TL;DR has_one 関連のモデルに対しては build は使えないので build_xxx を使う。 実例 Rails 4.2にて [1] pry(main)> shop = Shop.last [2] pry(main)> shop.products.build => #<Product:0x007fbc9a1468e8 id: nil, name: nil, price: nil, created_at: nil, updated_at: nil> [3] pry(main)> shop.owner.build NoMethodError: undefined method 'build' for nil:NilClass [4] pry(main)> shop.build_owner => #<Owner:0x007fbc7a8da840 id: nil, name: nil