get_dbh()  method.   This
       function  is  pure virtual and should retrieve the database handler, as returned by DBI->connect, for the
       database that you want to use.  You can use an interim package for that, as we did in our  example  above
       (MyDB).

       Then, each derived package will handle exactly one table, should setup columns and relationships.

API DOCUMENTATION

   "new()"
       Currently,  new() takes no arguments and constructs an empty object.  You normally shouldn't need to call
       this directly.

   "get_dbh()"
       This method should return a database handler, as returned by DBI->connect.   The  default  implementation
       croaks,  so  you  need  to  overwrite  it  in  your  subclasses.   To  write it only once, you can use an
       intermediate object.

   "table($table_name)"
       Call this method in each derived package to inform DBIx::OO of the table that you wish  that  package  to
       use.

           __PACKAGE__->table('Users')

   "columns(group[=> cols, ...])"
       Sets/retrieves the columns of the current package.

       Similarly  to  Class::DBI,  DBIx::OO uses a sort of column grouping.  The 'P' group is always the primary
       key.  The 'E' group is  the  essential  group--which  will  be  fetched  whenever  the  object  is  first
       instantiated.   You  can  specify  any  other  groups names here, and they will simply group retrieval of
       columns.

       Example:

           __PACKAGE__->columns(P => [ 'id' ],
                                E => [ 'name', 'description' ],
                                X => [ 'c1', 'big_content1', 'big_title1' ],
                                Y => [ 'c2', 'big_content2', 'big_title2' ]);

       The above code defines 4 groups.  When an object is first instantiated, it will fetch  'id',  'name'  and
       'description'.   When  you say $obj->c1, it will fetch 'c1, 'big_content1' and 'big_title1', because they
       are in the same group.  When you say $obj->c2  it  will  fetch  'c2',  'big_content2'  and  'big_title2'.
       That's pretty much like Class::DBI.

       To retrieve columns, you pass a group name.

       Notes

       •   Class::DBI  allows  you  to  call  columns() multiple times, passing one group at a time.  Our module
           should allow this too, but it's untested and might be buggy.  We suggest defining all groups  in  one
           shot, like the example above.

       •   Group 'P' is required.  I mean that.  We won't guess the primary key column like Class::DBI does.

   "clone_columns(@except)"
       Though public, it's likely you won't need this function.  It returns a list of column names that would be
       cloned  in  a  clone() operation.  By default it excludes any columns in the "P" group (primary keys) but
       you can pass a list of other names to exclude as well.

   "defaults(%hash)"
       Using this function you can declare some default values for your  columns.   They  will  be  used  unless
       alternative values are specified when a record is inserted (e.g. with create()).  Example:

           __PACKAGE__->defaults(created     => ['now()'],
                                 hidden      => 1,
                                 modified_by => \&get_current_user_id);

       You  can  specify  any  scalar  supported  by  SQL::Abstract's  insert operation.  For instance, an array
       reference specifies literal SQL (won't be quoted).  Additionally, you can pass code references, in  which
       case the subroutine will be called right when the data is inserted and its return value will be used.

   "get(field_name[, field_name, ...])"
       Retrieves  the  value  of one or more columns.  If you pass more column names, it will return an array of
       values, in the right order.

   "set(field => value[, field => value, ...])"
       Sets one or more columns to the specified value(s).

       This function calls "before_set" right before modifying the object data, passing a hash reference to  the
       new values.

   "before_set"
       By  default  this  function does nothing.  It will be called by the framework right before setting column
       values.  A hash reference with columns to be set will be passed.  You can modify this hash if  you  wish.
       For  example,  assuming  you have an Users table with a MD5 password and you want to create the MD5 right
       when the column is set, you can do this:

           package Users;

           ...

           sub before_set {
               my ($self, $h, $is_create) = @_;
               if (exists $h->{password}) {
                   $h->{password} = make_md5_passwd($h->{password});
               }
           }

           my $u = Users->retrieve('foo');
           $u->password('foobar');
           print $u->password;
           # be8cd58c70ad7dc935802fdb051869fe

       The $is_create argument will be true (1) if this function is called as a result of a create() command.

