UNIQUE constraint failed: matrixapp_bookplot.user_id

Hello everyone, 

when i am trying to add uuid primary key , I am getting this error UNIQUE constraint failed: matrixapp_bookplot.user_id

what is wrong here

class BookPlot(models.Model):
       
        ref_id = models.CharField(max_length=25)
        code = str(uuid.uuid4()).replace("-", "")[:4]
        user_id=models.CharField(primary_key=True,max_length=50, default=code, editable=False)
        plot_number = models.CharField(max_length=25)
        Payable_amout = models.IntegerField(null=True, blank=True)
        Mnthly_Installment = models.IntegerField(null=True, blank=True)
        number_of_Installment = models.IntegerField()
        name = models.CharField(max_length=100)
        father_name = models.CharField(max_length=100)
        mobile_no = models.IntegerField()
        payment_mode = models.CharField(max_length=10)
        remarks = models.CharField(max_length=100)
        receipt = models.ImageField(upload_to = 'receipt/', null= True, blank=True)
        joinig_date=models.DateField(auto_now_add=True,null=True,blank=True)

 

       
    def  bookplot(request):
       
     
        if request.method =="POST":
            ref_id = request.POST.get('ref_id')
           
            plot_number = request.POST.get('plot_number')
            amount = request.POST.get('amount')
            Mnthly_Installment = request.POST.get('Mnthly_installment')
            no_Installment = request.POST.get('no_Installment')
            name = request.POST.get('name')
            father_name = request.POST.get('father_name')
            mobile_number = request.POST.get('mobile_number')
            payment_mode = request.POST.get('payment_mode')
            remarks = request.POST.get('remarks')
            receipt = request.FILES.get('receipt')
            print(ref_id)
            book_plot = BookPlot(ref_id= ref_id,plot_number = plot_number, Payable_amout = amount,Mnthly_Installment = Mnthly_Installment, number_of_Installment = no_Installment, name = name,father_name = father_name , mobile_no = mobile_number, payment_mode = payment_mode ,remarks=remarks,receipt=receipt )
            book_plot.save()

 

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