Finding an Associated Table Oldest Record Filtering by One of Its Attributes

As developers, we often find ourselves dealing with complex relationships between tables in our databases. In this article, we’ll explore how to efficiently retrieve the oldest record from a related table based on a specific attribute.

Background and Problem Statement

Suppose you have two models: Subscription and Version. A Subscription has many Versions, and each Version has attributes like status, plan_id, and authorized_at date. When changes are made to a Subscription, the update often comes from a Version modification updating its parent Subscription. We need to find each Subscription's Version with the oldest authorized_at date where the plan_id changed to the current Subscription's plan_id.

Querying Approach

The provided SQL query uses a lateral subquery, which can be described as a “foreach loop” in SQL. This approach is highly performant and allows us to select columns from a single correlated record or aggregates from a group of related records.

Lateral Subquery Explanation

A lateral subquery is a type of SQL query that allows you to reference the rows being processed by the main query. In our case, for each row in subscriptions, we want to select a single row from versions ordered by authorized_at. The lateral join enables us to achieve this.

Here’s the relevant SQL excerpt:

SELECT "subscriptions".*,
       "latest_version"."authorized_at" AS current_version,
       "latest_version"."id" AS current_version_id 
FROM   "subscriptions" 
LATERAL
  (
     SELECT   "versions"."authorized_at", "versions"."id"
     FROM     "versions"
     WHERE    "versions"."subscription_id" = "subscriptions"."id" -- lateral reference
     AND      "versions"."plan_id" = "subscriptions"."plan_id"
     AND      "versions"."status" = 'processed'
     ORDER BY "versions"."authorized_at" ASC
     LIMIT 1
  ) latest_version ON TRUE

In this example, we’re using a lateral join to access the versions table for each row in subscriptions. We filter the results based on the conditions specified.

Implementing Lateral Subquery in Active Record

To create a similar query using Arel (a Rails framework) library, you can use the following code:

class Subscription < ApplicationRecord
  # Performs a lateral join and selects the 
  # authorized_at of the latest version 
  def self.with_current_version
    lateral = Version.arel_table.then do |v|
      v.project(
        v[:authorized_at],
        v[:id] # optional
      ).where(
        v[:subscription_id].eq(arel_table[:id])
          .and(v[:plan_id].eq(arel_table[:plan_id]) )
          .and(v[:status].eq('processed'))
      )
      .order(v[:authorized_at].asc)
      .take(1) # limit 1
      .lateral('latest_version ON TRUE')
    end
    lv = Arel::Table.new(:latest_version) # just a table alias
    select(
      *where(nil).arel.projections, # selects everything previously selected
      lv[:authorized_at].as("current_version"),
      lv[:id].as("current_version_id") # optional
    ).joins(lateral.to_sql)
  end
end

This code creates a lateral join using Arel and then projects the columns we want to select. The lateral method is used to enable the reference to the related table.

Choosing Between SQL Strings and Arel

You can use either SQL strings or Arel for creating lateral joins in Rails. Both approaches have their advantages:

• SQL Strings: Using raw SQL strings provides more flexibility, especially when working with complex queries.
• Arel: Arel offers a higher-level abstraction, making it easier to build and maintain complex queries.

Limitations of Lateral Join

While lateral joins provide great flexibility, they do have some limitations:

• Performance: Lateral joins can lead to performance issues if not optimized correctly. Make sure to use indexes and limit the number of rows being joined.
• Complexity: Complex queries with multiple lateral joins can be difficult to maintain.

Conclusion

In this article, we explored how to efficiently retrieve the oldest record from a related table based on a specific attribute using lateral subqueries. By leveraging Arel or SQL strings, you can create complex queries that provide the flexibility needed for your database needs.

When working with lateral joins, keep in mind their performance implications and strive to optimize them accordingly. With practice, you’ll become proficient in crafting efficient lateral join queries to tackle even the most challenging database problems.

Last modified on 2023-09-28