EAV containers

"Entity–attribute–value model (EAV) is a data model to describe entities where the number of attributes (properties, parameters) that can be used to describe them is potentially vast, but the number that will actually apply to a given entity is relatively modest. In mathematics, this model is known as a sparse matrix. EAV is also known as object–attribute–value model, vertical database model and open schema."

Source: Wikipedia.


Most people already get lost in that first paragraph, so let's try to explain this with an example.

You will typically use an EAV container when you have an entity (a record in ORM terms) that has a number of related attributes as its children. But for every record, these attibutes can be different. This makes it impossible to define these attributes as column in the entity's table, because there would be too many, most of them will not have data, and you can't deal with dynamic attributes at all (because columns need to be pre-defined).

To solve this issue 'relational' style, you would create a child table, and relate that to the 'entity' table using a One-to-Many relation, where every attribute would become a record in the child table. Downside of this approach however is that to be able to get a specific attribute value, you'll have to loop over all related records, compare the value of the attribute column with the attribute you look for, and if a match is found, get the contents of the value column.

The ORM EAV container uses this same implementation, but allows you to merge the attributes with the entity, so the attributes become properties of the entity record, thus emulating the variable number of columns that is required for an EAV implementation.


To enable the EAV container on a model, the model needs a child table with attribute and value column, which needs to be defined as either a One-to-Many or Many-to-Many relation.

Let's create an example of a hospital patient, of which the hospital keeps a record of statistics. Which depends on the illness of the patient, they can't be predicted up front.

class Model_Patient extends \Orm\Model
	// list of properties for this model, shortened for this example
	protected static $_properties = array(
		'id',					// primary key

	// set up the statistics relation the usual way
	protected static $_has_many = array(
		'statistics' => array(
			'key_from' => 'id',			// key in this model
			'model_to' => 'Model_Statistic',      // related model
			'key_to' => 'patient_id',		// key in the related model
			'cascade_save' => true,		// update the related table on save
			'cascade_delete' => true,		// delete the related data when deleting the parent

	// define the EAV container like so
	protected static $_eav = array(
		'statistics' => array(			// we use the statistics relation to store the EAV data
			'attribute' => 'key',		// the key column in the related table contains the attribute
			'value' => 'value',			// the value column in the related table contains the value

class Model_Statistic extend \Orm\Model
	// list of properties for this model
	protected static $_properties = array(
		'id',					// primary key
		'patient_id',				// foreign key
		'key',					// attribute column
		'value',				// value column

	// set up the patient relation the usual way
	protected static $_belongs_to = array(
	    'patient' => array(
	        'key_from' => 'patient_id',
	        'model_to' => 'Model_Patient',
	        'key_to' => 'id',
	        'cascade_save' => true,
	        'cascade_delete' => true,

You can define multiple EAV containers that link to different related tables. If you do, a request for an attribute will search through all containers in the order in which they are defined, until a match is found. As this is just an additional location of model properties, the handling of undefined properties in the model has remain unchanged: it will throw an exception.


Using the configuration above, let's assume we have the following datasets:

// SELECT * FROM patient
| id | name      |
|  1 | MisterIll |
|  2 | MissIll   |

// SELECT * FROM statistics
| id | patient_id | key           | value          |
+----+------------+---------------+ ---------------+
|  1 |          1 | Temperature   |           38.4 |
|  2 |          1 | Coughing      |            yes |
|  3 |          1 | Headache      |             no |
|  4 |          2 | Temperature   |           37.9 |
|  5 |          2 | Heartbeat     |             98 |

Given this data you can access the data like so:

// create some objects to work with
$mr = Patient::find(1);
$ms = Patient::find(2);

// you can now get the attributes directly
echo $mr->Temperature;		// '38.4'
echo $ms->Temperature;		// '37.9'
echo $mr->Headache;		// 'yes'
echo $ms->Headache;		// throws a property not found exception

// or set them directly
$mr->Temperature = '36.9';	// our patient seems to have improved
$mr->save();			// updates the patient record, and the updated EAV records

// you can still access the data the old-fashioned way
foreach ($mr->statistics as $statistic)
	// do something with each statistic value here...

Note that when your model has an EAV container defined, you can no longer use custom data properties for that model, as every new property will be seen as a new EAV key.