Zero or one association in ActiveRecord

Question

I am writing a Ruby on Rails application that has two models - User and Farm. A User is considered a farmer if their farmer field is set to true. However, there is no seperate class for farmers.

A User may have either one farm, or none at all. (I believe this is called a zero or one relationship). If I put:

has_one :farm

in the User model and

belongs_to :user

in the Farm model, this would create a one-to-one relationship between Users and Farms and mean that every User has a Farm. If I did the above, every User would have a Farm, and that would not make much sense since there are certain Users who cannot have a Farm.

In short, what I want is for a User to have a Farm only if their farmer boolean is set to true. Otherwise, the relationship shouldn't exist. Is their a way to do this using ActiveRecord the way it is meant to be used?

you could use validations ? http://guides.rubyonrails.org/active_record_validations_callbacks.html#validations-overview — MrYoshiji, Nov 13 '12 at 21:46
So I should create the farm as a regular field and not using any of the association helpers? And then create a farm only if the validation "farmer == true" passes? — Aaltan Ahmad, Nov 13 '12 at 21:51
Using an association is fine. It's not because you declare it that you have to fill it. It can be empty — Anthony Alberto, Nov 13 '12 at 21:57
@AnthonyAlberto will it impact the size/performance of the db if we have lots of Users with nil Farm associations? — Aaltan Ahmad, Nov 13 '12 at 22:02
Nope, there wont be an empty entry on the Farms table for each user who isn't a farmer, so its ok to do it that way. — ichigolas, Nov 14 '12 at 00:35
Will be an empty farm entry for every user in the User table? — Aaltan Ahmad, Nov 14 '12 at 04:43
Yeah but a null entry in SQL is like peanuts ... don't worry about it unless you have a lot of them — Anthony Alberto, Nov 14 '12 at 13:29
possible duplicate of [Can has\_one association be used when the model has one or zero instances of another model?](http://stackoverflow.com/questions/19005296/can-has-one-association-be-used-when-the-model-has-one-or-zero-instances-of-anot) — Eric Platon, Apr 11 '14 at 05:21

score 15 · Answer 1 · edited May 23 '17 at 11:53

15

has_one doesn't mean that you must have one related entity(here farm). has_one is used for relation where we have 0 or 1 linked records.

You may find the similar discussion here.

Can has_one association be used when the model has one or zero instances of another model?

edited May 23 '17 at 11:53

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answered Mar 21 '14 at 15:10

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Zero or one association in ActiveRecord

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