How to Convert SQL Queries to Rails ActiveRecord Commands

Learn how to effectively convert SQL statements into Rails ActiveRecord commands using Arel for an efficient querying experience.
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Converting SQL Statements to Rails Commands

When working with Ruby on Rails, sometimes developers encounter complex SQL queries that need to be translated into a more Rails-friendly format. This can be particularly challenging when dealing with specialized SQL functions, such as ROW_NUMBER(). In this guide, we’ll explore a solution for converting a specific SQL statement into an Active Record query, employing Arel to bridge the gap.

The Problem

Let's consider a scenario where we need to fetch a limited number of records based on certain criteria from an Active Record query response. The original SQL query given looks like this:

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

The goal is to understand how to express this complex SQL query using Rails’ Active Record capabilities.

Understanding the SQL Statement

Here's a breakdown of the SQL query:

Common Table Expression (CTE): The WITH some_table AS part of the query creates a temporary result set that can be referenced within the main query.

ROW_NUMBER() function: This function assigns a unique sequential integer to rows within a partition.

Filtering: The WHERE clause filters records under two conditions:

Records where column_a equals 'ABC' and the row number (rn) is less than or equal to 10.

OR records where column_b is not equal to 'AAA'.

The Solution: Using ActiveRecord and Arel

Active Record does not directly support Common Table Expressions (CTEs), but we can work around this limitation by employing some Arel, a SQL AST manager for Ruby that helps construct SQL queries programmatically.

Step 1: Setting Up the Arel Table

First, we need to set up the Arel table for the annotations model:

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

Step 2: Creating the Sub-query

Next, we build the sub-query that performs the equivalent of the ROW_NUMBER() functionality:

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

Step 3: Constructing the Final Query

Now, we can create the final query that incorporates the sub-query and the filtering conditions:

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

Resulting SQL Query

The resulting Active Record command will generate the following SQL statement:

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

Important Notes to Keep in Mind

While we briefly touched upon using a CTE here, it seems unnecessary for this specific case.

Various workarounds exist to transition ROW_NUMBER() to Arel, but those might complicate the query more than necessary.

Conclusion

Converting SQL statements into Active Record queries can seem daunting at times, especially with complex operations like window functions. However, by leveraging Arel, you can create robust queries that maintain the power of SQL within the Rails ecosystem. Understanding how to effectively use these tools will greatly enhance your query-making capabilities and streamline your code.

By implementing these techniques, you can efficiently utilize SQL's rich feature set while taking advantage of Rails' Active Record for a more expressive and maintainable approach.