https://ruby-doc.org/core-2.2.3/Enumerable.html#method-i-inject

  inject -  is that as you loop through each element of your enumerable object, a second accumulator value is passed in as well.
  The accumulator value is updated on each iteration and eventually returned.

  (1..5).inject(0){ |sum, num| sum + num } ==> returns 15
  This example simply keeps track of a sum variable and adds each number in the array to it and then returns the sum at the end.


  We are passing in an empty hash as the initial value for our accumulator.
  We are calling the merge method on our accumulator hash each iteration.
  This method returns an updated hash as our new accumulator.

  users = User.all
  users.inject({}) do |memo, user|
    memo.merge(user.email => user.email)
  end

  => {"schamane.vzl@gmail.com"=>"schamane.vzl@gmail.com",
   "jannet@gmail.com"=>"jannet@gmail.com",
   "supersimple2@mail.ru"=>"supersimple2@mail.ru",
   "schamane2.vzl@gmail.com"=>"schamane2.vzl@gmail.com",
   "schamane.vzl1@gmail.com"=>"schamane.vzl1@gmail.com",
   "admin@example.com"=>"admin@example.com"}

In inject, the order of the arguments are |accumulator, element|

https://ruby-doc.org/core-2.2.3/Enumerable.html#method-i-each_with_object

  users = User.all
  users.each_with_object({}) do |user, memo|
    memo[user.email] = user.email
  end

  => {"schamane.vzl@gmail.com"=>"schamane.vzl@gmail.com",
   "jannet@gmail.com"=>"jannet@gmail.com",
   "supersimple2@mail.ru"=>"supersimple2@mail.ru",
   "schamane2.vzl@gmail.com"=>"schamane2.vzl@gmail.com",
   "schamane.vzl1@gmail.com"=>"schamane.vzl1@gmail.com",
   "admin@example.com"=>"admin@example.com"}

The each_with_object method ignores the return value of the block and only passes the initial accumulator object along to the next iteration.
This is why we don’t need to use the merge method here.

in each_with_object the order of the arguments are |element, accumulator|

https://medium.com/@Fdel15/ruby-each-with-object-or-inject-a737bb90cdd8