Limit Foreignkey Choices based on already obtained values

Question

I have three models which are interlinked hierarchically (see #models.py). In my CreateTimeTable view, I create a timetable for a week and each period of a day takes a 'foreignkey' of a teacher who is inturn linked to his/her subject. Of course no teacher can teach two classes at the same time, so I have set the " 'grade' and 'each period' to be unique together ".

Eg:- let's assume that there are two sections in grade 12: -

Section A have 'Physics' period by 'Professor Albert Einstein' for monday's first period section B can have 'Physics' period on monday's first period, but it can not be the 'Physics' Period by 'Professor albert Einstein', that's why i have set the unique_together fields.....

Of course this works, no teacher can be linked to the same period at the same time for two different sections

Assuming that grade 12 section A's timetable has been created already and it's first period is 'Physics' by 'Professor Albert Einstein', then I don't want to display the choice of 'Physics' by 'Albert Einstein' in the first period of the form for the next section (i.e section B).

I have no idea how to do this. Please tell how to go about this

#models.py

class Subjects(models.Model):
    Subjects_available = (
        ('Maths','Maths'),
        ('English','English'),
        ('Computer','Computer'),
        ('Chemistry','Chemistry'),
        ('Physics','Physics'),
    )
    subject_name = models.CharField(max_length = 256,choices = Subjects_available,unique=True)

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

class Teachers(models.Model):
    classes_available = (
        ('Grade 1','Grade 1'),
        ('Grade 2','Grade 2'),
        ('Grade 3','Grade 3'),
        ('Grade 4','Grade 4'),
        ('Grade 5','Grade 5'),
        ('Grade 6','Grade 6'),
        ('Grade 7','Grade 7'),
        ('Grade 8','Grade 8'),
        ('Grade 9','Grade 9'),
        ('Grade 10','Grade 10'),
        ('Grade 11','Grade 11'),
        ('Grade 12','Grade 12'),
    )

    name = models.CharField(max_length = 256)
    subject = models.ForeignKey(Subjects,on_delete=models.CASCADE,related_name='teacher_subject')
    teaches_for_class = models.CharField(max_length=256,choices=classes_available)

    def  __str__(self):
        return str(self.subject) + ' - ' + self.name

class TimeTable(models.Model):
    standard_choice = (
        ('Grade 1','Grade 1'),
        ('Grade 2','Grade 2'),
        ('Grade 3','Grade 3'),
        ('Grade 4','Grade 4'),
        ('Grade 5','Grade 5'),
        ('Grade 6','Grade 6'),
        ('Grade 7','Grade 7'),
        ('Grade 8','Grade 8'),
        ('Grade 9','Grade 9'),
        ('Grade 10','Grade 10'),
        ('Grade 11','Grade 11'),
        ('Grade 12','Grade 12'),
    )

    section_choice = (
        ('A','A'),
        ('B','B'),
        ('C','C'),
        ('D','D'),
    )

    standard = models.CharField(max_length=256,choices=standard_choice)
    section = models.CharField(max_length=1,choices=section_choice)

    mon_1 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='mon_1_subject_teacher')
    mon_2 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='mon_2_subject_teacher')
    mon_3 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='mon_3_subject_teacher')
    mon_4 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='mon_4_subject_teacher')
    mon_5 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='mon_5_subject_teacher')

    tue_1 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='tue_1_subject_teacher')
    tue_2 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='tue_2_subject_teacher')
    tue_3 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='tue_3_subject_teacher')
    tue_4 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='tue_4_subject_teacher')
    tue_5 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='tue_5_subject_teacher')

    wednes_1 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='wed_1_subject_teacher')
    wednes_2 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='wed_2_subject_teacher')
    wednes_3 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='wed_3_subject_teacher')
    wednes_4 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='wed_4_subject_teacher')
    wednes_5 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='wed_5_subject_teacher')

    thurs_1 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='thurs_1_subject_teacher')
    thurs_2 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='thurs_2_subject_teacher')
    thurs_3 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='thurs_3_subject_teacher')
    thurs_4 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='thurs_4_subject_teacher')
    thurs_5 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='thurs_5_subject_teacher')

    fri_1 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='fri_1_subject_teacher')
    fri_2 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='fri_2_subject_teacher')
    fri_3 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='fri_3_subject_teacher')
    fri_4 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='fri_4_subject_teacher')
    fri_5 = models.ForeignKey(Teachers,on_delete=models.CASCADE,related_name='fri_5_subject_teacher')

    def  __str__(self):
        return self.standard+' '+self.section+ ' '+ 'TimeTable'

    # This works from one particular standard, but if some teachers take class for two grades , then you have 'ForeignKey'
    # the 'class they take' here and set unique_together constraint - hope it works
    class Meta():
        unique_together = [
        ['standard','section'],

        ['standard','mon_1'],
        ['standard','mon_2'],
        ['standard','mon_3'],
        ['standard','mon_4'],
        ['standard','mon_5'],

        ['standard','tue_1'],
        ['standard','tue_2'],
        ['standard','tue_3'],
        ['standard','tue_4'],
        ['standard','tue_5'],

        ['standard','wednes_1'],
        ['standard','wednes_2'],
        ['standard','wednes_3'],
        ['standard','wednes_4'],
        ['standard','wednes_5'],

        ['standard','thurs_1'],
        ['standard','thurs_2'],
        ['standard','thurs_3'],
        ['standard','thurs_4'],
        ['standard','thurs_5'],

        ['standard','fri_1'],
        ['standard','fri_2'],
        ['standard','fri_3'],
        ['standard','fri_4'],
        ['standard','fri_5'],
        ]

#forms.py

class TimeTableForm(models.TimeTable):
    class Meta():
        model = models.TimeTable
        fields = ('__all__')

#views.py

class CreateTimeTable(CreateView):
    model = models.TimeTable
    fields= ('__all__')
    template_name = 'TimeTableApp/create_time_table.html'
    success_url = reverse_lazy('TimeTableApp:inner_index')