Parsing multipart data using a nested serializer with a list can result in incorrect behavior. If the list contains more than one item, only the last item will be saved.

Here is an example:

classNestedSerializer(serializers.Serializer):example=serializers.MultipleChoiceField(choices=[1,2,3])classTestSerializer(serializers.Serializer):nested=NestedSerializer()

When you send the following data formatted as JSON everything will be fine:

{'nested': {'example': [1,2],    }}#   --->{'nested': {'example': [1,2],    }}

However, when you send the same data formatted as multipart data the serialized object will only take the last element of the list:

{'nested.example': [1,2]}#   --->{'nested' {'example':2,    }}

The bug is situated inparse_html_dict(dictionary, prefix='') inrest_framework.utils.html. This function takes and returns a newMultiValueDict. A key in aMultiValueDict can have multiple values, this is how a list is represented. When accessing a key in aMultiValueDict only the last element of the list is returned.

This can be easily solved by using .getlist() and .setlist() when getting and setting the value of theMultiValueDict.

Closes#3710.