How to create an object of "unknown" class?

Question

I have a MySQL database and a table tobjects where each record has its id, parameter, value (something like XML) and one can say that this parameter column determines the "type" of an object.

The objects are used in some other tables, depending on their types, so each of them should be handled in specific way.

Because "handling" is somewhat common (I use the same function) I created a TObject class (not abstract but could be) from which I inherit other classes; this inheritance method is very useful and that's the very reason I use object oriented programming. For example TObject has retrieve() method that gets from db all the necessary data, not those in tobjects table but others too, which are type dependent, so I override it in some classes.

The problem I encountered is that when I create an object I do not know what class should it be. Of course, I can SELECT Parameter FROM tobjects WHERE id=$id, and then (with switch) create object of the proper class, and use its retrieve() method (each class retrieves different data, only those from tobjects are common) to get data from the db, that causes me to run query two times and some part of work outside the class, which works, but is not gentle.

The best solution would be if I can create a TObject and then, upon retrieving, change the class of the object to the one I need and it would be TObject's descendant, but I'm almost sure it's not possible.

Is my solution, that I run the first query just to select one field from tobjects only to determine object's class right? Or is there a trick to change object's class in runtime?

Answer 1

If understand what you are doing correctly, here is the way I would approach this:

Passing PDO::FETCH_CLASS | PDO::FETCH_CLASSTYPE to the first argument of PDOStatement::fetch() will return an object of class PDOStatement::fetchColumn(0) - in other words, it determines the class name to instantiate from the value of the first column of the result set.

To leverage this, you would JOIN tobjects ON targetTable.objectType = tobjects.id and select tobjects.Parameter as the first column in the result set. If the Parameter column already holds a 1:1 mapping of database object types to class names, this is all you need to do, however I'm not sure whether this is the case, and it probably shouldn't be, because it makes it more difficult to substitute another class at a later date.

To overcome this limitation, I suggest you create a temporary table when you first connect the database, which maps Parameter values to class names, which you can JOIN onto the query to obtain the target class name.

So the flow would go something like this:

// Set up the connection
$db = new PDO('mysql:yourDSNhere');
$db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$db->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);

// Create a temp table to store the mapping
$db->query("
  CREATE TEMPORARY TABLE `objectMappings` (
    `Parameter` INT NOT NULL PRIMARY KEY,
    `ClassName` VARCHAR(255)
  ) ENGINE=MEMORY
");

// A mapping of Parameter IDs to class names
$classMap = array(
  1 => 'Class1',
  2 => 'Class2',
  3 => 'Class3',
  // ...
);

// Build a query string and insert
$rows = array();
foreach ($classMap as $paramId => $className) {
  // this data is hard-coded so it shouldn't need further sanitization
  $rows[] = "($paramId, '$className')";
}

$db->query("
  INSERT INTO `objectMappings`
    (`Parameter`, `ClassName`)
  VALUES
    ".implode(',
    ', $rows)."
");

// ...

// When you want to retrieve some data
$result = $db->query("
  SELECT m.ClassName, t.*
  FROM targetTable t
  JOIN tobjects o ON t.objectType = o.id
  JOIN objectMappings m ON o.Parameter = m.Parameter
  WHERE t.someCol = 'some value'
");

while ($obj = $result->fetch(PDO::FETCH_CLASS | PDO::FETCH_CLASSTYPE)) {
  // $obj now has the correct type, do stuff with it here
}