Problem:

You're getting something that says "ArgumentError: Unknown key(s): through" when you're using a has_one :something, :through => :something else code.

Explanation:

The :through option for Rails join models is really, really handy. Basically, it allows you to turn guy.thing.places into guy.places without having to change anything in the database.

How the models would be setup to allow for the more tedious way to associate places with a guy.

class Guy < ActiveRecord::Base
  has_one :thing
end


class Thing < ActiveRecord::Base
  belongs_to :guy
  has_many   :places
end

class Place < ActiveRecord::Base
  belongs_to :thing
end

The only alteration needed is turning the Guy class into:

class Guy < ActiveRecord::Base
  has_one  :thing
  has_many :places, :through => :thing
end

For a better example of how to use :through check out [Ariejan on Rails](http://ariejan.nl/2006/05/28/associative-join-model/) The problem with this, and I don't know why this is, is that this doesn't work with has\_one - only has\_many! If we were to change Thing to only have one Place we would then alter the joins thusly:

class Guy < ActiveRecord::Base
  has_one  :thing
  has_one  :place, :through => :thing
end

class Thing < ActiveRecord::Base
  belongs_to :guy
  has_one    :place
end

And our code would fail! The above code will barf out an `ArgumentError: Unknown key(s): through` error. #### Solution: You can use a has\_many join and make it act like a has_one join. Just use:

class Guy < ActiveRecord::Base
  has_one  :thing
  has_many :place, :through => :thing
end

Then, when you call guy.place you'll get an array (with just one element) of places. You can use `guy.place.first` to replace `guy.thing.place`

• bob said: er... guy.place.first isn't any better than guy.thing.place. clever. but not a solution.
• Danger said: Totally :-) I wrote this and immediately realized that there's a reason nobody ever wrote has_one :through - 'cause there's no need! You can just use the relationship chain to get what you want or define a shortcut in your model. But if somebody really wanted to use has_many :through (for some ridiculous purpose) they still could - they'd just get an array. I think I'll leave this assinine post up just to remind myself not to write stupid shit.

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