ERROR: insert or update on table "student_marks" violates foreign key constraint "student_marks_subjname_fkey"
DETAIL:  Key (subjname)=(38) is not present in table "student_subject".
My Model is:-

# -*- coding: utf-8 -*-

from odoo import models, fields, api, tools, _
from datetime import datetime, date
from odoo.tools import DEFAULT_SERVER_DATE_FORMAT
from odoo.exceptions import ValidationError

#Model of student
class StudentStudent(models.Model):
    _name = 'student.student'
    _order = "admission_no desc"

    name = fields.Char(string = 'Name', required = True)
    student_dob = fields.Date(string = "Date of Birth")
    age = fields.Integer(string = 'Age', compute = 'calcu_age',
        store = True)
    admission_no = fields.Integer('Admission No', required = True)
    place = fields.Char(string = 'Place', required = True)
    photo = fields.Binary(string = 'Image')
    phone_number = fields.Char(string = 'Phone Number', required = True)
    gender = fields.Selection([('male', 'Male'),
                            ('female', 'Female'),
                            ('others', 'Others')],
                            string = 'Gender')
    student_blood_group = fields.Selection([('A+', 'A+ve'),
                                            ('B+', 'B+ve'),
                                            ('O+', 'O+ve'),
                                            ('AB+', 'AB+ve'),
                                             ('A-', 'A-ve'),
                                             ('B-', 'B-ve'),
                                             ('O-', 'O-ve'),
                                             ('AB-', 'AB-ve')],
                                             string = 'Blood Group')
    nationality = fields.Many2one('res.country', string = 'Nationality')
    branchname = fields.Many2one('student.branch', string = 'Branchname')
    active = fields.Boolean(default = True)
    filteration = fields.Selection(selection = 'addfilter')
    sortingfunction = fields.Selection(selection = 'addsort')
    creating = fields.Char(compute = 'action_create')
    browsing = fields.Char(compute = 'action_browse')
    #Existing=fields.Char(compute='action_exists')
   
    #Method to display name and place using name_get method
    @api.multi
    def name_get(self):
        '''Method to display name and place'''
        return [(rec.id, '[' + rec.name + ']' + rec.place) for rec in self]

    # Validation of Phone Number
    @api.constrains('phone_number')
    def _check_phone_number(self):
        if len(self.phone_number) != 10:
            raise ValidationError(_("Invalid Phone Number..."))

    # Create Orm method for creating a record
    @api.multi
    def action_create(self):
        for record in self:
            if record.name == 'lilly':
                record.create({'name': "Joe",
                                'admission_no': 456,
                                'place': "rtr"})

    #Method to calculate age fom Date of Birth
    @api.depends('student_dob')
    def calcu_age(self):
        '''Method to calculate student age'''
        current_dt = datetime.today()
        for rec in self:
            if rec.student_dob:
                start = datetime.strptime(str(rec.student_dob), DEFAULT_SERVER_DATE_FORMAT)
                age_calc = ((current_dt - start).days / 365)
                # Age should be greater than 0
                if age_calc > 0.0:
                    rec.age = age_calc

    #Browse Orm method to browse recordset based on id
    @api.multi
    def action_browse(self):
        for record in self:
            c=self.env['student.student'].browse([2]).name
            print('\n', c)

    @api.multi
    def addfilter(self):
        print("\n\n\n\n")
        a = self.env['student.student'].search([('active', '=', True)])
        k = a.filtered(lambda r:r.age == 21)
        return [(m.name, (m.name).upper()) for m in k]

    @api.multi
    def addsort(self):
        print("\n\n\n\n")
        a = self.env['student.student'].search([('active', '=', True)])
        k = a.sorted(lambda r:r.age)
        return [(m.name, (m.name).upper()) for m in k]
   

    

#Model of Activity
class StudentActivities(models.Model):
    _inherit = 'student.student'
   
    activity_name = fields.Char(string = 'Activity Name',
        required = True)
    activity_score = fields.Integer(string = 'Activity Score',
        required = True)
 #Model of Teacher
class Teacher(models.Model):
    _name = 'student.teacher'
    _rec_name = 'nametr'

    nametr = fields.Char(string = 'Name', required = True)
    age = fields.Integer(string = 'Age')
    photo = fields.Binary(string = 'Image')
    gender = fields.Selection([('male', 'Male'),
                            ('female', 'Female'),
                            ('others', 'Others')],
                            string='Gender')
    nationality = fields.Many2one('res.country', string = 'Nationality')
    state = fields.Selection([
            ('draft', 'Draft'),
            ('in_progress', 'In Progress'),
            ('done', 'Done')],
            default = 'draft')

    @api.one
    def action_confirm(self):
        self.write({'state': 'in_progress'})
   
    @api.one
    def action_finished(self):
        self.write({'state': 'done'})

#Model of Marklist
class StudentMarkList(models.Model):
    _name = 'student.mark'
    _rec_name = 'student_id'

    student_id = fields.Many2one('student.student',
        string = 'StudentName')
    branch = fields.Many2one('student.branch',
        string = 'Branch Name')
    # Setting Related field
    #age = fields.Integer(related='student_id.age',string='Age')
    '''
    total = fields.Integer(string = 'Total Mark', store = True,
        compute = '_compute_total')'''
    subname = fields.One2many('student.marks', 'subjname',
        string = 'Subjectwise Marks')       
    hide_inv_button = fields.Boolean(copy = False, default = True)
    state = fields.Selection([('confirmed', 'Confirm'),
                            ('cancelled', 'Cancel')],
                            string = 'Status', readonly = True,
                            copy = False, index = True,
                            track_visibility = 'onchange')
                                                           
    #Method of Confirm Button
    @api.multi
    def action_confirm(self):
        self.state = 'confirmed'
        self.hide_inv_button = False
                           
#Model of subject
class StudentSubject(models.Model):
    _name = 'student.subject'
   
    name = fields.Char(string = 'Subject Name')   
    branch_id = fields.Many2one("student.branch", string = "Branch",
        index = True,readonly = "1")

#Model of Branch
class StudentBranch(models.Model):
    _name = 'student.branch'
    _rec_name = 'branchname'
   
    branchname = fields.Char(string = 'BranchName')
    branchcode = fields.Integer(string = 'Branch Code')
    subname = fields.One2many('student.subject', 'branch_id',
        string = 'Subject Name')

#Model of Marks
class StudentMarks(models.Model):
    _name = 'student.marks'
    _rec_name = 'subjname'
   
    subjname = fields.Many2one('student.subject', string = 'Subjectname')
    mark = fields.Integer(string = 'Mark')
    branchmark =fields.Many2one('student.branch', string = "Branch",)
                           
    #Dynamic dropdown to display subname based on branch
    @api.onchange('branchmark')
    def _branch_onchange(self):
        res = {}
        res['domain'] = {'subjname':[('branch_id', '=', self.branchmark.id)]}
        return res
   
# onchange handler for subjectwise branch change
@api.onchange('branchname')
def _onchange_branchname(self):
    # set auto-changing field
    self.branch_id = self.branchname
    # Can optionally return a warning and domains
    return {
        'warning': {
            'title': "Something bad happened",
            'message': "It was very bad indeed",
        }
    }

Please Help...

Thanks in Advance