   "id()"
       Returns the value(s) of the primary key(s).  If the primary key consists of  more  columns,  this  method
       will return an array with the values, in the order the PK column names were specified.

       Currently this is equivalent to $self->get(@{ $self->columns('P') }).

   "transaction_start()", "transaction_rollback()", "transaction_commit()"
       Use  these  functions  to  start,  commit  or  rollback  a DB transaction.  These simply call begin_work,
       rollback and commit methods on the DB handle returned by get_dbh().

   "get_accessor_name()"
       There are a few column names that we can't allow as accessor names.  This function receives a column name
       and returns the name of the accessor for that field.  By default it prefixes forbidden names with 'col_'.
       The forbidden names are:

         - id
         - can
         - our
         - columns
         - table
         - get
         - set
         - count

       If you don't like this behavior you can override this function in your classes to return something  else.
       However,  be  very  careful  about allowing any the above forbidden names as accessors--basically nothing
       will work.

   "get_fk_name"
       This function returns the name of a foreign key accessor, as  defined  by  has_a/has_many.   The  default
       returns "fk_$name"--thus prepending "fk_".

       If you want the Class::DBI behavior, you can override this function in your derived module:

           sub get_fk_name { return $_[1]; }

       (the first argument will be object ref. or package)

       I  think the Class::DBI model is unwise.  Many times I found my columns inflated to objects when I was in
       fact expecting to get an ID.  Having the code do implicit work for you is nice, but you can  spend  hours
       debugging  when  it  gets  it  wrong--which  is  why, DBIx::OO will by default prepend a "fk_" to foreign
       objects accessors.  You'll get use to it.

   "has_a/has_many"
           __PACKAGE__->has_a(name, type[, mapping[, order ]]);
           __PACKAGE__->has_many(name, type[, mapping[, order[, limit[, offset ]]]]);

       Creates a relationship between two packages.  In the simplest form, you call:

           __PACKAGE__->has_a(user => Users);

       This declaration creates a relation between __PACKAGE__ (assuming it  has  a  column  named  'user')  and
       'Users'  package.   It  is assuming that 'user' from the current package points to the primary key of the
       Users package.

       The declaration creates a method named 'fk_user', which you can call in order  to  retrieve  the  pointed
       object.  Example:

           package Pages;
           use base 'MyDB';
           __PACKAGE__->columns('P' => [ 'id' ],
                                'E' => [ 'user', ... ]);
           __PACKAGE__->has_a(user => 'Users');

           my $p = Pages->retrieve(1);
           my $u = $p->fk_user;
           print $u->first_name;

       In  more  complex  cases, you might need to point to a different field than the primary key of the target
       package.  You can call it like this:

           Users->has_many(pages => Pages, 'user');
           my $u = Users->retrieve('foo');
           my @pages = @{ $u->fk_pages };

       The above specifies that an User has many pages, and that they are determined by mapping the 'user' field
       of the Pages package to the primary key of the "Users" package.

       has_many() also defines an utility function that allows us to easily count the  number  of  rows  in  the
       referenced table, without retrieving their data.  Example:

           print $u->count_pages;

       You can specify an WHERE clause too, in SQL::Abstract syntax:

           print $u->count_pages(keywords => { -like => '%dhtml%' });

       The above returns the number of DHTML pages that belong to the user.

       In  even  more  complex cases, you want to map one or more arbitrary columns of one package to columns of
       another package, so you can pass a hash reference that describes the column mapping:

           ## FIXME: find a good example

       has_many() is very similar to has_a, but the accessor it creates simply returns multiple  values  (as  an
       array ref).  We can pass some arguments too, either to has_a/has_many declarations, or to the accessor.

           @pages = @{ $u->fk_pages('created', 10, 5) }

       The  above  will retrieve the user's pages ordered by 'created', starting at OFFSET 5 and LIMIT-ing to 10
       results.

       You can use has_a even if there's not a direct mapping.  Example, a page can have multiple revisions, but
       we can also easily access the first/last revision:

           Pages->has_many(revisions => 'Revisions', 'page');
           Pages->has_a(first_revision => 'Revisions', 'page', 'created');
           Pages->has_a(last_revision => 'Revisions', 'page', '^created');

       has_a() will LIMIT the result to one.  Ordering the results by 'created', we make sure that  we  actually
       retrieve  what  we  need.   Note that by prefixing the column name with a '^' character, we're asking the
       module to do a DESC ordering.

