Lesson 9.5: Composition over inheritance

In the previous lesson, we introduced type hierarchies into the data model for our book data. In this lesson, we’ll examine a case where composition is a better approach than inheritance.

Data table

ISBN-13	ISBN-10	Title	Format	Authors	Editors	Illustrators	Other contributors	Publisher	Year	City	State	Country	Page count	Genres	Price	Stock
9780008627843	0008627843	The Hobbit	ebook	J.R.R. Tolkien		J.R.R. Tolkien		Harper Collins	2023	New York City	New York	United States	310	fantasy;fiction	16.99
9780060929794	0060929790	One Hundred Years of Solitude	paperback	Garcia Marquez, Gabriel				Perennial	1998	New York City	New York	United States	458	fiction;historical fiction	6.12	4
9780195153446	0195153448	Classical Mythology	paperback	Lenardon, Robert J.;Morford, Mark P. O.				Oxford University Press	2002	New York City	New York	United States	820	history;nonfiction	34.98	12
9780375801679	0375801677	The Iron Giant	ebook	Hughes, Ted		Davidson, Andrew		Knopf Books for Young Readers	1999	New York City	New York	United States	79	fiction;children’s fiction	33.97
9780387881355	0387881352	Electron Backscatter Diffraction in Materials Science	hardback		Schwartz, Adam J.;Kumar, Mukul;Adams, Brent L.;Field, David P.			Springer	2009	New York City	New York	United States	425	nonfiction;technology	230.37	9
9780393045215	0393045218	The Mummies of Urumchi	paperback	Barber, Elizabeth Wayland				W.W. Norton & Company	1999	New York City	New York	United States	240	history;nonfiction	21.6	1
9780393634563	0393634566	The Odyssey	ebook	Homer			Wilson, Emily	W.W. Norton & Company	2017	New York City	New York	United States	656	fiction;classics	13.99
9780446310789	0446310786	To Kill a Mockingbird	paperback	Harper Lee				Grand Central Publishing	1988	New York City	New York	United States	281	fiction;historical fiction	21.64	16
9780451162076	0451162072	Pet Sematary	paperback	King, Stephen				Signet	1984	New York City	New York	United States	374	horror;fiction	93.22	1
9780500026557	0500026556	Hokusai’s Fuji	paperback	Wada, Kyoko		Katsushika, Hokusai		Thames & Hudson	2024	London		United Kingdom	416	art;nonfiction	24.47	11
9780500291221	0500291225	Great Discoveries in Medicine	paperback		Bynum, William;Bynum, Helen			Thames & Hudson	2023	London		United Kingdom	352	history;nonfiction	12.05	18
9780553212150	055321215X	Pride and Prejudice	paperback	Austen, Jane				Bantam Classics	1983	New York City	New York	United States	295	fiction;historical fiction	17.99	15
9780575104419	0575104414	Dune	ebook	Herbert, Frank				Hachette Book Group	2010	New York City	New York	United States	624	fiction;science fiction	5.49
9780671461492	0671461494	The Hitchhiker’s Guide to the Galaxy	paperback	Adams, Douglas				Pocket	1982	New York City	New York	United States	215	fiction;science fiction	91.47	9
9780679425601	0679425608	Under the Black Flag: The Romance and the Reality of Life Among the Pirates	hardback	Cordingly, David				Random House	1996	New York City	New York	United States	296	history;nonfiction	34.73	13
9780740748479	0740748475	The Complete Calvin and Hobbes	hardback	Watterson, Bill		Watterson, Bill		Andrews McMeel Publishing	2005	Kansas City	Missouri	United States	1451	comics;fiction	128.71	6
9781098108274	1098108272	Fundamentals of Data Engineering	ebook	Reis, Joe;Housley, Matt				O’Reilly Media	2022	Sevastopol	California	United States	450	nonfiction;technology;children’s fiction	47.99
9781489962287	148996228X	Interpretation of Electron Diffraction Patterns	paperback	Keown, Samuel Robert;Andrews, Kenneth William;Dyson, David John				Springer	1967	New York City	New York	United States	199	nonfiction;technology	47.17	15
9781859840665	1859840663	The Motorcycle Diaries: A Journey Around South America	paperback	Guevara, Ernesto			Wright, Ann	Verso	1996	London		United Kingdom	160	biography;nonfiction	14.52	4
9783319398778		Physical Principles of Electron Microscopy: An Introduction to TEM, SEM, and AEM	ebook	Egerton, R.F.				Springer	2016	London		United Kingdom	196	nonfiction;technology	19.5
9798691153570		Business Secrets of The Pharoahs	paperback	Crorigan, Mark				British London Publishing	2020	London		United Kingdom	260	business;nonfiction	11.99	8

