Object Oriented Data Models

Object Oriented Data Models L19

Topics • Objects in PHP • Generalisation • Polymorphism • Reusable classes • Objects and Databases • Next week • Data modelling patterns - Fowler

Tenets of the Relational model • Data in 1NF relations - no repeated fields • Data in 3NF relations - no redundant data • Field values drawn from small range of supplied data types • Relationships require join operation • Basic CRUD operations • Passive - changes only when UPDATEd • Balanced implementation- updates and read

Common applications • Core business processing • Accounting systems • Order processing • Stock control • Administrative systems • student records • admissions • bookings

Tricky application domains • Design systems - CAD, CAM, CASE • Text searching - search engine • Multi-media, hyper-media systems -images, video, audio, complex networks • Spatial data - Geographic Information Systems • Decision support systems - analysis of large body of ‘static’ data • Real-time active systems - air-traffic control

Complex entities • Electronic components in a Computer Aided Design (CAD) system • Mechanical components in a Computer Aided Manufacturing (CAM) system • Software requirements, specifications and components in a CASE environment • Multi-media documents - text, images and sounds in multimedia document managers • Spatial or geographic data, such as geometric figures and maps in a Geographic Information System (GIS)

Objects and entities • Can view as extension of Entity-Relationship modelling • Entity type = = class • roughly = table definition • part of schema • Entity instance = = object • roughly= row • part of factbase • + OID Object Identifier to ensure uniqueness • + type ref so Object is not tied to table

Class • Defines the attributes of the objects of that kind • attributes may be • basic types - strings, numbers, dates • array or object encapsulated in object • references to other objects • methods • constructor - to create a new object • accessors - to access attributes, compute derived data • modifiers - to change attributes

A PHP class • <?php • class emp { • var $name, $sal, $comm; • function emp($n,$s,$c) { • $this->name=$n; • $this->sal=$s; • $this->comm=$c; • } • function yearsal() { • return $this->sal * 12 + $this->comm; • } • } • $x = new emp('Jones', 200, 5); • print_r($x); • $xsal = $x->yearsal(); • print ("$x->name earns $xsal per annum");

PHP v Java • Object orientation in PHP4 is rather minimal by comparison with Java • PHP variables are untyped • All variables publicly accessible • PHP $this === Java self (and no default) • PHP -> === Java .

Inheritance • Salaried staff and contract staff are exclusive categories, but they overlap in specification – all are ‘employees’ and share some common characteristics Common attributes Common methods employee Is a Is a Local attributes Local methods (new or replacement) Salaried staff contractor

Generalisation and Specialisation class employee { var $name; function yearsal() { return false; } function initial() { return substr($this->name,0,1); } } class salaried extends employee { var $sal, $comm; function salaried($n,$s,$c) { $this->name=$n; $this->sal=$s; $this->comm=$c; } function yearsal() { return $this->sal * 12 + $this->comm; } } class contractor extends employee { var $rate, $days; function contractor($n,$r,$d) { $this->name=$n; $this->rate=$r; $this->days=$d; } function yearsal() { return $this->rate * $this->days; } }

Creating and using Objects $x = new salaried ('Fred', 100, 99); $xsal= $x->yearsal(); $i = $x->initial(); print ("$i $x->name earns $xsal per annum "); $y = new contractor('Joe', 10, 300); $ysal= $y->yearsal(); print ("$y->name earns $ysal per annum "); Output is F Fred earns 1299 per annum Joe earns 3000 per annum

Polymorphism • Many shapes • An ‘employee’ object may be really a ‘salaried’ object or a ‘contractor’ object • Sometimes we want to treat as an instance of the generalisation (employee), sometimes as the special case (salaried or contractor)

Power of Polymorphism class company { var $name, $staff; function company($n) { $this->name = $n; } function employ($s) { $this->staff[] = $s; } function yearsal() { $tot = 0; foreach($this->staff as $person) { $tot = $tot + $person->yearsal(); } return $tot; } } $c = new company("University of the West of England"); $c->employ($x); $c->employ($y); $csal= $c->yearsal(); print ("$c->name pays $csal per annum "); University of the West of England pays 4299 per annum

How does it work? • In the calculation loop, each employee is accessed in turn. • When the function annualsal() is called, it is the version appropriate to the objects base class which is used, so the appropriate calculation is performed.

Object References • In a relational model, rows are related by shared values • the deptno in an Emp row = the deptno in a Dept row • Objects have a system-generated unique identifier (across all objects, not just the rows in a single table like an auto_increment column) • References can be stored as an attribute of another object

Emps and Depts class dept { var $name, $location, $manager; function dept($n, $l, $m) { $this->name = $n; $this->location = $l; $this->manager = $m; } } $d = new dept("Accounting","Swindon", & $x); // print("Department $d->name is managed by $d->manager->name "); // cant chain in PHP 4.3 $boss = $d->manager; print("Department $d->name is managed by $boss->name "); Department Accounting is managed by Fred

Reflection • Reflection refers to the ability of a program to find out about itself • For example, there are functions to: • Get the class of an object • Get the superclass of an object • Get the names of the class methods (functions) • Get the names of the class variables • Classes could be called ‘self-describing’

