Death by a Million Recursions
Tags: python, sql
Published: 2023-04-22 00:27:56

Serialize rules can be an incredibly useful tool for making sure that only the necessary information is displayed in a serialized object. However, there are times when using serialize rules can lead to recursion errors.

Recursion errors occur when there is an infinite loop of serialization, where an object is being serialized repeatedly because it is a property of another object. This can happen when two objects have a many-to-many relationship and both objects contain the serialized representation of each other.

Let's look at an example, 

In the case of our three tables, Pizza, Restaurant, and RestaurantPizza, we can see how recursion errors might occur. Let's say we want to serialize all the Pizza objects, but we also want to include all the Restaurant objects that have a relationship with each Pizza. We might define our serialize rules for Pizza to include the Restaurant objects:

`class Pizza(db.Model, SerializerMixin):
    serialize_rules = ('-restaurants', '-restaurant_pizzas', '-created_at', '-updated_at')`

But since Restaurant also has a relationship with Pizza, we might define our serialize rules for Restaurant to include the Pizza objects:


`class Restaurant(db.Model, SerializerMixin):
    serialize_rules = ('-pizzas', '-restaurant_pizzas', '-created_at', '-updated_at')`

This creates a recursion error because both Pizza and Restaurant are trying to include each other in their serialized representations.

To avoid recursion errors, we need to be mindful of how we use serialize rules. In some cases, we might need to exclude certain relationships altogether. In other cases, we might need to include only a subset of related objects, rather than all related objects.

In our example, we might decide to exclude the Restaurant objects from the serialized representation of Pizza:

`class Pizza(db.Model, SerializerMixin):
    serialize_rules = ('-restaurant_pizzas', '-created_at', '-updated_at')`

This way, we only include the RestaurantPizza objects that relate to each Pizza, rather than including the Restaurant objects themselves.

Alternatively, we might decide to include only a subset of the Pizza objects in the serialized representation of each Restaurant. For example, we might only include the Pizza objects that are the most popular:

`class Restaurant(db.Model, SerializerMixin):
    serialize_rules = ('pizzas', '-restaurant_pizzas', '-created_at', '-updated_at')
    @property
    def pizzas(self):
        return sorted(self.pizzas, key=lambda pizza: pizza.popularity)[:3]`

In this case, we define a property that returns only the three most popular Pizza objects for each Restaurant, and we include only this property in the serialized representation of each Restaurant.

By being mindful of our use of serialize rules and taking care to avoid recursion errors, we can use this powerful tool to create clean, concise serialized representations of our database objects.

Serialize rules are a powerful tool for creating serialized representations of database objects in SQLAlchemy. By using serialize rules, we can control which properties of each object are included in the serialized representation, and we can exclude properties that are not relevant or that might cause recursion errors.

In this article, we looked at an example of how to use serialize rules in SQLAlchemy to create serialized representations of three tables with a many-to-many relationship. We also discussed some best practices for using serialize rules, including being mindful of recursion errors and using properties to include only a subset of related objects when necessary.

With these best practices in mind, we can use serialize rules to create clean, concise serialized representations.