Reflection and object/relational mapping

[allspiritseve]

I was browsing the reflection docs in the php manual, and noticed that you can get and set properties of an object:

    public mixed getValue(stdclass object)
    public void setValue(stdclass object, mixed value) 

What do you guys think about using this to construct and deconstruct objects in a data mapper? It's breaking encapsulation, but the data mapper should know about the internals of the object anyways. That would mean that if the object isn't used heavily by the view, you wouldn't necessarily need a getter and setter for every single property in a given object... right? Or am I missing something?

[Amenthes]

I'll assume you were talking about non-public members, otherwise setters and
getters would make no sense. So, if that's the case, setValue() won't work as
expected as far as I know. A week ago or so, Sebastian Bergmann wrote
on php.internals that he couldn't use setValue() with non-public members and
reported it as a bug. I haven't tested it though. It may have been solved for 5.3

On breaking encapsulation I would say that, yes, the mapper should know about
the structure of the object it constructs, nevertheless, there may be business
logic in there too. Just to save you from repeating that logic in the mappers.
Like validating a certain business rule on a field or something.

[allspiritseve]

I'll assume you were talking about non-public members, otherwise setters and
getters would make no sense. So, if that's the case, setValue() won't work as
expected as far as I know. A week ago or so, Sebastian Bergmann wrote
on php.internals that he couldn't use setValue() with non-public members and
reported it as a bug. I haven't tested it though. It may have been solved for 5.3

It looks like you need to call $prop->setAccessible(true); before you can access private properties.

On breaking encapsulation I would say that, yes, the mapper should know about
the structure of the object it constructs, nevertheless, there may be business
logic in there too. Just to save you from repeating that logic in the mappers.
Like validating a certain business rule on a field or something.

That's a good point.

[Amenthes]

It looks like you need to call $prop->setAccessible(true); before you can access private properties.

It appears the bug concerns exactly the aspect you mentioned.

http://bugs.php.net/bug.php?id=46718

[Ren]

I think using Reflection is a bit overkill.

You can simple cast an object to an array to get all its attributes. Its just a matter of filtering, and rekeying that array to turn it into something you can use with PDO.

<?php

class TinyObjectMapper
{
    protected $className;
    protected $mapOut;

    function __construct($className, $mapOut) { $this->className = $className; $this->mapOut = $mapOut; }

    function getClassName() { return $this->className; }

    function mapOut($object)
    {
        $array = (array)$object;
        return array_combine($this->mapOut, array_intersect_key($array, $this->mapOut));
    }

    function mapIn($row)
    {
        $r = array();
        foreach($this->mapOut as $propertyName => $columnName)
            $r[substr($propertyName, 3)] = $row[$columnName];
        return call_user_func(array($this->className, '__set_state'), $r);
    }
}

class Person
{
    protected $id;
    protected $firstName;
    protected $lastName;

    function __construct($firstName, $lastName, $id = null)
    {
        $this->id = $id;
        $this->firstName = $firstName;
        $this->lastName = $lastName;
    }

    static function __set_state(array $values)
    {
        return new self($values['firstName'], $values['lastName'], $values['id']);
    }
}


class PersonMapper extends TinyObjectMapper
{
    function __construct($className = 'Person', array $mapOut = array())
    {
        $mapOut["\0*\0id"] = 'p_ID';
        $mapOut["\0*\0firstName"] = 'p_FirstName';
        $mapOut["\0*\0lastName"] = 'p_LastName';
        parent::__construct($className, $mapOut);
    }
}

    $personMapper = new PersonMapper();

    $homer = new Person('Homer', 'Simpson');

    var_dump($personMapper->mapOut($homer));

    $homer = $personMapper->mapIn(array('p_ID' => 1, 'p_FirstName' => 'Homer', 'p_LastName' => 'Simpson'));

    var_dump($homer);

[allspiritseve]

You can simple cast an object to an array to get all its attributes.

What about settings it's attributes?

[Ren]

What about settings it's attributes?

Used the __set_state() static method, as it takes an array of attribute values and returns an new instance.

[allspiritseve]

Used the __set_state() static method, as it takes an array of attribute values and returns an new instance.

If I used reflection I wouldn't need any such method in my domain objects though...

[Ren]

If I used reflection I wouldn't need any such method in my domain objects though...

Its just not worth using Reflection, imo. Though it is easier now that can set private/protected attributes via Reflection, I suppose.

[allspiritseve]

Its just not worth using Reflection, imo. Though it is easier now that can set private/protected attributes via Reflection, I suppose.

I guess I'm just a fan of keeping all non-business logic out of my domain objects... what you described above sounds like an array implementation of the builder pattern. It's introducing a little complexity into the domain object; I'd rather keep all that complexity in the mapper.

[kyberfabrikken]

It looks like you need to call $prop->setAccessible(true); before you can access private properties.

Note that it's not available until 5.3

[Ren]

I guess I'm just a fan of keeping all non-business logic out of my domain objects... what you described above sounds like an array implementation of the builder pattern. It's introducing a little complexity into the domain object; I'd rather keep all that complexity in the mapper.

I have written a Reflection based mapper.

The mapper had a method with mapped attribute names to SqlColumn objects or another Mapper for value objects (things like Money).

For setting search the constructor for argument of with the same name as the attribute, then look for get<AttributeName>() and set<AttributeName>() methods trying to determine how to build the object.

Also had a subclass of the Mapper which did class table inheritance (ie when a domain object is split over multiple database tables.)

Nice exercise, but all pretty slow, and pointless.

There is no reason why couldn't move the __set_state() method to the Mapper. The only reason it is there, is because var_export()ing model objects would work too.