How to Retrieve the Most Recent Visit Time for Each Unique User in MySQL with ActiveRecord?

Using DISTINCT ON with MySQL in ActiveRecord

When dealing with voluminous data, it's often necessary to extract unique information while performing database operations. One such scenario is identifying the latest visit of each distinct user.

Question:

While attempting to retrieve the most recent visit time for each unique user using the DISTINCT ON syntax, an ActiveRecord::StatementInvalid error is encountered. The query employed was:

<code class="ruby">Event.order(time: :desc).select('DISTINCT ON(user_id) user_id, time')</code>

Solution:

The DISTINCT ON syntax is specific to PostgreSQL and doesn't apply to MySQL. To achieve the desired result in MySQL using ActiveRecord, the query should be modified:

<code class="ruby">Events.group(:user_id).maximum(:time)</code>

Output:

This modified query will produce the expected output, where the keys represent unique user IDs, and the values represent the time of their most recent visits:

{21=>Tue, 18 Dec 2018 11:15:24 UTC +00:00, 23=>Thu, 20 Dec 2018 06:42:10 UTC +00:00}

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