models.py

from django.db import models

class users(models.Model):
    first_name = models.CharField(max_length = 50)
    surname = models.CharField(max_length = 50)

    def publish(self):
        self.save()

    def __str__(self):
        return self.first_name + "  "+ self.surname

class libary(models.Model):
    NameOfBook = models.CharField(max_length = 50)
    Author = models.CharField(max_length = 50)

    def publish(self):
        self.save()

    def __str__(self):
        return self.NameOfBook +"  "+ self.Author

class loan(models.Model):
    User = models.ForeignKey(users, on_delete = models.CASCADE)
    Book = models.ForeignKey(libary, on_delete = models.CASCADE)

    def publish(self):
        self.save()