Reflection code $class= get_class($x); $super = get_parent_class($x); print ("$x->name is a $class ($super) "); $class= get_class($y); $super = get_parent_class($y); print ("$y->name is a $class ($super) "); Fred is a salaried (employee) Joe is a contractor (employee)

User-defined types • Codd’s relational model has ‘domains’ • commercial RDBMS and programming languages provide only standard types with standard functions (integer, string, date) • Applications require • restrictions on standard types e.g. restricted values - can sometimes use Constraints to enforce • types defined by several values bound together • eg international currency requires amount and currency code • functions which operate on these types • eg to convert between 2 international currencies

Beaufort Scale 0 < 1 Calm 1 1 - 3 Light air 2 4 - 6 Light breeze 3 7 - 10 Gentle breeze 4 11 - 16 Moderate breeze 5 17 - 21 Fresh breeze 6 22 - 27 Strong breeze 7 28 - 33 Near gale 8 34 - 40 Gale 9 41 - 47 Strong gale 1048 - 55 Storm 1156 - 63 Violent storm 12>= 64 Hurricane

Degree classification • 0 - 34 fail • 35 - 39 pass • 40 - 49 3rd • 50 - 59 2.1 • 60 - 69 2.2 • 70 - 100 1st

Relational DB using a theta join • Grade table class min max fail 0 34 pass 35 39 3rd 40 49 • Student table name mark fred 38 sally 57 • Getting the class select name, class from student, grade where mark between min and max;

Common problems • Converting a wind speed in Beaufort scale • 25 knots is Force 6 • Converting average mark on degree to an Honours classification • 56 marks is a 2.1

classlist Honours Grades class name 1st topmark class 2.1 2.2 3rd classification pass classInterval fail 0 34 39 49 59 69 100 mark

Classification name : varchar getClass(mark : number) : string 1..* 1..* classInterval topMark : number class : String

Reusable Types • This Classification type is OK for Honours grades • But can we use it for Beaufort scale too?. • Three approaches • Use Classification type (but the names will all be wrong) • Create another similar type with ‘wind’ name (wasted work) • Generalise the Classification type with more neutral names(can be hard to find good names and not so readable) • We can generalise the names • classInterval > range • topmark > limit • class > value • classification > rangeFunction • getClass() > getValue()

rangeFunction name : string getValue(val : number) : string 1..* 1..* range limit : number value : String

class range { var $limit, $value; function range($l,$v) { $this->limit = $l; $this->value = $v; } } class rangeFunction { var $name, $ranges; function rangeFunction($n) { $this->name= $n; } function addRange($r) { $this->ranges[]=$r; } function getValue($val) { foreach($this->ranges as $r ) { if ($val <= $r->limit) { return $r->value; } } return false; } }

$pr = new rangeFunction("Prices"); $r = new range(9 , 9); $pr->addRange ($r); $r = new range(19 , 7); $pr->addRange ($r); $r = new range(99 , 5); $pr->addRange ($r); $r = new range(9999999 , 4); $pr->addRange ($r); //print_r($pr); for($i=0;$i<200;$i+=5) { $price = $pr->getValue($i); $cost = $i * $price; print("price for $i at £$price is £$cost "); } price for 0 at £9 is £0 price for 5 at £9 is £45 price for 10 at £7 is £70 price for 15 at £7 is £105 price for 20 at £5 is £100 price for 25 at £5 is £125

Where can we use this class? • Honours grades • Beaufort scale • Salary grades • Discount rates for multiple purchase • Tax rate bands • ...

Objects and Databases • Objects don’t fit well into a RDBMS • How many tables would the Beaufort Scale require? • Some RDBMS have stored procedures but not integrated with data • Object-Relational databases (e.g. Oracle 9i) try to bridge the gap by adding types, methods, inheritance, references but not popular • J2EE and .NET do the mapping to and from Objects in the Middle tier.

Row to Object mapping class person { var $usercode,$familyname, $givenname, $job, $dept, $ext, $homephone, $room, $fullname; var $dblink; function person($db) { $this->dblink = $db; } function get($username) { $res = mysql_query("select * from staff where username='$username'",$this->dblink); if (!$e = mysql_fetch_object($res) ) return false; $this->username = $e->username; $this->givenname = $e->givenname; $this->familyname = $e->familyname; $this->ext = $e->ext; $this->homephone =''; $this->room = $e->room; $this->fullname = $this->givenname . ' ' . $this->familyname; } function put() { $res = mysql_query("update staff set givenname='$this->givenname', familyname = '$this->familyname', ext= '$this->ext', room= '$this->room' where username='$this->username'", $this->dblink); return $res; } function show() { print(" $this->fullname is in Room $this->room on Ext. $this->ext "); } }

Testing $x = new person( $db); $x->get('CW'); $x->show(); $x->ext= $x->ext+1; $x->put($db); $y = new person($db); $y->get('CW'); $y->show(); Chris Wallace is in Room 3P14 on Ext. 83176 Chris Wallace is in Room 3P14 on Ext. 83177

Tutorial work • Exercises • Update the Rose model to show the OO features • A new subtype for Volunteers who get paid a flat annual fee • Salgrade table as an object in PHP

Object Oriented Data Models