update

app/models/customer.py

@@ -20,6 +20,13 @@ class Customer(db.Model):
         back_populates="customer",
     )
 
+    loans = relationship(
+        "Loan",
+        primaryjoin="Customer.id == Loan.customer_id",
+        foreign_keys="Loan.customer_id", 
+        back_populates="customer",
+    )
+
     @classmethod
     def is_valid_customer(cls, customer_id):
         customer = cls.query.filter_by(id=customer_id).first()
@@ -44,11 +51,20 @@ class Customer(db.Model):
                 account_type=account_type
             )
             
-            db.session.commit()
         except IntegrityError as err:
-            db.session.rollback()
             raise ValueError(f"Database integrity error: {err}")
         return customer
+    
+    @classmethod
+    def get_customer(cls, customer_id):
+        """
+        Get customer by  ID.
+        """
+        customer = cls.query.filter_by(id=customer_id).first()
+
+        if not customer:
+            raise ValueError(f"Customer does not exist")        
+        return customer
 
     def __repr__(self):
         return f'<Customer {self.id}>'