# Question about has\_one **URL:** \ **Category:** Rails\ **Created:** [September 27, 2006, 8:35pm UTC](https://www.ruby-forum.com/t/question-about-has-one/70263 "2006-09-27T20:35:10Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![nelson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/dec6dc/32.png) [@nelson](https://www.ruby-forum.com/u/nelson)\ **Post date:** [September 27, 2006, 8:35pm UTC](https://www.ruby-forum.com/t/question-about-has-one/70263/1 "2006-09-27T20:35:10Z")
I have a question about regarding the use of ‘has\_one’ in this scenario: Schema: ## User id first\_name last\_name ## UserMessage user\_id message\_id ## Message id Assuming business rules dictate that a Message can have at most ONE User (let’s assume a message can be created without a user associated to it), these are my questions: 1. How would I use has\_one in my model (if that’s the proper association to use) to indicate that a message can have at most one User (without having a nullable user\_id column in Message)? BTW, I chose to have the UserMessage mapping in its own table because if I had user\_id in the Message table, I would be forced to use the NULL value to indicate that a Message has no User associated with it, and if there are tons of Messages with no User, I cringe at the amount of NULL values. (But if my logic is wrong, please let me know.) 1. I know in Rails that a ‘has\_one’ association means that if a Message has\_one User, then Rails expects a ‘message\_id’ in the User table. Is the schema even properly designed for Rails as there is a UserMessage table that contains the mappings, and I would tend to want to use ‘:through’, but that option is not available in a has\_one relationship. Of course, if the correct way is indeed to use a nullable user\_id column, then this question is moot, and has\_one/belongs\_to is probably the way to go. Thanks in advance! Nelson ---
**Author:** ![nelson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/dec6dc/32.png) [@nelson](https://www.ruby-forum.com/u/nelson)\ **Post date:** [September 27, 2006, 9:05pm UTC](https://www.ruby-forum.com/t/question-about-has-one/70263/2 "2006-09-27T21:05:51Z")
> ## UserMessage > > user\_id > message\_id > > ## Message > > id > I’m not sure that it’s ‘has\_one’ that you need here. ‘has\_one’ implies a > one-to-one relationship - like Order -\> Invoice. Also, I may be wrong, but i don’t think you need that joiner table. The joiner table suggests a many-to-many relationship: a message can belong to many users, a user can have many messages. I think what you’re looking for is a one-to-many relationship: A user can have many messages, a message belongs to a user. ## User id first\_name last\_name ## Message id user\_id This is wired up in the models like this: class User \< ActiveRecord::Base has\_many :messages end class Message \< ActiveRecord::Base belongs\_to :user end Hope that helps, Steve ---
**Author:** ![nelson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/dec6dc/32.png) [@nelson](https://www.ruby-forum.com/u/nelson)\ **Post date:** [September 27, 2006, 9:14pm UTC](https://www.ruby-forum.com/t/question-about-has-one/70263/3 "2006-09-27T21:14:18Z")
Thanks for your quick response. The way that you wired it up makes sense. However, this means that the Message table must have a user\_id column, and if I want to indicate that a Message has no User, then user\_id must be NULL. And that would mean that if I had a lot of Messages with no User, then I’d have store quite a bit of NULL values, whereas using a joiner table, I wouldn’t. Generally, I like to avoid NULLs - is there a way to design the schema/model so that I can use the joiner table? If not, I guess I’ll settle and use a nullable user\_id column in Message. Thanks, Nelson ---
**Author:** ![nelson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/dec6dc/32.png) [@nelson](https://www.ruby-forum.com/u/nelson)\ **Post date:** [September 27, 2006, 9:57pm UTC](https://www.ruby-forum.com/t/question-about-has-one/70263/4 "2006-09-27T21:57:29Z")
Nelson H. wrote: > However, this means that the Message table must have a user\_id > column, and if I want to indicate that a Message has no User, then > user\_id must be NULL. Why would you have a message created without a user? Who owns the message at that point? ---
**Author:** ![nelson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/dec6dc/32.png) [@nelson](https://www.ruby-forum.com/u/nelson)\ **Post date:** [September 27, 2006, 10:08pm UTC](https://www.ruby-forum.com/t/question-about-has-one/70263/5 "2006-09-27T22:08:02Z")
> Thanks for your quick response. The way that you wired it up makes > sense. However, this means that the Message table must have a user\_id > column, and if I want to indicate that a Message has no User, then > user\_id must be NULL. And that would mean that if I had a lot of > Messages with no User, then I’d have store quite a bit of NULL values, > whereas using a joiner table, I wouldn’t. Generally, I like to avoid > NULLs - is there a way to design the schema/model so that I can use the > joiner table? If not, I guess I’ll settle and use a nullable user\_id > column in Message. > I’m not sure why you would want a message to not have a user, but I > guess that depends on the requirements of your application. Personally, I would favour NULLs over a joiner table in this instance - but I’d also question the relationship between User and Message if you’re planning on having messages with no associated user. Steve ---
**Author:** ![nelson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/dec6dc/32.png) [@nelson](https://www.ruby-forum.com/u/nelson)\ **Post date:** [September 27, 2006, 11:28pm UTC](https://www.ruby-forum.com/t/question-about-has-one/70263/6 "2006-09-27T23:28:47Z")
Stephen B. wrote: > I’m not sure why you would want a message to not have a user, but I > guess that depends on the requirements of your application. > > Personally, I would favour NULLs over a joiner table in this instance - > but I’d also question the relationship between User and Message if > you’re planning on having messages with no associated user. > > Steve I apologize because Message and User isn’t a good example, but I guess I’m trying to model a zero\_to\_many relationship using a joiner table rather than a nullable column.