_form.html.erb

<%= f.fields_for :urls do |url_fields| %>
    <div class="field">
       <%= f.label :url %><br>
      <%= url_fields.text_field :url %>
    </div>
  <% end %> 

DOI controller

def new
    @digital_object_identifier = current_user.digital_object_identifiers.build
    @digital_object_identifier.urls << Url.new

  end

error line 2

"urls_attr
ibutes"=>{"0"=>{"url"=>"www.something.com"}}}, "commit"=>"Create Digital object
identifier"}
  ←[1m←[35mUser Load (1.0ms)←[0m  SELECT "users".* FROM "users" WHERE "users"."i
d" = 2 ORDER BY "users"."id" ASC LIMIT 1
Unpermitted parameters: urls_attributes

gistfile1.txt

params.require(:digital_object_identifier).permit(:string_identifier, :integer_identifier, :description, url_attributes: [ :url ] )