I updated mongoid to 3.0.0 from 2.4.9 and have a problem with validation, there's:

1.9.3p125 :040 > p = Elite::Post.create(

{title: "sample title", short: "short message"}

)
=> #<Elite::Post _id: 4ffe6d189e3e72c50d00000c, _type: nil, created_at: 2012-07-12 06:22:16 UTC, updated_at: 2012-07-12 06:22:16 UTC, title: "sample title", short: "short message">
1.9.3p125 :041 > p = Elite::Post.new(

{title: "Sample Title", short: "short message"}

)
=> #<Elite::Post _id: 4ffe6d209e3e72c50d00000d, _type: nil, title: "Sample Title", short: "short message">
1.9.3p125 :042 > p.valid?
=> true #must be false
1.9.3p125 :043 > p.save
=> true
1.9.3p125 :044 > Elite::Post.all.map(&:title)
=> ["sample title", "Sample Title"] # The posts a dublicated
1.9.3p125 :064 > p = Elite::Post.new(

{title: "Sample Title", short: "short message"}

)
=> #<Elite::Post _id: 4ffe6eab9e3e72c50d00000e, _type: nil, title: "Sample Title", short: "short message">
1.9.3p125 :065 > p.valid?
=> false
1.9.3p125 :066 > p.save
=> false

In the model I have:

validates :title,
presence: true,
uniqueness: true

or

validates_uniqueness_of :title