inverse_of が必須なケース

inverse_of.rb

class User < ApplicationRecord
  has_one :post
end

class Post < ApplicationRecord
  belongs_to :fuga, class_name: 'User'
end

u = User.new
post = u.build_post
post.valid? #=> false
post.errors #=> #<ActiveModel::Errors:0x007f9b5caf2400 @base=#<Post id: nil, user_id: nil, title: nil, body: nil, created_at: nil, updated_at: nil>, @messages={:fuga=>["must exist"]}, @details={:fuga=>[{:error=>:blank}]}>
post.fuga #=> nil

# 次のようにすると valid? が true になる
class User < ApplicationRecord
  has_one :post, inverse_of: :fuga 
end