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Description
#8979 prevents list serializers from having data and instances with different lengths:
https://github.com/encode/django-rest-framework/pull/8979/files#diff-c33f1f011c9f5cf3ed1357b809ebf2b4ed0b808b227c6c007291bf9b259422ecR612-R617
This design choice breaks some use cases. Let's say we want the list serializer to delete from queryset all elements that are not provided in the data during an update. The queryset will be bigger than the data in this case:
The intention of the original PR is good, it solves the problem of not passing initial data and instances to child serializers correctly. But instead of indexing, it's better to relate the elements by id, so we can have different lengths. This PR addresses the issue.
refs #8926 #8979