       (Of course, it's a lot faster if we had first_revision and last_revision as columns in  the  Pages  table
       that link to Revision id, but we just wanted to point out that the above is possible ;-)

       Join

       has_a()  will  additionally  create  a join function.  It allows you to select data from 2 tables using a
       single SQL query.  Example:

           package MyDB::Users;
           MyDB::Users->table('Users');
           MyDB::Users->has_a(profile => 'Profiles');

           package MyDB::Profiles;
           MyDB::Profiles->table('Profiles');

           @data = Users->search_join_profile;
           foreach (@data) {
               my $user = $_->{Users};        # the key is the SQL B<table> name
               my $profile = $_->{Profiles};
               print $user->id, " has address: ", $profile->address;
           }

       The above only does 1 SELECT.  Note that the join search function returns an array  of  hashes  that  map
       from the SQL table name to the DBIx::OO instance.

       You can pass additional WHERE, ORDER, LIMIT and OFFSET clauses to the join functions as well:

           @data = Users->search_join_profile({ 'Users.last_name' => 'Doe' },
                                              'Users.nickname',
                                              10);

       The above fetches the first 10 members of the Doe family ordered by nickname.

       Due to lack of support from SQL::Abstract side, the JOIN is actually a select like this:

           SELECT ... FROM table1, table2 WHERE table1.foreign = table2.id

       In  the  future  I  hope to add better support for this, that is, use "INNER JOIN" and eventually support
       other JOIN types as well.

       Notes

       1.  The "fk_" accessors will actually retrieve data at each call.  Therefore:

               $p1 = $user->fk_pages;
               $p2 = $user->fk_pages;

           will retrieve 2 different arrays, containing different sets of objects (even if  they  point  to  the
           same  records),  hitting  the  database twice.  This is subject to change, but for now you have to be
           careful about this.  It's best to keep a reference to the  returned  object(s)  rather  than  calling
           fk_pages() all over the place.

       2.  has_many()  creates  accessors  that select multiple objects.  The database will be hit once, though,
           and multiple objects are created from the returned data.  If this isn't desirable, feel free to LIMIT
           your results.

   "might_have()"
       Alias to has_a().

   "has_mapping(name, type, maptype, map1, map2, order, limit, offset)"
       You can use has_mapping to map one object to another using an intermediate table.   You  can  have  these
       tables:

           Users: id, first_name, etc.
           Groups: id, description, etc.
           Users_To_Groups: user, group

       This  is quite classical, I suppose, to declare many-to-many relationships.  The Users_To_Groups contains
       records that map one user to one group.  To get the ID-s of all groups that a certain  user  belongs  to,
       you would say:

           SELECT group FROM Users_To_Group where user = '$user'

       But since you usually need the Group objects directly, you could speed things up with a join:

           SELECT Groups.id, Groups.description, ... FROM Groups, Users_To_Groups
                  WHERE Users_To_Groups.group = Groups.id
                    AND Users_To_Groups.user = '$user';

       The relationship declared with has_mapping() does exactly that.  You would call it like this:

           package Users;
           __PACKAGE__->table('Users');
           __PACKAGE__->columns(P => [ 'id' ], ...);

           __PACKAGE__->has_mapping(groups, 'Groups',
                                    'Users_To_Groups', 'user', 'group');

           package Groups;
           __PACKAGE__->table('Groups');
           __PACKAGE__->columns(P => [ 'id' ], ...);

           # You can get the reverse mapping as well:
           __PACKAGE__->has_mapping(users, 'Users',
                                    'Users_To_Groups', 'group', 'user');

           package Users_To_Groups;
           __PACKAGE__->table('Users_To_Groups');
           __PACKAGE__->columns(P => [ 'user', 'group' ]);

       Note  that Users_To_Groups has a multiple primary key.  This isn't required, but you should at least have
       an unique index for the (user, group) pair.

