How to Perform a Recursive SQL Query with Ecto.Query in Elixir

Learn how to efficiently fetch books based on category IDs and their children using a recursive SQL query in Ecto.Query.
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This video is based on the question https://stackoverflow.com/q/67304563/ asked by the user 'Kourosh Alinaghi' ( https://stackoverflow.com/u/11451505/ ) and on the answer https://stackoverflow.com/a/67305733/ provided by the user 'eshirvana' ( https://stackoverflow.com/u/1367454/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Recursive SQL Queries with Ecto.Query

When dealing with databases, particularly ones that have hierarchical data like categories and subcategories, you may often encounter the need to pull data not just from a single entity, but also from its related children. This is where recursive SQL queries come into play.

In this guide, we’ll tackle a common problem: how to efficiently query a list of books based on a list of category IDs, including those categories’ children, using Ecto.Query in Elixir. Let’s dive into it!

Problem Description

Assume we have two tables:

Categories Table

id

name

parent_id (nullable)

Books Table

category_id

The objective is to create a function that takes a list of category IDs (category_ids) and returns a query that retrieves all books belonging to the specified categories or their subcategories.

The Challenge

The initial approach involved using a recursive SQL statement, but it only worked when passing a single integer ID. When trying to pass a list of category IDs, an error occurred:

[[See Video to Reveal this Text or Code Snippet]]

Thus, we need a reliable method to handle this without sending multiple queries to the database.

Solution Breakdown

Here’s how we can construct a recursive query to effectively gather this information.

Step 1: Create a Recursive Function in SQL

We first define a SQL function that accepts an array of integers as category IDs. This allows us to handle the incoming list efficiently.

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Implement the Ecto.Query in Elixir

After establishing your SQL function, you can define a function in your Elixir application to utilize this recursive query with Ecto.

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Usage

Now, when you call category_search_query/2 with your list of category IDs, it efficiently retrieves the books associated with those categories and all of their children in a single query.

Conclusion

By using a recursive SQL query, you can handle hierarchical data structures efficiently. With proper query structuring and adaptation in Ecto.Query, you can eliminate multiple database requests and optimize your application's performance.

By following the steps outlined above, you can effectively resolve the issue of recursively fetching related entities in an organized manner. Now, when working with categories and books in your projects, you have the tools to tackle it with ease!

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