Eager Loading Associations of the Same Class in Rails

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In this article, we will explore how to eagerly load associations of the same class in Rails. We’ll use the provided Stack Overflow question as a starting point and delve into the intricacies of eager loading with scopes.

Understanding Associations and Scopes

Before diving into the solution, let’s review some fundamental concepts:

• Associations: In Rails, an association is a relationship between two models. This can be a one-to-many (e.g., has_many), many-to-one (e.g., belongs_to), or many-to-many (e.g., has_and_belongs_to_many) relationship.
• Scopes: A scope in Rails is a way to define a custom query that can be applied to a model. Scopes are typically used with scope methods, which are defined on the model.

Eager Loading Associations

Eager loading associations allows you to load related objects along with the main object when it’s fetched from the database. This can significantly improve performance by reducing the number of database queries.

Using includes

To eager load an association, you use the includes method on the model. For example:

Category.includes(:products).where(kind: 'category')

This will fetch all categories and include their associated products in the result set.

Eager Loading with Scopes

When using scopes to filter data, eager loading can help by fetching related objects at the same time.

The Challenge

The original question presents a challenge because it involves two associations of the same class: Category has an association with itself through the has_many :sub_categories method. We also have a scope with_products on both models that filters data based on related products.

Solution 1: Using includes and Joins

One way to achieve this is by using includes and joins. Here’s an example:

Category.includes(:products, :sub_categories).where(kind: 'category')

In this code:

• We use includes with the :products and :sub_categories associations.
• The scope with_products will be applied to both the products and sub-categories associations.

However, there’s a catch. When using joins with eager loading, we need to ensure that the association is not being loaded multiple times. In this case, since Category has an association with itself through has_many :sub_categories, using includes directly won’t work as expected because it will load the sub-categories associations multiple times.

Solution 2: Using load and Joins

Another approach is to use the load method on the model, which allows us to define custom loading logic. Here’s an example:

class Category < ApplicationRecord
  # ...

  def self.with_products_and_sub_categories(kind)
    where(kind: kind).joins(
      [:products, :sub_categories]
    ).select('categories.*, products.*', 'sub_categories.*)'
  end

  def sub_categories_with_products
    joins(:products).select('sub_categories.*')
  end
end

In this code:

• We define a scope with_products_and_sub_categories that filters data based on the given kind.
• Inside the scope, we use the joins method to join the category associations with its products and sub-categories.
• We also define a method sub_categories_with_products that returns the related sub-categories with their associated products.

Solution 3: Using eager_load on has_and_belongs_to_many

Since we’re dealing with has_and_belongs_to_many associations, we can use the eager_load option to specify which associations should be loaded eagerly. Here’s an example:

class Product < ApplicationRecord
  # ...

  has_and_belongs_to_many :categories,
    class_name: 'Category',
    source: :categories,
    eager_load: [:sub_categories]

  def sub_categories_with_products
    joins(:sub_categories).select('sub_categories.*')
  end
end

class Category < ApplicationRecord
  # ...

  has_and_belongs_to_many :products,
    class_name: 'Product',
    source: :products,
    eager_load: [:with_products]
end

# Usage:
Category.with_products.includes(:sub_categories).where(kind: 'category')

In this code:

• We use the eager_load option to specify which associations should be loaded eagerly.
• For the product association, we load the sub-categories with their associated products using the eager_load option.

Conclusion

Eager loading associations of the same class in Rails can be achieved through various methods. The approach you choose depends on your specific requirements and the structure of your models. By understanding how scopes work, using includes and joins carefully, and leveraging the load method or eager_load option for has_and_belongs_to_many, you can optimize your database queries and improve performance in your Rails applications.

Last modified on 2024-12-24