       Arguments

       I started with an example  because  the  function  itself  is  quite  complicated.   Here  are  arguments
       documentation:

       name
           This is used to name the accessors.  By default we will prepend a "fk_" (see get_fk_name).

       type
           The type of the target objects.

       maptype
           The  mapping object type.  This is the name of the object that maps one type to another.  Even though
           you'll probably never need to instantiate such an object, it still has to be declared.

       map1
           Specifies how we map from current package (__PACKAGE__) to the  "maptype"  object.   This  can  be  a
           scalar  or  an  hash ref.  If it's a scalar, we will assume that __PACKAGE__ has a simple primary key
           (not multiple) and "map1" is the name of the column from "maptype" that we should map  this  key  to.
           If  it's  a  hash  reference,  it  should  directly  specify the mapping; the keys will be taken from
           __PACKAGE__ and the values from "maptype".  If that sounds horrible, check the example below.

       map2
           Similar to "map1", but "map2" specifies the mapping from  "maptype"  to  the  target  "type".   If  a
           scalar,  it  will be the name of the column from "maptype" that maps to the primary key of the target
           package (assumed to be a simple primary key).  If a hash reference, it specifies the full mapping.

       order, limit, offset
           Similar to has_many, these can specify default ORDER BY and/or LIMIT/OFFSET clauses for the  resulted
           query.

       Example

       Here's the mapping overview:

                            map1                      map2
            __PACKAGE__     ===>    C<maptype>        ===>       C<type>
          current package        table that holds           the target package
                                   the mapping

   "create"
           my $u = Users->create({ id          => 'foo',
                                   first_name  => 'John',
                                   last_name   => 'Doe' });

       Creates  a  new  record  and  stores it in the database.  Returns the newly created object.  We recommend
       passing a hash reference, but you can pass a hash by value as well.

   clone(@except)
       Clones an object, returning a hash (reference) suitable for create().  Here's how you would call it:

         my $val = $page->clone;
         my $new_page = Pages->create($val);

       Or, supposing you don't want to copy the value of the "created" field:

         my $val = $page->clone('created');
         my $new_page = Pages->create($val);

   "init_from_data($data)"
       Initializes one or more objects from the given data.  $data can be a hashref  (in  which  case  a  single
       object  will be created and returned) or an arrayref (multiple objects will be created and returned as an
       array reference).

       The hashes simply contain the data, as retrieved from the database.  That is, map column  name  to  field
       value.

       This  method  is convenient in those cases where you already have the data (suppose you SELECT-ed it in a
       different way than using DBIx::OO) and want to initialize DBIx::OO objects without the penalty  of  going
       through the DB again.

   "retrieve"
           my $u = Users->retrieve('foo');

       Retrieves  an  object from the database.  You need to pass its ID (the value of the primary key).  If the
       primary key consists on more columns, you can pass the values in order as an array, or  you  can  pass  a
       hash reference.

       Returns undef if no objects were found.

   "search($where, $order, $limit, $offset)"
           $a = Users->search({ created => [ '>=', '2006-01-01 00:00:00' ]});

       Searches  the database and returns an array of objects that match the search criteria.  All arguments are
       optional.  If you pass no arguments, it will return an array containing  all  objects  in  the  DB.   The
       syntax of $where and $order are described in SQL::Abstract.

       In scalar context it will return a reference to the array.

       The $limit and $offset arguments are added by DBIx::OO and allow you to limit/paginate your query.

       UPDATE 0.0.7:

       Certain  queries  are  difficult  to express in SQL::Abstract syntax.  The search accepts a literal WHERE
       clause too, but until version 0.0.7 there was no way to specify bind variables.  For example, now you can
       do this:

           @admins = Users->search("mode & ? <> 0 and created > ?",
                                   undef, undef, undef,
                                   MODE_FLAGS->{admin},
                                   strftime('%Y-%m-%d', localtime)).

       In order to pass bind variables, you must pass order, limit and offset (give  undef  if  you  don't  care
       about them) and add your bind variables immediately after.

   "retrieve_all()"
       retrieve_all() is an alias to search() -- since with no arguments it fetches all objects.

   "update"
           $u->set(first_name => 'Foo',
                   last_name => 'Bar');
           $u->update;

       Saves any modified columns to the database.

