Filament finds records that can still be attached to a BelongsToMany relationship with a pivot-table sub-select (whereNotExists(select 1 from pivot ... whereColumn ...)), which throws on MongoDB at render time. It offers this hook on the relationship class instead (filamentphp/filament#20442):

public function newUnattachedQuery(): \Illuminate\Database\Eloquent\Builder

Our BelongsToMany keeps the attached related keys as an array on the parent document, so the implementation is a fresh query on the related model with {{whereNotIn(relatedKey, parent->

)}}, preserving the relationship's own constraints and leaving the query modifiable by Filament. Must be a real public method on MongoDB\Laravel\Relations\BelongsToMany.

Filament features this enables: TableSelect and ModalTableSelect form fields with ignoreRelatedRecords(), and AttachAction::tableSelect() in relation managers. Today these break the whole form page on MongoDB.