When using fields_for to build DOM structure for nested models, the id hidden field gets emitted implicitly and the developer has no control over the placement which can result in invalid HTML. I''ve submitted the patch below which adds a method, #hidden_fields, to FormBuilder to explicitly place the hidden fields. If this methods gets called in the #fields_for block the FormBuilder won''t implicitly emit the hidden fields. https://rails.lighthouseapp.com/projects/8994/tickets/3259-allow-explicit-placement-of-hidden-fields-with-nested-models#ticket-3259-1