Personal Catalogue: Django & PostgreSQL — Chapter 2, Exercise 2
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TASK
Explain, in your own words, why a shared Item model with a generic
creator field was chosen over four separate per-type models, and name
the one real feature that decision makes simpler.

SOLUTION

A fully "correct" normalized design would give each item type its own
model — a Book model with an author field, a MusicCD model with an
artist field, a Movie model (covering both DVD and Blu-ray) with a
director field, and so on. Every field on every model would then
genuinely apply to every row of that model, with nothing left
meaningless or blank.

The real cost of that design shows up specifically when trying to
answer the app's own central question: "do I already own this?" With
four separate models, answering that means running a real query
against each of the four models separately and then combining the
results in Python - four queries and some merging logic, every single
time a search happens.

The shared Item model, with a generic creator field standing in for
author/artist/director depending on the row's own item_type, avoids
that entirely. A single Django ORM query - something like
Item.objects.filter(title__icontains=query) - searches across every
item type at once, because every item type's data already lives in
the same table.

THE ONE FEATURE THIS MAKES SIMPLER

Search-across-everything - the app's real core "do I already own
this?" feature, covered in full in Chapter 7 - becomes a single ORM
query instead of four separate queries merged together. That's the
concrete, specific payoff of accepting a slightly less "pure" schema:
a few fields (like creator on a DVD row) are meaningless for some rows,
in exchange for the app's own most important feature staying genuinely
simple to implement and reason about.

WHY THIS WORKS AS AN ANSWER
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It compares the real query cost of both designs specifically for the
search feature, rather than treating "shared model" as simply a stylistic
preference, and names the one concrete feature (cross-type search) the
choice was actually optimizing for.