Rails - May 2006 - can I fake has_one :through?

I''m having a difficult time wrapping my head around a set of models  
and their associations and seeing how they map into Rails. I''ll try  
to lay out my problem using the has_many and belongs_to structures. I  
have a RuleSpace, a Rule, and a Subject.

A RuleSpace has_many Rules
A RulesSpace has_many Subjects through Rules

A Rule belongs_to (many) RuleSpaces
A Rule has_many Subjects

A Subject has_one Rule
A Subject has_one RuleSpace

So given a single Subject I should be able to find a single Rule and  
a single RuleSpace. Going the other way, given a RuleSpace I should  
find many Subjects.

I''m thinking the model definitions should look like this:

class RuleSpace < AR::Base
   has_many :rules
   has_many :subjects, :through => :rules
end

class Rule < AR::Base
   belongs_to :rule_space
   belongs_to :subject
end

class Subject < AR::Base
   has_one :rule
   has_many :rule_spaces, :through => :rules
end

I''m not sure at all that a call to Subject.rule_spaces will return  
only a single element. Will #has_one enforce this? Is this legal?

cr

On May 11, 2006, at 10:46 PM, cremes.devlist@mac.com wrote:
> I''m having a difficult time wrapping my head around a set of
models
> and their associations and seeing how they map into Rails. I''ll
try
> to lay out my problem using the has_many and belongs_to structures.  
> I have a RuleSpace, a Rule, and a Subject.
>
> A RuleSpace has_many Rules
> A RulesSpace has_many Subjects through Rules
>
> A Rule belongs_to (many) RuleSpaces
> A Rule has_many Subjects
>
> A Subject has_one Rule
> A Subject has_one RuleSpace
>
> So given a single Subject I should be able to find a single Rule  
> and a single RuleSpace. Going the other way, given a RuleSpace I  
> should find many Subjects.
>
> I''m thinking the model definitions should look like this:
>
> class RuleSpace < AR::Base
>   has_many :rules
>   has_many :subjects, :through => :rules
> end
>
> class Rule < AR::Base
>   belongs_to :rule_space
>   belongs_to :subject
> end
>
> class Subject < AR::Base
>   has_one :rule
>   has_many :rule_spaces, :through => :rules
> end
>
> I''m not sure at all that a call to Subject.rule_spaces will return
> only a single element. Will #has_one enforce this? Is this legal?
Answering my own question here... I continued to search the list  
archives and found a similar question on March 31 2006.

Josh Susser wrote:
> You can simulate has_one with a has_many with an :order and :limit  
> => 1
> (though the naming is still plural, not singular).
So I guess that means I change my Subject model to look like this:

class Subject < AR::Base
   has_many :rules
   has_many :rule_spaces, :through => :rules, :order => ASC, :limit =>
1
end

I''m going to code this up for real and run it through some tests to  
see what happens. I wonder what interesting ways it will break. :-)

cr