BUGFIX many many through not sorting by join table

[micmania1]

Resolves #7981

Todo:

• Write new tests to cover default sort on main dataobject as well as join table
• Add docs on sorting many many lists

many_many joins work by doing a join with two separate queries.

eg.

SELECT ...
 FROM "Fluent_Locale" INNER JOIN (
     SELECT ...
     FROM "Fluent_FallbackLocale"
     WHERE ("Fluent_FallbackLocale"."ParentID" = ?)
     ORDER BY "Fluent_FallbackLocale"."Sort" ASC
 ) AS "Fluent_FallbackLocale" ON "Fluent_FallbackLocale"."LocaleID" = "Fluent_Locale"."ID"
 ORDER BY "Fluent_Locale"."Locale" ASC

As you can see, the inner query is sorting correctly on the join table (by Sort ASC). However, the main query has its own sort which overrides that when you get the final set of results.

This inner query is built by SilverStripe using the standard ORM functions and assumes the default sort of the join DataObject (FluentFallbackLocale in this case). Likewise for the main query.

With @robbieaverill's fix it will sort correctly in cases where no default sort is set on the main DataObject and default is set on the join table data object. However, if the default sort is set on the main Dataobject, the fix will essentially be bypassed since sort is already set which is what we're seeing with Fluent.

The fix i've added applies the sort order much earlier, when it first starts building the ORM query objects. It will check to see if there is config which defines a default sort on the join table (this should also work for standard many-many sort too) and applies that if so. If that has no value, it will fallback to whatever sort SilverStripe gives it (default sort on dataobject or natural database sort).

This happens early enough that ->sort() will function as normal if the user decides to sort by something else and early enough that updateManymanyComponents can be hooked into.

[mfendeksilverstripe]

I tested this solution with the Fluent module which uses MMTL for fallbacks. It works fine, however we will need to add private static $default_sort = '"Fluent_FallbackLocale"."Sort" ASC'; to the FallbackLocale to use this new solution.

[tractorcow]

I had an idea once where you could declare the sort directly on the many_many array config, so you could override the model-specific sorts, but just for that relation.

e.g.

private static $many_many = [
    'Items' => [
        'through' => Relation::class,
        'from' => 'Parent',
        'to' => 'Child',
        'sort' => '"App_Relation"."Sort" ASC',
    ],
];

[mfendeksilverstripe]

+1 for Damian's idea. However, at this point, we will be happy with whatever solution that works.

[ScopeyNZ]

Code LGTM. Tried it locally and it worked.

Thanks for the tests and doc updates 👍