The Power of the Query Object Pattern in Ruby on Rails

The Query Object Pattern is a powerful solution for simplifying query code, making ActiveRecord models cleaner, and enhancing readability and maintainability. Instead of overloading the model or controller with complex queries and multiple filters, the Query Object Pattern allows you to extract this logic into a dedicated class that is easy to reuse and test. Each Query Object represents a specific query task, contributing to a more organized architecture.

The Problem

When we start adding complex business logic to the model, especially in queries and scopes, we quickly end up with hard-to-understand and maintain code. This problem becomes even more apparent in controllers implementing search filters with multiple parameters, such as date, status, and customer IDs. Let's explore these cases.

Example of Complex Queries in the Model

In this example, the Order model has a complex query for fetching completed orders within a date range. All the query logic is within the model, making it bulkier and less modular.

This approach makes it difficult to reuse the query in other application parts and clutters the model.

Example of Complex Search Logic in the Controller

In our OrdersController, a search action must apply several filters based on the provided parameters. Applying this logic directly in the controller results in lengthy, less organized code.

The filtering logic makes the controller more complex, more challenging to maintain, and more difficult to test.

Solution with the Query Object Pattern

Using the Query Object Pattern, we create a new class that encapsulates the query logic. This Query Object can be reused by both the model and the controller, delegating complex logic to a specialized structure.

Refactoring the Model with the Query Object Pattern

Let’s extract the query to a CompletedOrdersQuery class, which applies filters for completed orders within a date range.

Now, the code that needs to access these orders can instantiate the Query Object and call the call method:

Refactoring the Controller with the Query Object Pattern

Similarly, to improve the index action of the controller, we extract the filtering logic into the OrderSearchQuery Query Object:

Now, the index action in the OrdersController is straightforward, delegating query logic to the OrderSearchQuery:

Benefits of the Query Object Pattern

1. Separation of Responsibilities: The model focuses on data management, and the controller orchestrates the flow, while the Query Object handles filtering and query logic.
2. Reusability and Flexibility: Query Objects can be reused across various application parts, avoiding code duplication.
3. Testability: Since the query logic is in a dedicated class, it can be easily tested in isolation without interfering with model or controller tests.

Conclusion

The Query Object Pattern is a powerful solution for keeping Rails application code clean and modular, especially for complex queries and filters. It promotes a?better separation of responsibilities, adhering to the?SOLID principle of single responsibility, making the application easier to maintain and scale.