How to Get the Latest Visits of All Unique Users in MySQL ActiveRecord?

How to Use the Equivalent of DISTINCT ON in MySQL ActiveRecord

The requirement is to retrieve the latest visits of all distinct users from the events table. Using the DISTINCT ON(user_id) clause with MySQL in ActiveRecord results in a syntax error.

The MySQL equivalent of DISTINCT ON is to use GROUP BY with the MAX() function:

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

This query will group the results by user_id and return the maximum value for the time column for each group. The output will be a hash with the user_id as the key and the latest visit time as the value:

{
  21 => "2018-12-18 09:44:59",
  42 => "2018-12-19 12:08:59"
}

Note: DISTINCT ON(columns) is a PostgreSQL-specific syntax. MySQL does not support this syntax directly.

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