Rails 6 has added ActiveRecord::Relation#extract_associated for extracting associated records from a relation.

#### Before

Trying to access collections from related collections on objects, is a common use case in many applications. A simple example is if we want to fetch all authors of posts from a given category. Previously, if we wanted to extract such associated records from a relation we would make use of preload and collect.

This usage of preload(:author).collect(&:author) can get repetitive and does not describe the intent of usage properly.

#### extract_associated method

Relation#extract_associated has now been introduced as a shorthand for preload and collect. It extracts a named association from the relation. The named association is first preloaded, then the individual association records are collected from the relation.

extract_associated returns collection of associated collections in case of belongs_to and has_many associations.