form.rb

# Use an instance for the top-level form_for
form_for @parent do |f|

  # Always use a symbol to access the nested association from f.object
  f.fields_for :child do |fc|
    # generates: params[:parent][:child_attributes][:field]
  end

  # Optionally, provide an instance as a *second* argument
  f.fields_for :relative, (@other_parent or Person.new) do |fr|
    # generates: params[:parent][:relative_attributes][:field]
  end

  # Not like this:
  f.fields_for @child do |fc|
    # generates: params[:parent][:child][:field]
    # error on nested attribute assignment in controller:
    # Child(#2172589160) expected, got HashWithIndifferentAccess(#2156610500)
  end
end

parent.rb

class Person < ActiveRecord::Base
  has_one :child
  has_one :relative
  accepts_nested_attributes_for :child, :relative
end