class Thing
  include Mongoid::Document
  field :bar, type: String
end

class Stuff
  include Mongoid::Document
  field :b, type: String, as: :bar
end

thing = Thing.create!(bar: 'foo')
thing.bar
thing[:bar]
thing['bar']
thing.read_attribute('bar')
thing.read_attribute(:bar)
thing.send('bar')

stuff = Stuff.create!(bar: 'foo')
stuff.bar
stuff[:bar]                       # <---- doesn't work
stuff['bar']                      # <---- doesn't work
stuff.read_attribute('bar')       # <---- doesn't work
stuff.read_attribute(:bar)        # <---- doesn't work
stuff.send('bar')

irb(main):042:0> thing.bar
=> "foo"
irb(main):043:0> thing[:bar]
=> "foo"
irb(main):044:0> thing['bar']
=> "foo"
irb(main):045:0> thing.read_attribute('bar')
=> "foo"
irb(main):046:0> thing.read_attribute(:bar)
=> "foo"
irb(main):047:0> thing.send('bar')
=> "foo"

irb(main):049:0> stuff.bar
=> "foo"
irb(main):050:0> stuff[:bar]
=> nil
irb(main):051:0> stuff['bar']
=> nil
irb(main):052:0> stuff.read_attribute('bar')
=> nil
irb(main):053:0> stuff.read_attribute(:bar)
=> nil
irb(main):054:0> stuff.send('bar')
=> "foo"
irb(main):055:0> stuff.read_attribute(:b)
=> "foo"
irb(main):056:0> stuff.read_attribute('b')
=> "foo"
irb(main):057:0> stuff['b']
=> "foo"
irb(main):058:0> stuff[:b]
=> "foo"