MidCOM Database Abstraction Layer

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MidCOM's Database Abstraction Layer (DBA) provides an interface to MgdSchema functionality of the Midgard storage backend. They are part of the MidCOM Base Classes an act like a wrapper and provide Midgard functionality to the MidCOM framework.

Internally, this is imlemented with the midcom_core_dbaobject base class that acts as a decorator to Midgard objects. DBA classes have to extend this base class to get a PHP object representing the database table row.

[edit] Main Features

One important part of this layer is midcom_core_querybuilder, which provides access to the storage backend. In all DBA-based classes, methods like new_query_builder() and functionality like the MidCOM ACL system is automatically available.

There are also a number of "event handler" functions that are automatically triggered when a DBA object is updated, created or deleted. One of them is called before the operation, another one after it is completed. For deletion, they look like this:

function _on_deleting()
{
   //your code here
   //you can use $this to operate on the object in question
   return $proceed //false cancels the operation, true causes it to proceed
}
 
function _on_deleted()
{
   //your code here. 
   //you can use $this to operate on the object in question
}

Apart from these methods, the more generic MidCOM Watches allow a Component (or the MidCOM core) to react to changes done to other objects as well, for example a Midgard Replicator run can be triggered when an object is updated or deleted.

[edit] Example

If you have a database table mytable consisting of two columns, mystring and myfloat, and a correctly defined Mgdschema file to go along, the DBA system will provide you with a PHP object with both columns as properties and all the standard methods, so code like this is possible:

//create a new entry in the database:
$object = new mytable_dba_class();
$object->mystring = "something";
$object->myfloat = 2.3456;
$object->create();
 
//retrieve entries from the db:
$qb = mytable_dba_class::new_query_builder();
$qb->add_constraint('myfloat', '>', 3);
$qb->add_order('mystring');
$entries = $qb->execute();

[edit] Creating a Component with a Custom Object Schema

A small howto from the midgard-users forum:

create component skeleton using for example the scaffold
create mgdschema.xml
symlink the .xml from MIDGARD_PREFIX/share/midgard/schema/ using the component name as filename.
stop apache
run midgard-schema database_name to get the SQL imported and then metadata etc columns added
start apache
in manifest.inc add 'class_definitions' => Array('midcom_dba_classes.inc'),
edit/create file midcom_dba_classes.inc (in the config directory. See the following example of a class

array(
    'table' => 'org_routamc_positioning_location',
    'old_class_name' => null,
    'new_class_name' => 'org_routamc_positioning_location',
    'midcom_class_name' => 'org_routamc_positioning_location_dba'
),

in component root create my_class.php, below is a basic example based on the definition above:

class org_routamc_positioning_location_dba extends __org_routamc_positioning_location_dba
{
    function org_routamc_positioning_location_dba($id = null)
    {
        return parent::__org_routamc_positioning_location_dba($id);
    }
}