NAME¶
Alzabo::Runtime::Schema - Schema objects
SYNOPSIS¶
use Alzabo::Runtime::Schema qw(some_schema);
my $schema = Alzabo::Runtime::Schema->load_from_file( name => 'foo' );
$schema->set_user( $username );
$schema->set_password( $password );
$schema->connect;
DESCRIPTION¶
Objects in this class represent schemas, and can be used to retrieve data from
that schema.
This object can only be loaded from a file. The file is created whenever a
corresponding "Alzabo::Create::Schema" object is saved.
INHERITS FROM¶
"Alzabo::Schema"
Note: all relevant documentation from the superclass has been merged into this
document.
METHODS¶
load_from_file ( name => $schema_name )¶
Loads a schema from a file. This is the only constructor for this class. It
returns an "Alzabo::Runtime::Schema" object. Loaded objects are
cached in memory, so future calls to this method may return the same object.
Throws: "Alzabo::Exception::Params",
"Alzabo::Exception::System"
set_user ($user)¶
Sets the username to use when connecting to the database.
user¶
Return the username used by the schema when connecting to the database.
set_password ($password)¶
Set the password to use when connecting to the database.
password¶
Returns the password used by the schema when connecting to the database.
set_host ($host)¶
Set the host to use when connecting to the database.
host¶
Returns the host used by the schema when connecting to the database.
set_port ($port)¶
Set the port to use when connecting to the database.
port¶
Returns the port used by the schema when connecting to the database.
set_referential_integrity ($boolean)¶
Turns referential integrity checking on or off. If it is on, then when
"Alzabo::Runtime::Row" objects are deleted, updated, or inserted,
they will report this activity to any relevant
"Alzabo::Runtime::ForeignKey" objects for the row, so that the
foreign key objects can take appropriate action.
This defaults to false. If your RDBMS supports foreign key constraints, these
should be used instead of Alzabo's built-in referential integrity checking, as
they will be much faster.
referential_integrity¶
Returns a boolean value indicating whether this schema will attempt to maintain
referential integrity.
set_quote_identifiers ($boolean)¶
If this is true, then all SQL constructed for this schema will have quoted
identifiers (like `Table`.`column` in MySQL).
This defaults to false. Turning this on adds some overhead to all SQL
generation.
connect (%params)¶
Calls the "Alzabo::Driver->connect" method for the driver owned by
the schema. The username, password, host, and port set for the schema will be
passed to the driver, as will any additional parameters given to this method.
See the "Alzabo::Driver->connect() method" for more details.
disconnect¶
Calls the "Alzabo::Driver->disconnect()" method for the driver
owned by the schema.
join¶
Joins are done by taking the tables provided in order, and finding a relation
between them. If any given table pair has more than one relation, then this
method will fail. The relations, along with the values given in the optional
where clause will then be used to generate the necessary SQL. See
"Alzabo::Runtime::JoinCursor" for more information.
This method takes the following parameters:
- •
- join => <see below>
This parameter can either be a simple array reference of tables or an array
reference of array references. In the latter case, each array reference
should contain two tables. These array references can also include an
optional modifier specifying a type of join for the two tables, like
'left_outer_join', an optional foreign key object which will be used to
join the two tables, and an optional where clause used to restrict the
join.
If a simple array reference is given, then the order of these tables is
significant when there are more than 2 tables. Alzabo expects to find
relationships between tables 1 & 2, 2 & 3, 3 & 4, etc.
For example, given:
join => [ $table_A, $table_B, $table_C ]
Alzabo would expect that table A has a relationship to table B, which in
turn has a relationship to table C. If you simply provide a simple array
reference, you cannot include any outer joins, and every element of the
array reference must be a table object.
If you need to specify a more complicated set of relationships, this can be
done with a slightly more complicated data structure, which looks like
this:
join => [ [ $table_A, $table_B ],
[ $table_A, $table_C ],
[ $table_C, $table_D ],
[ $table_C, $table_E ] ]
This is fairly self explanatory. Alzabo will expect to find a relationship
between each pair of tables. This allows for the construction of
arbitrarily complex join clauses.
For even more complex needs, there are more options:
join => [ [ left_outer_join => $table_A, $table_B ],
[ $table_A, $table_C, $foreign_key ],
[ right_outer_join => $table_C, $table_D, $foreign_key ] ]
In this example, we are specifying two types of outer joins, and in two of
the three cases, specifying which foreign key should be used to join the
two tables.
It should be noted that if you want to join two tables that have more than
one foreign key between them, you must provide a foreign key object
when using them as part of your query.
