Premier League Predictor: Django & MySQL — Chapter 5, Exercise 2
====================================================

TASK
Explain why db_persist=True is specifically required for this
GeneratedField to work correctly on MySQL, and what real database
feature it corresponds to.

SOLUTION

Django's GeneratedField supports two underlying storage modes, chosen
via db_persist:

- db_persist=True compiles to a real, physically STORED generated
  column — the computed value is actually written to disk on every
  insert or update, exactly like an ordinary column, just with its
  value derived automatically from the expression instead of supplied
  directly.
- db_persist=False (the default when omitted) compiles to a VIRTUAL
  generated column instead — the value is computed on read, not
  stored on disk at all.

This distinction matters specifically for MySQL because MySQL's own
support for indexing a generated column depends on which of these two
kinds it is. A VIRTUAL generated column has real, documented
limitations around being indexed directly on older MySQL versions —
historically it could only be indexed as part of a secondary index in
InnoDB, with real restrictions on primary keys and certain storage
engines. A STORED generated column behaves like any other real,
materialized column for indexing purposes, with no such caveats.

Since unique_key exists purely so a UniqueConstraint can be placed on
it, the column has to be reliably indexable — that's the entire point
of building it. Choosing db_persist=True sidesteps MySQL's own
VIRTUAL-column indexing caveats entirely by making unique_key a real,
stored column from the start, at the modest cost of a small amount of
extra disk space per row (the value is now genuinely duplicated on
disk rather than computed fresh on each read).

MySQL has supported STORED generated columns since 5.7, which is why
Chapter 1's own MySQL setup didn't need to specify any particular
newer MySQL version for this feature specifically to work.

WHY THIS WORKS AS AN ANSWER
----------------------------
It distinguishes STORED from VIRTUAL generated columns, explains the
real MySQL indexing limitation that makes VIRTUAL columns a worse fit
here, and ties db_persist=True directly to the actual reason
unique_key needs to be reliably indexable at all.