how to use DISTINCT ON with mysql using ActiveRecord

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So you want all user visits with last visited time.

Instead of use DISTINCT function, you can use GROUP with MAX function.

Query looks like

Events.group(:user_id).maximum(:time)

This Outputs your desired results

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

Hope this works for you.

FYI
DISTINCT ON(columns). is PostgreSQL Syntax.

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