The way an outer join is interpreted is that this:
[ left_outer_join => $table_A, $table_B ]
is interepreted to mean
SELECT ... FROM table_A LEFT OUTER JOIN table_B ON ...
Table order is relevant for right and left outer joins, obviously.
However, for regular (inner) joins, table order is not important.
It is also possible to apply restrictions to an outer join, for example:
join => [ [ left_outer_join => $table_A, $table_B,
# outer join restriction
[ [ $table_B->column('size') > 2 ],
'and',
[ $table_B->column('name'), '!=', 'Foo' ] ],
] ]
This corresponds to this SQL;
SELECT ... FROM table_A
LEFT OUTER JOIN table_B ON ...
AND (table_B.size > 2 AND table_B.name != 'Foo')
These restrictions are only allowed when performing an outer join, since
there is no point in using them for regular inner joins. An inner join
restriction has the same effect when included in the "WHERE"
clause.
If the more multiple array reference of specifying tables is used and no
"select" parameter is provided, then the order of the rows
returned from calling "Alzabo::Runtime::JoinCursor->next()"
is not guaranteed. In other words, the array that the cursor returns will
contain a row from each table involved in the join, but the which row
belongs to which table cannot be determined except by examining the
objects. The order will be the same every time
"Alzabo::Runtime::JoinCursor->next()" is called, however. It
may be easier to use the
"Alzabo::Runtime::JoinCursor->next_as_hash()" method in this
case.
- •
- select => "Alzabo::Runtime::Table" object or objects
(optional)
This parameter specifies from which tables you would like rows returned. If
this parameter is not given, then the "distinct" or
"join" parameter will be used instead, with the
"distinct" parameter taking precedence.
This can be either a single table or an array reference of table
objects.
- •
- distinct => "Alzabo::Runtime::Table" object or objects
(optional)
If this parameter is given, it indicates that results from the join should
never contain repeated rows.
This can be used in place of the "select" parameter to indicate
from which tables you want rows returned. The "select"
parameter, if given, supercedes this parameter.
For some databases (notably Postgres), if you want to do a "SELECT
DISTINCT" query then all of the columns mentioned in your "ORDER
BY" clause must also be in your SELECT clause. Alzabo will make sure
this is the case, but it may cause more rows to be returned than you
expected, though this depends on the query.
NOTE: The adding of columns to the SELECT clause from the ORDER BY
clause is considered experimental, because it can change the expected
results in some cases.
- •
- where (optional)
See the documentation on where clauses for the Alzabo::Runtime::Table
class.
- •
- order_by (optional)
See the documentation on order by clauses for the Alzabo::Runtime::Table
class.
- •
- limit (optional)
See the documentation on limit clauses for the Alzabo::Runtime::Table
class.
If the "select" parameter specified that more than one table is
desired, then this method will return n JoinCursor object representing the
results of the join. Otherwise, the method returns a RowCursor object.
Throws: "Alzabo::Exception::Logic",
"Alzabo::Exception::Params"
one_row¶
This method takes the exact same parameters as the "join()" method but
instead of returning a cursor, it returns a single array of row objects. These
will be the rows representing the first row (a set of one or more table's
primary keys) that is returned by the database.
Throws: "Alzabo::Exception::Logic",
"Alzabo::Exception::Params"
function and select¶
These two methods differ only in their return values.
They both take the following parameters:
- •
- select => $function or [ scalars, SQL functions and/or
"Alzabo::Column" objects ]
If you pass an array reference for this parameter, it may contain scalars,
SQL functions, or column objects. For example:
$schema->function( select =>
[ 1,
$foo->column('name'),
LENGTH( $foo->column('name') ) ],
join => [ $foo, $bar_table ],
);
This is equivalent to the following SQL:
SELECT 1, foo.name, LENGTH( foo.name )
FROM foo, bar
WHERE ...
- •
- join
See the documentation on the join parameter for the join method.
- •
- where
See the documentation on where clauses for the Alzabo::Runtime::Table
class.
- •
- order_by
See the documentation on order by clauses for the Alzabo::Runtime::Table
class.
- •
- group_by
See the documentation on group by clauses for the Alzabo::Runtime::Table
class.
- •
- having
This parameter is specified in the same way as the "where"
parameter, but is used to generate a "HAVING" clause. It only
allowed when you also specify a "group_by" parameter.
- •
- limit
See the documentation on limit clauses for the Alzabo::Runtime::Table
class.
These methods are used to call arbitrary SQL functions such as 'AVG' or 'MAX',
and to select data from individual columns. The function (or functions) should
be the return values from the functions exported by the SQLMaker subclass that
you are using. Please see Using SQL functions for more details.
