class User < ActiveRecord::Base
  has_many :posts, inverse_of: :user
end

class Post < ActiveRecord::Base
  belongs_to :user, inverse_of: :posts
  belongs_to :category
end

user = User.first

puts "without eager_loading:"
puts user.object_id == user.posts.first.user.object_id
puts ""

puts "with eager_loading:"
puts user.object_id == user.posts.eager_load(:category).first.user.object_id

without eager_loading:
true

with eager_loading:
true

without eager_loading:
true

with eager_loading:
false