   "delete"
           $u = Users->retrieve('foo');
           $u->delete;

       Removes  the  object's  record  from  the database.  Note that the Perl object remains intact and you can
       actually revive it (if you're not losing it) using undelete().

   "undelete"
           $u = Users->retrieve('foo');
           $u->delete;    # record's gone
           $u->undelete;  # resurrected

       This function can "ressurect" an object that has been deleted (that is, it re-INSERT-s  the  record  into
       the  database),  provided  that you still have a reference to the object.  I'm not sure how useful it is,
       but it helped me test the delete() function. :-)

       Other (useless) thing you can do with it is manually emulating the create() function:

           $u = new Users;
           $u->{values}{id} = 'foo';
           $u->first_name('Foo');
           $u->last_name('Bar');
           $u->undelete;

       Note we can't call the column accessors, nor use set/get, before we have a primary key.

       This method is not too useful in itself, but it helps understanding the internals of  DBIx::OO.   If  you
       want to read more about this, see "under the hood".

   "revert", or "discard_changes"
           $u = Users->retrieve('foo');
           $u->first_name(undef);
           $u->revert;

       Discards any changes to the object, reverting to the state in the database.  Note this doesn't SELECT new
       data, it just reverts to values saved in the "modified" hash.  See "under the hood" for more info.

       "discard_changes()" is an alias to "revert()".

   get_sql_abstract
       Returns  the  instance  of  SQL::Abstract::WithLimit (our custom derivative) suitable for generating SQL.
       This is cached (will be created only the first time get_sql_abstract is called).

   count
       Returns the result of an SQL COUNT(*) for the specified where clause.  Call this as a package method, for
       example:

           $number_of_romanians = Users->count({ country => 'RO' });

       The argument is an SQL::Abstract where clause.

   "disable_fk_checks()", "enable_fk_checks()"
       Enable or disable foreign key checks in the backend DB server.  These are hard-coded in MySQL syntax  for
       now so be careful not to use them with other servers. ;-)

   "autocreate(@packages)"
       You  can  use  this  facility  to  automatically  create / upgrade your database.  It takes a very simple
       (rudimentary even) approach, but we found it to be useful.  Here's the "big" idea.

           package MyDB::Users;
           use base 'MyDB';

           __PACKAGE__->table('Users');
           __PACKAGE__->columns(P => [ 'id' ],
                                E => [qw/ first_name last_name /]);

           sub get_autocreate_data {q{
           #### (users:0) ####

           CREATE TABLE Users ( id VARCHAR(32) NOT NULL PRIMARY KEY,
                                first_name VARCHAR(64),
                                last_name VARCHAR(64) );

           # you can put Perl comments too.

           CREATE INDEX idx_Users_first_name ON Users(first_name)
           }}

       OK, now you can write this make_database.pl script:

           /usr/bin/perl -w

           use MyDB;
           MyDB->autocreate(qw( MyDB::Users ));

       When you run this script the first time, it will create the Users table.  (An internal  _dbix_oo_versions
       table  gets  created  as  well;  we're  using it inside DBIx::OO in order to keep track of existing table
       versions).  Note that if you run it again, it doesn't do anything--the database is up to date.

       Later.  You sold a billion copies of your software, customers are happy but they are crying loud  for  an
       "email" field in their user profiles, also wondering what was your idea to index on first_name and not on
       last_name!  In order to make it easy for them to upgrade their databases, you need to modify MyDB::Users.
       Besides   declaring  the  'email'  column  using  __PACKAGE__->columns,  append  the  following  to  your
       get_autocreate_data section:

           #### (users:1) ####

           # (note that we incremented the version number)

           # add the 'email' field
           ALTER TABLE Users ADD (email VARCHAR(128));

           # index it
           CREATE UNIQUE INDEX idx_Users_email ON Users(email);

           # and add that last_name index
           CREATE INDEX idx_Users_last_name ON Users(last_name);

       Now you can just tell your users to run make_database.pl again and everything gets updated.

