Cannot serialize an object of class ActionDispatch::Http::UploadedFile into BSON

Question

I'm following GridFS with Mongoid and CarrierWave to implement a simple has_many polymorphic relationship and when I attempt to create a new user, with an avatar, through nested attribute assignment, I get:

Cannot serialize an object of class ActionDispatch::Http::UploadedFile into BSON

Has anyone else encountered this? I noticed a few individuals posted replies to the "GridFS with Mongoid and CarrierWave" article but I was unable to find anyone with an answer.

# app/models/asset.rb
class Asset
    include Mongoid::Document
    include Mongoid::Timestamps

    mount_uploader :file, AssetUploader

    field :name, type: String

    referenced_in :attachable, polymorphic: true
end

# app/models/user.rb
class User
    include Mongoid::Document
    include Mongoid::Timestamps

    references_one :avatar, as: :attachable

    accepts_nested_attributes :avatar
end

# config/initializers/carrierwave.rb
CarrierWave.configure do |config|
    config.grid_fs_connection = Mongoid.database
    config.storage = :grid_fs
    config.grid_fs_access_url = "/images"
end

# app/uploaders/asset_uploader.rb
class AssetUploader < CarrierWave::Uploader::Base  
end

# app/views/users/new.html.haml
= semantic_form_for(@user, html: { multipart: true }) do |f|
    = f.inputs do
    = f.semantic_fields_for :avatar do |af|
        = af.input :file, as: :file
    = f.buttons do
        = f.commit_button "Upload"

Answer 1

It's all about naming the fields.

Had the same issue with mongo_mapper and a field I have renamed on the form.

The form fields have to have the same names of the fields in you model.