I don’t know why it is not mentioned any where in rails api, it could be very useful for all.

This is straight from rails\activesupport\lib\active_support\core_ext\module\deligation.rb :

Provides a delegate class method to easily expose contained objects’ methods
as your own. Pass one or more methods (specified as symbols or strings)
and the name of the target object as the final :to option (also a symbol
or string). At least one method and the :to option are required.

Delegation is particularly useful with Active Record associations:

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