Lesson 9.4 schema

define
book sub entity,
    abstract,
    owns isbn-13,
    owns isbn-10,
    owns title,
    owns page-count,
    owns genre,
    owns price,
    plays contribution:work,
    plays publishing:published;
paperback sub book,
    owns stock;
hardback sub book,
    owns stock;
ebook sub book;
contributor sub entity,
    owns name,
    plays contribution:contributor;
author sub contributor;
editor sub contributor;
illustrator sub contributor;
publisher sub entity,
    owns name,
    plays publishing:publisher;
place sub entity,
    abstract,
    owns name,
    plays locating:location,
    plays locating:located;
city sub place,
    plays publishing:location;
state sub place;
country sub place;
contribution sub relation,
    relates contributor,
    relates work;
publishing sub relation,
    relates publisher,
    relates published,
    relates location,
    owns year;
locating sub relation,
    relates located,
    relates location;
isbn-13 sub attribute, value string;
isbn-10 sub attribute, value string;
title sub attribute, value string;
page-count sub attribute, value long;
genre sub attribute, value string;
price sub attribute, value double;
stock sub attribute, value long;
name sub attribute, value string;
year sub attribute, value long;

Currently, the entity type contributor has three specialized subtypes: author, editor, and illustrator. But what happens if a single person carries out more than one of these functions, either for the same book or different books? For example, in the dataset, the edition of The Hobbit is both written and illustrated by J.R.R. Tolkien. We could create separate instances of author and illustrator, but then these two objects would have different identities, despite representing the same concept.

The problem here lies in that the subtypes of contributor are not mutually exclusive. As a data instance can only have a single type, using an inheritance hierarchy to model the types of contributor was a poor choice. In fact, if we consider the different ways the people can contribute to books, these are better described as behaviours of those people rather than inherent properties of them. As such, they are best implemented with interfaces. More specifically, these behaviours are roles that contributors fulfil, so we will use roles in relations to model them.

To implement this change, we will create three subtypes of contribution: authoring, editing, and illustrating, and then remove the subtypes of contributor: author, editor, and illustrator.

define
contributor sub entity,
    plays contribution:contributor;
contribution sub relation,
    relates contributor,
    relates work;
authoring sub contribution;
editing sub contribution;
illustrating sub contribution;

As we saw in Lesson 5.2, the role interface contribution:contributor is inherited by the subtypes of contribution, so they expose it as well. Because contributor implements this interface, this allows instances of contributor to play the role in instances of all three subtypes (or the supertype).

match
$hobbit isa book, has isbn-13 "9780008627843";
$tolkien isa contributor, has name "J.R.R. Tolkien";
insert
($hobbit, $tolkien) isa authoring;
($hobbit, $tolkien) isa illustrating;

Now we can easily record contributors in different roles.

A data instance can only have a single type. If a concept displays multiple simultaneous capabilities, for example a person being an author and an editor, then this indicates that the concept should be modeled as a single type implementing multiple interfaces, rather than as multiple types. This is in line with the OOP principle of composition over inheritance.

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Lesson 9.5: Composition over inheritance