GETTING AVERAGE OF A STUDENT IN DIFFERENT YEARS

Hello Friends, i have the following models

=========================================

class Person(models.Model):
fname=models.CharField(verbose_name="First Name",max_length=20,blank=False)
lname = models.CharField(verbose_name="Last Name", max_length=20, blank=False)
class Meta:
ordering =('id','fname','lname')
verbose_name="Person"
verbose_name_plural="Person"
def __str__(self):
return self.fname +" "+self.lname

class Year (models.Model):
year = models.PositiveIntegerField(verbose_name="Year",unique=True )

class Meta:
verbose_name = "Year"
verbose_name_plural = "Year"


def __str__(self):
return str(self.year)

class Courses (models.Model):
course = models.CharField(verbose_name="Course", max_length=20, blank=False,unique=True)
year = models.ForeignKey(Year,verbose_name="Year",on_delete=models.SET_NULL,null=True )

class Meta:
ordering = ('id', 'course','year')
verbose_name = "Coures"
verbose_name_plural = "Coures"

def __str__(self):
return str(self.course)

class PersonCourse(models.Model):
person = models.ForeignKey(Person, verbose_name="Person", on_delete=models.SET_NULL,null=True)
course = models.ForeignKey(Courses,verbose_name="Course",on_delete=models.SET_NULL,null=True)
year = models.ForeignKey(Year, verbose_name="Year", on_delete=models.SET_NULL, null=True)

class Meta:
ordering = ('id', 'person','course')
verbose_name = "PersonCourse"
verbose_name_plural = "PersonCourse"

def __str__(self):
return self.person.lname +" "+self.person.lname
class Grade(models.Model):
person = models.ForeignKey(Person, verbose_name="Person", on_delete=models.SET_NULL,null=True)
course = models.ForeignKey(Courses,verbose_name="Course",on_delete=models.SET_NULL,null=True)
year = models.ForeignKey(Year, verbose_name="Year", on_delete=models.SET_NULL, null=True)
mark=models.PositiveIntegerField(verbose_name="Marks" )
class Meta:
ordering = ('id', 'person','course')
verbose_name = "Grade"
verbose_name_plural = "Grade"

def __str__(self):
return self.person.lname +" "+self.person.lname +" "+self.course.course

===================== function ==========

def Average(self,obj):
results=Grade.objects.filter(person=obj).aggregate(Avg("mark"))
return results['mark__avg']
Average.short_description="Average"

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if i run the function it returns the average of all the subjects in the different year.

However i want it only to get the overage for a student in a separate year

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