Tuesday, 28 June 2011

ISA hierarchy of tables inside a database and how to implement a single form

Please consider the following: Imagine I have a simple ISA hierarchy: parent A with children B and C. So let's say we have the corresponding tables named "a", "b", "c" and (created with the help of the gii model creator) the classes A, B, C The following is a screenshot from MySQL workbench:

The tables b and c have a primary key which ALSO plays the role of the foreign key to the primary key of table a (as expected)
I have an attribute which are common among the children classes B and C .. BUT for other reasons (as you can see other foreign keys) I cannot move these properties to the parent class A.
Initially I names these attributes with different names. This proved to be bad design! So I got back to the database and named them with the same name "protocol"
Now to access these attributes from the parent class A you have to create a custom property inside class A.
A custom property is not a variable but a combination of get/set methods to read and write to a variable. If you have getAttr and setAttr then the attribute is named "attr".
What I did was to create a getChild method (but no setChild method was necessary) Remember the parent class (created from gii) has already access to its children from the public function relations. So exploit this to get the child object.
Note: that this solution works only if row of A has ONLY ONE child, either B or C
The goal is to create a common form to fill the database tables a and b or c So you need an attribute to differentiate the category. Is model A a parent of B or a parent of C? Once more you use custom properties. I used getCateg and setCateg methods. In my case if model A is a newRecord then the category is null but you may want to use a different approach

I create with the help of CHtml a drop down list so user can select B or C This drop down list has values (0, 1) which correspond to class B or C The extra bonus is that this "categ" custom property is an attribute that can be used from the form widget and be validated
$attribute = "pvCat";
print $form->labelEx($model, $attribute);
print $form->dropDownList( $model, $attribute, $model::getCatDescs() );
print $form->error($model, $attribute);
As you see you have a seamless portion of code as the rest of the attributes which are real columns in table "a"
So the dropdown list feeds the "categ" attribute
This way the used has chosen the class. So you can now add a $protocol variable in your class A. This protocol is only temporary and plays the same role for both class B and C. This way you create a textbox to fill protocol for either class B or class C in a seamless way.

$attribute = "protocol";
print $form->labelEx($model, $attribute);
print $form->textField($model, $attribute);
print $form->error($model, $attribute);

I also edited the "rules" method of class A. For instance the "categ" has the 'in' validator with a specific range
But currently I use the controller to call explicitly $modelB->save() or $modelC->save() There is an idea to override save() function in class A to automagically save the child model (since in object oriented terms the class/model A is an abstract class) Please let me know if that works better for you

Authored by George Pligor
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Stergios and George on 21 December 2011 at 21:01 said...

FROM a user inside Yii framework community called Say_Ten: Would be to create a FormModel class that presents a common interface for dealing with models B and C. This form has an attribute about which class type we are saving and add in a save() method that then deals with the two different interfaces transparently to the client form etc. Do a search on the Proxy Design Pattern.

Stergios and George on 21 December 2011 at 21:01 said...

Say_Ten suggests this solution: http://en.wikipedia.org/wiki/Proxy_pattern which is also a very nice presentation of php interfaces If implementation would start from the beginning probably a new class would be best. I currently simulated the solution on A class. I have added an attributes which stores (by calculation) which is the category (B or C) of the A class object. I will transfer this addition inside wiki as soon as possible.

Although I was not aware of the proxy pattern I indeed tried to create a new class that would handle the B and C. BUT I didn't want to deviate away from the CActiveForm and the way that form elements are created. The error handler and all this stuff (this was just a choice, not sure it is the right one) When I created the new class in order to use CActiveForm I extended it from CActiveRecord but then of course an exception was thrown since the name of this class was not found as a table inside the database. Probably I should extend it from some other of the Yii's base classes or use another solution. In case someone has explored Yii's classes more thoroughly would know the answer. Thank you!

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