       The #### (foo:N) #### syntax is meant simply to declare an  ID  and  a  version  number.   "foo"  can  be
       anything  you  want -- it doesn't have to be the table name.  You can actually create multiple tables, if
       you need to.

   autopopulate
       This is supposed to initialize tables.  Untested and may not work -- don't use it.

   get_autocreate_data
       See the documentation of "autocreate".

CAVEATS

       There are a number of problems you might encounter, mostly related  to  the  fact  that  we  don't  cache
       objects.

   Concurrent objects
           $u1 = Users->retrieve('foo');
           $u2 = Users->retrieve('foo');

       $u1  and  $u2 now point to different objects, but both point to the same record in the database.  Now the
       problem:

           $u1->first_name('Foo');
           $u2->first_name('Bar');
           $u1->update;

       Which one gets set?  'Foo', but $u2 has uncommitted changes.  When you further say $u2->update,  it  will
       set  the name to 'Bar'.  If you say $u2->revert, it will revert to whatever was there before 'Foo'.  This
       can lead to potential problems.

       Class::DBI (almost) doesn't have this problem (it can appear when you have multiple  processes  accessing
       the database concurrently, such as httpd processes).

UNDER THE HOOD

       A DBIx::OO object is a hash blessed into the DBIx::OO package.  The hash currently contains 2 keys:

       values
           A hash containing the field => value pairs that are currently retrieved from the database.

       modified
           Another  hash  that maps field_name => 'original value' for the fields that were modified and not yet
           committed of the current object.

       If a field is not present in values and is requested with get(), then the database will be queried for it
       and for all other fields that aren't present in "values" but are listed in the Essential group.

       If a field is present in modified, then it will be saved in the DB on the next update() call.  An  object
       can  discard  these operations with the discard() method.  Discard restores the values using those stored
       in the "modified" hash.

       Each operation plays around these hashes.  For instance, when you call search(), a single  SQL  will  run
       and  then  we'll  iterate  over the results, create objects and assign the SELECT-ed values to the values
       hash.

       A retrieve() operation creates a new object and assign the passed value to its primary key, then it  will
       call  the  internal _retrieve_columns([ 'P', 'E' ]) function in order to fetch essential object data from
       the DB.  Note that a call to _retrieve_columns is not actually necessary, since it will happen anyway the
       first time you want to retrieve a field that doesn't exist in values -- but it's good to call it  because
       retrieve() should return undef if the object can't be found in the DB.

BUGS

       Yeah, the documentation sucks.  Other bugs?

SEE ALSO

       SQL::Abstract, Class::DBI, DBIx::Class

AUTHOR

       Mihai Bazon, <mihai.bazon@gmail.com>
           http://www.dynarch.com/
           http://www.bazon.net/mishoo/

COPYRIGHT

       Copyright (c) Mihai Bazon 2006.  All rights reserved.

       This  module  is  free  software;  you  can redistribute it and/or modify it under the same terms as Perl
       itself.

THANKS

       I'd like to thank irc.n0i.net -- our small but wonderful community that's always there when you need it.

DISCLAIMER OF WARRANTY

       BECAUSE THIS SOFTWARE IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE SOFTWARE,  TO  THE  EXTENT
       PERMITTED  BY  APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER
       PARTIES PROVIDE THE SOFTWARE "AS  IS"  WITHOUT  WARRANTY  OF  ANY  KIND,  EITHER  EXPRESSED  OR  IMPLIED,
       INCLUDING,  BUT  NOT  LIMITED  TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
       PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF  THE  SOFTWARE  IS  WITH  YOU.  SHOULD  THE
       SOFTWARE PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR, OR CORRECTION.

       IN  NO  EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY
       OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE SOFTWARE AS PERMITTED BY THE ABOVE LICENCE, BE  LIABLE
       TO  YOU  FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES ARISING OUT OF
       THE USE OR INABILITY TO USE THE SOFTWARE (INCLUDING BUT NOT  LIMITED  TO  LOSS  OF  DATA  OR  DATA  BEING
       RENDERED  INACCURATE  OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE SOFTWARE TO OPERATE
       WITH ANY OTHER SOFTWARE), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF  SUCH
       DAMAGES.

perl v5.34.0                                       2022-06-13                                      DBIx::OO(3pm)