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The `remove()` and `clear()` methods of the related managers created by `ForeignKey`, `GenericForeignKey`, and `ManyToManyField` suffered from a number of issues. Some operations ran multiple data modifying queries without wrapping them in a transaction, and some operations didn't respect default filtering when it was present (i.e. when the default manager on the related model implemented a custom `get_queryset()`). Fixing the issues introduced some backward incompatible changes: - The implementation of `remove()` for `ForeignKey` related managers changed from a series of `Model.save()` calls to a single `QuerySet.update()` call. The change means that `pre_save` and `post_save` signals aren't called anymore. - The `remove()` and `clear()` methods for `GenericForeignKey` related managers now perform bulk delete so `Model.delete()` isn't called anymore. - The `remove()` and `clear()` methods for `ManyToManyField` related managers perform nested queries when filtering is involved, which may or may not be an issue depending on the database and the data itself. Refs. #3871, #21174. Thanks Anssi Kääriäinen and Tim Graham for the reviews.
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