added str method in Author and created Books class in models.py

bellapanghulan_reading/bookshelf/models.py

 from django.db import models
+from django.core.exceptions import ValidationError
 
 # Create your models here.
 class Author(models.Model):
@@ -7,3 +8,23 @@ class Author(models.Model):
     age = models.IntegerField(default = 0)
     nationality = models.CharField(max_length = 50)
     bio = models.TextField(max_length = 700)
+
+    def __str__(self):
+        return self.first_name + " " + self.last_name
+
+class Books(models.Model):
+    title = models.CharField(max_length = 100)
+    author = models.ForeignKey(Author, on_delete=models.CASCADE)
+    publisher = models.CharField(max_length = 100)
+    year_published = models.DateTimeField('date published')
+    ISBN = models.CharField(max_length=13, validators=[validate_ISBN])
+    blurb = models.TextField(max_length = 200, min_length = 100)
+    
+    def validate_ISBN(value):
+        if len(value) != 13:
+            raise ValidationError('Must be exactly 13 digits.')
+        if not value.isdigit():
+            raise ValidationError('Must only be digits.')
+    
+    def __str__(self):
+        return self.title
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