center.rb

class Center < ActiveRecord::Base
  belongs_to :thing
  validates :thing_id, :presence => true
end

thing.rb

class Thing < ActiveRecord::Base
  has_many :centers
  has_one  :default_center, :class_name => 'Center', :conditions => ['centers.default = ?', true]
  accepts_nested_attributes_for :default_center
end

thing_spec.rb

describe "default_centers" do
  fit "should create a default center with nested attributes" do
    params = HashWithIndifferentAccess.new({
      "thing"=>{
        "default_center_attributes"=>{
          "name"=>"Very very good sample name",
          "default" => true
         }
       })
    thing = Thing.create!(params[:thing])
    thing.should be_valid
  end
end