text: from django.contrib.auth.models import User
  from django.db import models
  
  from products.models import Product
  
  
  class Contact(models.Model):
      buyer = models.ForeignKey(User, on_delete=models.CASCADE, related_name='contact_buyer')
      seller = models.ForeignKey(User, on_delete=models.CASCADE, related_name='contact_seller')
      product = models.ForeignKey(Product, on_delete=models.CASCADE, related_name='contact_product')
  
      class Meta:
          constraints = [
              models.UniqueConstraint(fields=['buyer', 'seller', 'product'], name='unique_contact')
          ]
  