Self-referential :has_many :through relationship?

Rails
1

Hi All,

I’m building an app that needs to make a comparison between two people
and I’m wondering how to best build the models, and if I’m doing this
correctly.

I’m representing each partner as a User, so I wanted to keep just a user
table and a comparisons table (the comparisons table will keep all the
comparison data, the user model will hold all the data about that
individual user)

I’m thinking of making a User-to-User self-referential join through a
“comparison” model.

So, I’d have two models: the user model, and a comparison model

The user model would look something like this:
class User < ActiveRecord::Base
belongs_to :partner, :through => :comparisons
end

and the comparisons:

class Comparison < ActiveRecord::Base
belongs_to :user
has_one :partner, :class_name => “User”, :foreign_key => “partner_id”
end

I honestly have no idea if this will work… does this look close?

Thanks in advance!
Dustin

2

Dustin A. wrote:

I’m building an app that needs to make a comparison between two people
and I’m wondering how to best build the models, and if I’m doing this
correctly.

I’m representing each partner as a User, so I wanted to keep just a user
table and a comparisons table (the comparisons table will keep all the
comparison data, the user model will hold all the data about that
individual user)

I’m thinking of making a User-to-User self-referential join through a
“comparison” model.

So, I’d have two models: the user model, and a comparison model

The user model would look something like this:
class User < ActiveRecord::Base
belongs_to :partner, :through => :comparisons
end

and the comparisons:

class Comparison < ActiveRecord::Base
belongs_to :user
has_one :partner, :class_name => “User”, :foreign_key => “partner_id”
end

I honestly have no idea if this will work… does this look close?

Did you get this to work yet? I just posted an updated example of a
self-referential has_many :through on my blog:
http://blog.hasmanythrough.com/2007/10/30/self-referential-has-many-through
You might find that helpful.

If you really need an asymmetric join model, I’m interested in seeing
what you put together. I was thinking about blogging an example of that
too, but I couldn’t think up a good use for it.

–
Josh S.
http://blog.hasmanythrough.com

3

On 10/28/07, Dustin A. [email protected] wrote:

individual user)

I’m thinking of making a User-to-User self-referential join through a
“comparison” model.

So, I’d have two models: the user model, and a comparison model

The user model would look something like this:
class User < ActiveRecord::Base
belongs_to :partner, :through => :comparisons

:through is not a valid option on belongs_to, you can only use it on
:has_many

…

I honestly have no idea if this will work… does this look close?

Sorry, don’t think so.

Maybe with a little more insight on what you’re trying to do with the
comparison Model, someone can move you towards the goal.

Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/

4

Did you get this to work yet? I just posted an updated example of a
self-referential has_many :through on my blog:
has_many :through - Self-referential has_many :through associations
You might find that helpful.

If you really need an asymmetric join model, I’m interested in seeing
what you put together. I was thinking about blogging an example of that
too, but I couldn’t think up a good use for it.

–
Josh S.
http://blog.hasmanythrough.com

Well, what I’m trying to do is compare two people - through a photo…
I’m using attachment_fu. Here’s what I have at this point… I actually
did away with the User model all together and just made the comparisons
hold all the foreign keys:

Comparisons model:
belongs_to :male_partner, :polymorphic => true
belongs_to :female_partner, :polymorphic => true

(Fields for comparisons):
t.column “male_partner_id”, :integer
t.column “male_partner_type”, :string
t.column “female_partner_id”, :integer
t.column “female_partner_type”, :string
t.column “comparison_value”, :float

MalePartner
has_many :comparisons, :as => :male_partner

FemalePartner
has_many :comparisons, :as => :female_partner

That’s it, again, no idea if it will work… The reason I made the
male_partner and female_partner polymorphic is to be able to use an
image from the web instead of an uploaded image. This is for a dating
site.

Thanks for the blog Josh, I checked it out… very cool - a little more
that I need for this (I think), but nonetheless, it’s cool to see that
all those self-referential joins can be done.