fields_for :profile_or_build

build_for_has_one

module Extensions
  module ActiveRecord
    module BuildForHasOne
      module InstanceMethods
        def self.included(base)
          base.send :alias_method_chain, :method_missing, :build
        end
      private
        def method_missing_with_build(method, *args, &block)
          relation_name = method.to_s.scan(/(\w+)_or_build$/).flatten.first
          return method_missing_without_build(method, *args, &block) unless relation_name.present? && respond_to?(relation_name.to_sym)

          reflection = self.class.reflect_on_association(relation_name.to_sym)
          unless reflection.collection?
            self.class_eval do
              name = reflection.name
              define_method method do
                return send(name) || send(:"build_#{name}")
              end
            end

            send(method)
          end
        end
      end
    end
  end
end
ActiveRecord::Base.send :include, Extensions::ActiveRecord::BuildForHasOne::InstanceMethods

form_builder extension

module Extensions
  module ActionView
    module Helpers
      module FormBuilder
        def self.included(klass)
          klass.class_eval do
            def fields_for_with_association(record_name, record_object = nil, fields_options = {}, &block)
              if record_object.is_a?(Hash) && record_object.has_key?(:build_if_empty)
                fields_options, record_object = record_object, nil
                fields_options.delete(:build_if_empty)

                association_name = record_name.is_a?(Array) ? ActiveModel::Naming.param_key(record_name.last) : record_name
                record_object = @object.send("#{association_name}_or_build") unless record_object
              end

              fields_for_without_association(record_name, record_object, fields_options, &block)
            end
            alias_method_chain :fields_for, :association
          end
        end
      end
    end
  end
end
ActionView::Helpers::FormBuilder.send :include, Extensions::ActionView::Helpers::FormBuilder if defined?(ActionView)

the view

<%= f.fields_for :profile_or_build, {build_if_empty: true}  do |profile|%>
   <%= profile.label :name %>
   <%= profile.text_field :name %>
<% end %>