Throws: "Alzabo::Exception::Logic",
"Alzabo::Exception::Params"
function() return values
The return value of this method is highly context sensitive.
If you only requested a single element in your "select" parameter,
such as "DISTINCT(foo)", then it returns the first value in scalar
context and all the values as an array in list context.
If you requested multiple functions such as "AVG(foo), MAX(foo)", then
it returns a single array reference, the first row of values, in scalar
context and a list of array references in list context.
select() return values
This method always returns a new "Alzabo::DriverStatement" object
containing the results of the query. This object has an interface very similar
to the Alzabo cursor interface, and has methods such as "next()",
"next_as_hash()", etc.
row_count¶
This method is simply a shortcut to get the result of COUNT('*') for a join. It
equivalent to calling "function()" with a "select"
parameter of "COUNT('*')".
Throws: "Alzabo::Exception::Logic",
"Alzabo::Exception::Params"
prefetch_all¶
This method will set all the tables in the schema to prefetch all their columns.
See the lazy column loading section in "Alzabo::Runtime::Table" for
more details.
prefetch_all_but_blobs¶
This method will set all the tables in the schema to prefetch all their
non-blob-type columns.
This method is called as soon as a schema is loaded.
prefetch_none¶
This method turns of all prefetching.
name¶
Returns a string containing the name of the schema.
tables (@optional_list)¶
If no arguments are given, this method returns a list of all
"Alzabo::Runtime::Table" objects in the schema, or in a scalar
context the number of such tables. If one or more arguments are given, returns
a list of table objects with those names, in the same order given (or the
number of such tables in a scalar context, but this isn't terribly useful).
An "Alzabo::Exception::Params" exception is throws if the schema does
not contain one or more of the specified tables.
table ($name)¶
Returns an "Alzabo::Runtime::Table" object representing the specified
table.
An "Alzabo::Exception::Params" exception is throws if the schema does
not contain the table.
has_table ($name)¶
Returns a boolean value indicating whether the table exists in the schema.
begin_work¶
Starts a transaction. Calls to this function may be nested and it will be
handled properly.
rollback¶
Rollback a transaction.
commit¶
Finishes a transaction with a commit. If you make multiple calls to
"begin_work()", make sure to call this method the same number of
times.
run_in_transaction ( sub { code... } )¶
This method takes a subroutine reference and wraps it in a transaction.
It will preserve the context of the caller and returns whatever the wrapped code
would have returned.
driver¶
Returns the "Alzabo::Driver" object for the schema.
rules¶
Returns the "Alzabo::RDBMSRules" object for the schema.
sqlmaker¶
Returns the "Alzabo::SQLMaker" object for the schema.
JOINING A TABLE MORE THAN ONCE¶
It is possible to join to the same table more than once in a query. Table
objects contain an "alias()" method that, when called, returns an
object that can be used in the same query as the original table object, but
which will be treated as a separate table. This faciliaties queries similar to
the following SQL::
SELECT ... FROM Foo AS F1, Foo as F2, Bar AS B ...
The object returned from the table functions more or less exactly like a table
object. When using this table to set where clause or order by (or any other)
conditions, it is important that the column objects for these conditions be
retrieved from the alias object.
For example:
my $foo_alias = $foo->alias;
my $cursor = $schema->join( select => $foo,
join => [ $foo, $bar, $foo_alias ],
where => [ [ $bar->column('baz'), '=', 10 ],
[ $foo_alias->column('quux'), '=', 100 ] ],
order_by => $foo_alias->column('briz') );
If we were to use the $foo object to retrieve the 'quux' and 'briz' columns then
the join would simply not work as expected.
It is also possible to use multiple aliases of the same table in a join, so that
this will work properly:
my $foo_alias1 = $foo->alias;
my $foo_alias2 = $foo->alias;
This information is never saved to disk. This means that if you're operating in
an environment where the schema object is reloaded from disk every time it is
used, such as a CGI program spanning multiple requests, then you will have to
make a new connection every time. In a persistent environment, this is not a
problem. For example, in a mod_perl environment, you could load the schema and
call the "set_user()" and "set_password()" methods in the
server startup file. Then all the mod_perl children will inherit the schema
with the user and password already set. Otherwise you will have to provide it
for each request.
You may ask why you have to go to all this trouble to deal with the user and
password information. The basic reason was that I did not feel I could come up
with a solution to this problem that was secure, easy to configure and use,
and cross-platform compatible. Rather, I think it is best to let each user
decide on a security practice with which they feel comfortable.
In addition, there are a number of modules aimed at helping store and use this
sort of information on CPAN, including "DBIx::Connect" and
"AppConfig", among others.
AUTHOR¶
Dave Rolsky, <autarch@urth.org>