class CompositeNested(fields.Nested):

    def _deserialize(self, value, attr, data):
        data = super(CompositeNested, self)._deserialize(value, attr, data)
        return tuple(data[k] for k in self.schema.fields.keys())

class ValueWithUnitSchema(Schema):
    value = fields.Float()
    unit = fields.String()

    class Meta:
        strict = True
        ordered = True

class MonthRangeValueWithUnitSchema(Schema):
    from_month = fields.Integer()
    to_month = fields.Integer()
    value = CompositeNested(ValueWithUnitSchema)

    class Meta:
        strict = True
        ordered = True

class MySchema(ModelSchema):
    field_of_model = fields.List(CompositeNested(MonthRangeValueWithUnitSchema), required = True)