In case the Intervention model is only used to facilitate creating many Uploads (with one uploaded file each) at one time, there is a simpler way without an intermediate model.

My Document model has has_one_attached :file. I want to create multiple documents with one form which uploads multiple attachments. The controller specifies an array parameter without a corresponding db field:

params.require(:document).permit(..., multiple_files: [])

Note that this array parameter has to be the last item in the strong param list, otherwise you get syntax error.

The new-document form has f.file_field(:multiple_files, multiple: true).

The Documents#create action has:

def create
  params[:document][:multiple_files].each do |upload|
    next unless upload.present?

    @document = Document.new
    @document.file.attach(upload)
    # check for errors
    @document.valid?
    flash.now[:danger] = @document.errors.full_messages
    # save, etc.
  end
end