Rails: Using build with a has_one association in rails
The build method signature is different for has_one and has_many associations. class User < ActiveRecord::Base has_one :profile has_many :messages end The build syntax for has_many association: user.messages.build The build syntax for has_one association: user.build_profile # this will work user.profile.build # this will throw error Read the has_one association documentation for more details.