Change SQL generation when primary_key=True and unique=True

[Nis Jørgensen <nis@…>]

At the moment, these two models, which are logically equivalent, generate different sql:

class Country(models.Model):
    isocode = CharField(max_length=3, primary_key=True, unique=True)

class Country(models.Model):
    isocode = CharField(max_length=3, primary_key=True)

• at least for backends other than Oracle

I would like to suggest that we do not create explicit UNIQUE constraints for (single field) primary keys.

On the other hand, I think it would be nice if field.unique returned True for a primary key field no matter if it was defined as such or not.

I will write a patch doing this, but am of course open to alternative suggestions.

[Nis Jørgensen <nis@…>]

Patch doing both of the things mentioned in the article

[Nis Jørgensen <nis@…>]

Patch doing both of the things mentioned in the ticket

[Nis Jørgensen <nis@…>]

Oops - double submit. The patches are identical.

The patch does the following:

If a field is declared as primary_key, it is automatically flagged as unique

If a field is declared as primary_key, no UNIQUE is output in the SQL.

All checks of the type

if f.primary_key or f.unique:

are replaced by

if f.unique:

[Malcolm Tredinnick]

The concept seems reasonable. The patch introduces some random formatting changes that are not part of the change needed, but otherwise looks reasonable from just eyeballing it. I haven't tested it on all (or, in fact, any) db backends, so I'm not going to move it to almost ready-for-checkin just yet.

[Malcolm Tredinnick]

(In [7790]) Make sure we only create the minimum number of table indexes for MySQL.

This patch simplifies a bunch of code for all backends and removes some
duplicate index creation for MySQL, in particular (versions 4.x and later).
Patch from Nis Jørgensen.

Fixed #5671, #5680, #7170, #7186.