Before Rails 6, if we want to extract associated records from an ActiveRecord::Relation, we would use preload and collect.
For example, we want to fetch subscriptions of some users. The query would look as shown below.
1User.where(blocked: false).preload(:subscriptions).collect(&:subscriptions) 2 3=> # returns collection of subscription records
ActiveRecord::Relation#extract_associated provides a shorthand to achieve same result and is more readable than former.
1User.where(blocked: false).extract_associated(:subscriptions) 2 3=> # returns the same collection of subscription records
Here's the relevant pull request for this change.