Django ORM Count Without Duplicate Column - django

Question

I have a table called Reading History. I want to find the average number of reads in this table. I wrote the following method for this, but I can't get the right result. Records are made in the table once (student-book records with the same values). When the same record comes, the counter value is increased by one.
For example, suppose that two different students read two different books. I expect total reads / 2 but the result I get is total reads / 4 because there are 4 rows in the table. How can I calculate this? For example, if a student reads 4 different books once, the average will be 1, but the average should be 4.
I tried to use distinct and values but I couldn't get the result I wanted. Maybe I didn't manage to use it correctly. Also I tried to use Avg. When avg didn't give the correct result, I tried to break it down and get the values myself. Normally, Avg was written in Sum.
Serializer
class ClassReadingHistoryReportSerializer(ModelSerializer):
instructor = InstructorForClassSerializer(source="instructor.user")
students = StudenListReadingHistoryReportSerializer(many=True,
source="student_list_class")
avg_read_book = serializers.SerializerMethodField()
class Meta:
model = Class
exclude = [
"created_at",
"updated_at",
"school",
]
def get_avg_read_book(self, obj):
sum_read_book = Class.objects.filter(id = obj.id).aggregate(sum_read=Sum('student_list_class__child__child_reading_history__counter'))
count_child_record = Class.objects.filter(id = obj.id).aggregate(count_child=Count('student_list_class__child__child_reading_history'))
return None
Models
class ChildProfile(models.Model):
user = models.OneToOneField(
settings.AUTH_USER_MODEL,
on_delete=models.CASCADE,
primary_key=True,
verbose_name=AccountStrings.ChildProfileStrings.user_verbose_name,
related_name="user_child")
city = models.ForeignKey(
"country.City",
on_delete=models.SET_NULL,
null=True,
blank=True,
verbose_name=AccountStrings.ChildProfileStrings.city_verbose_name,
related_name="city_child_profiles")
hobbies = models.CharField(
max_length=500,
null=True,
blank=True,
verbose_name=AccountStrings.ChildProfileStrings.hobbies_verbose_name)
class Meta:
verbose_name = AccountStrings.ChildProfileStrings.meta_verbose_name
verbose_name_plural = AccountStrings.ChildProfileStrings.meta_verbose_name_plural
#property
def get_full_name(self):
return f"{self.user.first_name} {self.user.last_name}"
def __str__(self):
return self.get_full_name
def clean(self) -> None:
"""
This method will check if the user type is a child during creation.
"""
if self.user.user_type != 2:
raise ValidationError(AccountStrings.ChildProfileStrings.user_type_error)
class School(AbstractSchoolBaseModel):
city = models.ForeignKey(
"country.City",
on_delete=models.DO_NOTHING,
related_name="city_schools",
verbose_name=SchoolStrings.SchoolStrings.city_verbose_name)
name = models.CharField(
max_length=250,
verbose_name=SchoolStrings.SchoolStrings.name_verbose_name)
address = models.CharField(
max_length=250,
verbose_name=SchoolStrings.SchoolStrings.address_verbose_name)
website = models.CharField(
max_length=250,
verbose_name=SchoolStrings.SchoolStrings.website_verbose_name)
class Meta:
verbose_name = SchoolStrings.SchoolStrings.meta_verbose_name
verbose_name_plural = SchoolStrings.SchoolStrings.meta_verbose_name_plural
def __str__(self):
return self.name
class Class(AbstractSchoolBaseModel):
school = models.ForeignKey(
"school.School",
on_delete=models.CASCADE,
related_name="classes_school",
verbose_name=SchoolStrings.ClassStrings.school_verbose_name)
instructor = models.ForeignKey(
"account.InstructorProfile",
on_delete=models.CASCADE,
related_name="instructors_school",
verbose_name=SchoolStrings.ClassStrings.instructor_verbose_name)
name = models.CharField(
max_length=50,
verbose_name=SchoolStrings.ClassStrings.name_verbose_name)
grade = models.IntegerField(
verbose_name=SchoolStrings.ClassStrings.grade_verbose_name)
class Meta:
verbose_name = SchoolStrings.ClassStrings.meta_verbose_name
verbose_name_plural = SchoolStrings.ClassStrings.meta_verbose_name_plural
ordering = ["name", "grade"]
def __str__(self):
return f"{self.school.name} - {self.name} - Grade: {self.grade}"
def clean(self) -> None:
"""
This method checks whether the teacher trying to be assigned to the class is working in that school.
"""
if self.instructor.school != self.school:
raise ValidationError(SchoolStrings.ClassStrings.instructor_not_working_at_this_school_error)
class StudentList(AbstractSchoolBaseModel):
school_class = models.ForeignKey(
"school.Class",
on_delete=models.CASCADE,
related_name="student_list_class",
verbose_name=SchoolStrings.StudentListStrings.school_class_verbose_name
)
child = models.ForeignKey(
"account.ChildProfile",
on_delete=models.CASCADE,
related_name="student_list_children",
verbose_name=SchoolStrings.StudentListStrings.child_verbose_name)
class Meta:
verbose_name = SchoolStrings.StudentListStrings.meta_verbose_name
verbose_name_plural = SchoolStrings.StudentListStrings.meta_verbose_name_plural
def __str__(self):
return f"{self.school_class.name} : {self.child.user.first_name} {self.child.user.last_name}"
def clean(self) -> None:
result = StudentList.objects.filter(school_class = self.school_class, child = self.child)
if not self.pk and result.exists():
raise ValidationError(SchoolStrings.StudentListStrings.child_already_added_to_this_class_error)
class ReadingHistory(AbstractBookBaseModel):
IS_FINISHED = ((False,
BookStrings.ReadingHistoryStrings.is_finished_false),
(True, BookStrings.ReadingHistoryStrings.is_finished_true))
book = models.ForeignKey(
"book.Book",
on_delete=models.CASCADE,
related_name="book_reading_history",
verbose_name=BookStrings.ReadingHistoryStrings.book_verbose_name)
child = models.ForeignKey(
"account.ChildProfile",
on_delete=models.CASCADE,
related_name="child_reading_history",
verbose_name=BookStrings.ReadingHistoryStrings.child_verbose_name)
is_finished = models.BooleanField(
choices=IS_FINISHED,
verbose_name=BookStrings.ReadingHistoryStrings.is_finished_verbose_name
)
counter = models.PositiveIntegerField(
verbose_name=BookStrings.ReadingHistoryStrings.counter_verbose_name,
default=0)
class Meta:
verbose_name = BookStrings.ReadingHistoryStrings.meta_verbose_name
verbose_name_plural = BookStrings.ReadingHistoryStrings.meta_verbose_name_plural
def __str__(self):
return f"{self.child.user.first_name} {self.child.user.last_name} - \"{self.book.name}\" "
def clean(self) -> None:
result = ReadingHistory.objects.filter(child=self.child,
book=self.book)
if not self.pk and result.exists():
raise ValidationError(
BookStrings.ReadingHistoryStrings.exists_error)
I forgot to add the counter column in the BookReadingHistory table to the diagram. Diagram and Table data are shown in the images below.
Table
Diagram