gistfile1.rb

class User
  has_and_belongs_to_many :elearning_requests
end

class ElearningRequest
  include Mongoid::Document
  include Mongoid::Timestamps::Short
  include Mongoid::Enum

  # ----------------------------------------------------------------------------
  # Fields ---------------------------------------------------------------------
  # ----------------------------------------------------------------------------

  field :title,       type: String, localize: true
  field :description, type: String, localize: true
  enum  :status,      [:pending, :approved, :declined], default: :pending

  # ----------------------------------------------------------------------------
  # Relations ------------------------------------------------------------------
  # ----------------------------------------------------------------------------

  belongs_to :user
  has_and_belongs_to_many :documents
end


class Document
  include Mongoid::Document
  include Mongoid::Timestamps::Short

  mount_uploader :file, DocumentUploader

  belongs_to :elearning_request

end

class DocumentUploader < CarrierWave::Uploader::Base

  def store_dir
    "/elearning_request/documents/#{model.id}"
  end

  def extension_white_list
    %w(pdf)
  end


end

# dans la forme 
#   = form_for [:account, elearning_request] do |f|
# ...
#                   = f.fields_for :documents do |fo|
#                           = fo.file_field :file, name: 'elearning_request[documents][]'

# Et enfin les logs du serveur: 

17:37:02 web.1  | Processing by Account::ElearningRequestsController#create as HTML
17:37:02 web.1  |   Parameters: {"utf8"=>"✓", "authenticity_token"=>"uNIaFg3x+UJ4oYLPQrMGb276WpQa8InST7swv8O1tk4=", "elearning_request"=>{"title_translations"=>{"fr"=>"dfsfds", "en"=>"fdsfds"}, "description_translations"=>{"fr"=>"fdsfds", "en"=>"fdsfds"}, "documents"=>[#<ActionDispatch::Http::UploadedFile:0x007f7f78f7ca20 @tempfile=#<Tempfile:/tmp/RackMultipart20141029-29535-zfijfh>, @original_filename="CraftingRails4App.pdf", @content_type="application/pdf", @headers="Content-Disposition: form-data; name=\"elearning_request[documents][]\"; filename=\"CraftingRails4App.pdf\"\r\nContent-Type: application/pdf\r\n">]}, "locale"=>"fr"}
17:37:02 web.1  |   MOPED: 127.0.0.1:27017 COMMAND      database=admin command={:ismaster=>1} runtime: 0.4398ms
17:37:02 web.1  |   MOPED: 127.0.0.1:27017 QUERY        database=travel_development collection=users selector={"$query"=>{"_id"=>BSON::ObjectId('5450e84f63626d02a1040000')}, "$orderby"=>{:_id=>1}} flags=[] limit=-1 skip=0 batch_size=nil fields=nil runtime: 0.4677ms
17:37:02 web.1  | Document ... #<ActionDispatch::Http::UploadedFile:0x007f7f78f7ca20>
17:37:02 web.1  |   MOPED: 127.0.0.1:27017 UPDATE       database=travel_development collection=documents selector={"$and"=>[{"_id"=>{"$in"=>[]}}]} update={"$pull"=>{"elearning_request_ids"=>BSON::ObjectId('545117ae63626d735f140000')}} flags=[:multi]
17:37:02 web.1  |                          COMMAND      database=travel_development command={:getlasterror=>1, :w=>1} runtime: 0.7122ms
17:37:02 web.1  | Completed 500 Internal Server Error in 17ms
17:37:02 web.1  | 
17:37:02 web.1  | NoMethodError - undefined method `id' for #<ActionDispatch::Http::UploadedFile:0x007f7f78f7ca20>:
17:37:02 web.1  |   mongoid (4.0.0) lib/mongoid/relations/targets/enumerable.rb:62:in `<<'
17:37:02 web.1  |   mongoid (4.0.0) lib/mongoid/relations/referenced/many_to_many.rb:209:in `append'

C'est réglé ça ?