ValueError: Cannot assign must be a "Survey" instance

Hello.
I want to submit a nested report, and I am facing one error, don’t have any idea how to solve it.

Error output-------------------------------------------------------------------

*** ValueError: Cannot assign "OrderedDict([('questions', [OrderedDict([('answers', [<Answer: Answer object (1)>])]), OrderedDict([('answers', [<Answer: Answer object (5
)>])]), OrderedDict([('answers', [<Answer: Answer object (9)>, <Answer: Answer object (10)>])]), OrderedDict([('answers', [<Answer: Answer object (16)>])]), OrderedDict(
[('answers', [<Answer: Answer object (18)>]), ('solutions', [<Answer: Answer object (21)>, <Answer: Answer object (24)>])])]), ('user', OrderedDict())])": "Report.survey
" must be a "Survey" instance.

My serializers.py-------------------------------------------------------------------

class ReportAnswerField(serializers.SlugRelatedField):
    def to_representation(self, answers):
        return AnswerSerializer(answers).data


class ReportQuestionSerializer(serializers.ModelSerializer):
    answers = ReportAnswerField(
        slug_field='key',
        queryset=Answer.objects.all(),
        many=True
    )
    solutions = ReportAnswerField(
        slug_field='key',
        queryset=Answer.objects.all(),
        required=False,
        many=True
    )

    class Meta:
        model = Question
        fields = [
            'answers',
            'solutions'
        ]


class ReportSurveySerializer(serializers.ModelSerializer):
    questions = ReportQuestionSerializer(many=True)
    user = CurrentUserSerializer()

    class Meta:
        model = Survey
        fields = ['questions', 'user']


class ReportSerializer(serializers.ModelSerializer):
    survey = ReportSurveySerializer()
    # Error is coming from here
    class Meta:
        model = Report
        fields = ['survey']

My views.py-------------------------------------------------------------------

class ReportsViewSet(viewsets.ModelViewSet):
    queryset = Report.objects.all()
    serializer_class = ReportSerializer

My models.py-------------------------------------------------------------------

class Survey(ProjectModel):
    DEFAULT = 'default'

    KEYS = (
        (DEFAULT, 'Default survey'),
    )

    user = models.ForeignKey(
        to=User,
        on_delete=models.CASCADE,
        related_name='surveys',
        null=True
    )
    questions = models.ManyToManyField(
        to=Question,
        related_name='surveys',
    )

    key = models.CharField(choices=KEYS, max_length=255)


class Report(ProjectModel):
    survey = models.ForeignKey(
        to=Survey,
        related_name='reports',
        on_delete=models.CASCADE
    )
    user = models.ForeignKey(
        to=User,
        on_delete=models.CASCADE,
        null=False
    )

Hello,
could you please add information what is serializers, viewsets, models? …so people do not have to guess. Also which line of the code is in the backtrace of the exception? Maybe the whole backtrace can be useful?