Lesson 9.2: Determining object types

Determining entity types

Data might come from a variety of sources, from tabulated data files to real-time data streams from microservices. In this lesson, we’ll examine how we might go about modeling tabulated book data for the bookstore database using the PERA model, as shown here.

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

The first step is to determine what entity types exist in the data. If we examine the data table, it might appear that each row represents a book entity and every value in that row represents an attribute of the book. This would be a perfectly valid model, but it wouldn’t be particularly expressive.

define
book sub entity,
    owns isbn-13,
    owns isbn-10,
    owns title,
    owns format,
    owns author,
    owns editor,
    owns illustrator,
    owns other-contributor,
    owns publisher,
    owns year,
    owns city,
    owns state,
    owns country,
    owns page-count,
    owns genre,
    owns price,
    owns stock;

For simplicity, throughout most of Lesson 9, we’ll be omitting the attribute type definitions in schema definitions where not required.

The lack of expressivity causes several problems. Consider, for instance, if we had an author that was listed as having published different books under different names, as often is the case. We would not be able to tell which names (and therefore books) corresponded to the same author. Similarly, if two different authors published books under the same name, we would not be able to tell that they were written by different authors.

In order to distinguish cases like these, we need a notion of identity for authors. The identity of a concept is independent of any properties it has. In this case, an author remains the same author no matter what different names they are known by, and two authors are not the same person simply because they have the same name. Concepts with identities are generally best categorised as belonging to entity types. This would indicate that authors should be entities, and we can apply the same logic to editors, illustrators, and other contributors. We can also apply it to publishers, cities, states, and countries, when considering the following:

A publisher may have multiple imprints.
Two cities may have the same name.
A state or country may change its name.

These facts indicate that these concepts have identities not determined by their names (or any other property), and so should likewise be considered entities.

define
book sub entity,
    owns isbn-13,
    owns isbn-10,
    owns title,
    owns format,
    owns year,
    owns page-count,
    owns genre,
    owns price,
    owns stock;
author sub entity,
    owns name;
editor sub entity,
    owns name;
illustrator sub entity,
    owns name;
other-contributor sub entity,
    owns name;
publisher sub entity,
    owns name;
city sub entity,
    owns name;
state sub entity,
    owns name;
country sub entity,
    owns name;

Here we’ve made use of interface polymorphism to have several unrelated types implement ownership of name.

Determining relation types

Now that properties have been moved from book to other entity types, we need a way of recording the dependencies between book and those types so that we can query those properties. For instance, we need a way to record the dependency between a book and an author, indicating that the author wrote the book, allowing us to retrieve the author’s name from the book. We establish these dependencies with relation types. To begin with, we’ll link each entity type to book using a relation type. It is fairly evident how we should do so for some of those types.

define
contribution sub relation,
    relates contributor,
    relates work;
publishing sub relation,
    relates publisher,
    relates published;
book plays contribution:work,
    plays publishing:published;
author plays contribution:contributor;
editor plays contribution:contributor;
illustrator plays contribution:contributor;
other-contributor plays contribution:contributor;
publisher plays publishing:publisher;

Once again, we’ve leveraged interface polymorphism to re-use the same contribution relation types to represent the interdependencies between book and several contributor types: author, editor, illustrator, and other-contributor.

However, the connection from city, state, and country to book is less obvious. What should this relation be? Examining the dataset, we can tell that these fields refer to the location in which the book was published. As such, it makes sense that the publishing relation is appropriate, and it follows that we should extend publishing to be a ternary relation type to account for this.

define
publishing relates location,
    owns year;
city plays publishing:location;
state plays publishing:location;
country plays publishing:location;

We have also moved the year attribute from book to publishing, as it is more accurately a property of the publication event than it is the book itself. This is a good first iteration, but there are still many ways this model can be improved. We’ll explore them in the next few lessons.