1.9.3p125 :010 > p = Elite::Post.includes(:author)
=> #<Mongoid::Criteria
  selector: {},
  options: {:sort=>{"created_at"=>1}},
  class: Elite::Post,
  embedded: false>

1.9.3p125 :011 > p.map(&:inspect)
=> ["#<Elite::Post _id: 500533339e3e720063000002, _type: nil, created_at: 2012-07-17 09:41:07 UTC, updated_at: 2012-07-17 09:41:07 UTC, title: \"hello\", short: \"wassup\", full: \"\", keywords: \"\", description: \"\", user_id: \"4fc9fb1cbd7f1602ca000001\">"]
1.9.3p125 :012 > p.map(&:inspect)
=> ["#<Elite::Post _id: 500533339e3e720063000002, _type: nil, created_at: nil, updated_at: nil, title: \"\", short: nil, full: \"\", keywords: nil, description: nil, user_id: \"4fc9fb1cbd7f1602ca000001\">"]
1.9.3p125 :013 > p.any?
=> true
1.9.3p125 :014 > p = Elite::Post.includes(:author)
=> #<Mongoid::Criteria
  selector: {},
  options: {:sort=>{"created_at"=>1}},
  class: Elite::Post,
  embedded: false>

1.9.3p125 :015 > p.any?
=> true
1.9.3p125 :016 > p.map(&:inspect)
=> ["#<Elite::Post _id: 500533339e3e720063000002, _type: nil, created_at: nil, updated_at: nil, title: \"\", short: nil, full: \"\", keywords: nil, description: nil, user_id: \"4fc9fb1cbd7f1602ca000001\">"]
1.9.3p125 :017 > p = Elite::Post.all
=> #<Mongoid::Criteria
  selector: {},
  options: {:sort=>{"created_at"=>1}},
  class: Elite::Post,
  embedded: false>

1.9.3p125 :018 > p.any?
=> true
1.9.3p125 :019 > p.map(&:inspect)
=> ["#<Elite::Post _id: 500533339e3e720063000002, _type: nil, created_at: 2012-07-17 09:41:07 UTC, updated_at: 2012-07-17 09:41:07 UTC, title: \"hello\", short: \"wassup\", full: \"\", keywords: \"\", description: \"\", user_id: \"4fc9fb1cbd7f1602ca000001\">"]
1.9.3p125 :020 > p.map(&:inspect)
=> ["#<Elite::Post _id: 500533339e3e720063000002, _type: nil, created_at: 2012-07-17 09:41:07 UTC, updated_at: 2012-07-17 09:41:07 UTC, title: \"hello\", short: \"wassup\", full: \"\", keywords: \"\", description: \"\", user_id: \"4fc9fb1cbd7f1602ca000001\">"]
1.9.3p125 :021 >

When we get collection with includes method and use like array (any?, map etc.) document fields are set to nil but only when field not recognized in .includes(:author) method.

And the issue of Case sensitive validation still not solved.