fix: custom validators for loans

loans/models.py

 from django.db import models
-from django.utils import timezone
 
 from book.models import Book
 from users.models import Member
+from .validators import validate_due_date, validate_loan_date
 
 
 class BookLoan(models.Model):
     book = models.ForeignKey(Book, on_delete=models.CASCADE)
     member = models.ForeignKey(Member, on_delete=models.CASCADE)
-    loan_date = models.DateTimeField()
-    due_date = models.DateTimeField()
+    loan_date = models.DateTimeField(validators=[validate_loan_date])
+    due_date = models.DateTimeField(validators=[validate_due_date])
     return_date = models.DateTimeField(blank=True, null=True)
     created_at = models.DateTimeField(auto_now_add=True)
     updated_at = models.DateTimeField(auto_now=True)

loans/validators.py

+from django.core.exceptions import ValidationError
+from django.utils.translation import gettext_lazy as _
+from django.utils import timezone
+
+
+def validate_loan_date(value):
+    now = timezone.now()
+    loan_date = value
+    if loan_date > now:
+        raise ValidationError(
+            _("Loan date cannot be later than today"),
+            params={"value": value},
+        )
+
+
+def validate_due_date(value):
+    due_date = value
+    loan_date = timezone.now()
+    if due_date < loan_date:
+        raise ValidationError(
+            _("Due date cannot be less than loan date"),
+            params={"value": value},
+        )