The validates_inclusion_of is not using the coerced value. Is this intentional? Here is an example:

require 'mongoid'

class Book
  include Mongoid::Document
  field :genre, type: Symbol
  validates_inclusion_of :genre, in: [:drama, :adventure]
end

b = Book.new

b.genre = :drama
b.valid? # true

b.genre = 'drama'
b.valid? # false

Mongoid used to return true for the second case too. This code was tested in the newest gem version (currently 3.1.0), but I don't know exactly when this was changed.