[5.7] Prevent breaking eager loading with string keys

[staudenmeir]

The eager loading improvement introduced in #26434 and #26453 broke some applications with string keys.

The documentation advices users with string keys to set both $incrementing = false and $keyType = 'string'. However, the affected users only adjusted $incrementing and left the default $keyType = 'int' unchanged. This has been working fine up to the latest release, as the $keyType is only used for auto-incrementing keys.

With this PR, the performance improvement is only applied to auto-incrementing primary keys. As a result, users with non-incrementing integer keys don't benefit from it anymore (but there probably aren't that many cases).

[stayallive]

So this broke my app 💯 never noticed that $keyType should be set... whoops!

Thanks for finding this!

[vlakoff]

By any chance, can you think of a way to get back the optimization with non-incrementing integer keys?

[staudenmeir]

No, not without some kind of flag that users would set in their models. So basically $keyType but "for real"...

You can apply the optimization to non-incrementing integer keys by extending the relationships and overriding whereInMethod(). But that's probably only worth the effort if you really benefit from raw queries.

[miklcct]

I think you should revert to the 5.7.14 behaviour in 5.8 for non-incrementing integer keys, and mention in the upgrade guide as a "breaking change", i.e. in 5.8 all integer keys should get the performance improvement and mention in the upgrade guide that the user must set $keyType for non-integer keys.