# Linking two belongs\_to associations through a common class

**URL:** <https://www.ruby-forum.com/t/linking-two-belongs-to-associations-through-a-common-class/180216>\
**Category:** Rails\
**Created:** [December 15, 2009, 1:09am UTC](https://www.ruby-forum.com/t/linking-two-belongs-to-associations-through-a-common-class/180216 "2009-12-15T01:09:46Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![toddq](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/977dab/32.png) [@toddq](https://www.ruby-forum.com/u/toddq)\
**Post date:** [December 15, 2009, 1:09am UTC](https://www.ruby-forum.com/t/linking-two-belongs-to-associations-through-a-common-class/180216/1 "2009-12-15T01:09:46Z")

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I’m new and I just trying to figure out something about associations.  
The following ruby code works but does not work in my actual rails  
application. Am I doing something wrong or is there a reason why I  
need to always use an extra table for a has\_many\_and\_belongs\_to  
relationship? When I try this is Rails its willing to do a part of  
this  
(e.g, e.department) but not the last ‘.customers’ part? I could of  
course just to x = e.department and then do x.customers (which does  
work in Rails) but why do I need to do this in two steps instead of  
one? I can see that for large datasets why the additional table would  
help but if the dataset is small why do you need the extra table/  
steps?

require ‘rubygems’;require ‘active\_record’; require ‘mysql’;  
ActiveRecord::Base.establish\_connection(:adapter =\>  
‘mysql’, :database =\> “idea”, :username =\> “myname”, :password =\>  
‘mypswd’)

class Employee \< ActiveRecord::Base  
belongs\_to :department  
end  
class Department \< ActiveRecord::Base  
has\_many :departments  
has\_many :comments  
end  
class Customers \< ActiveRecord::Base  
belongs\_to :department  
end  
e = Employee.find(1)  
#to find the number of customers associated with a particular employee  
within a department  
p e.department.customers.size

---

<div class="post-metadata">

**Author:** ![toddq](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/977dab/32.png) [@toddq](https://www.ruby-forum.com/u/toddq)\
**Post date:** [December 15, 2009, 12:57pm UTC](https://www.ruby-forum.com/t/linking-two-belongs-to-associations-through-a-common-class/180216/2 "2009-12-15T12:57:39Z")

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Not sure if you missed something out in your code above but you’ll need  
the has\_many :customers in the Department class to do this e.g.

class Department \< ActiveRecord::Base  
has\_many :customers  
end

Then you can do:  
e = Employee.find(1)  
p e.department.customers.size

Don’t forget to add has\_many :employees too if you want to do the  
reverse (i.e. cust.department.employees)

Cheers  
Luke

---

<div class="post-metadata">

**Author:** ![toddq](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/977dab/32.png) [@toddq](https://www.ruby-forum.com/u/toddq)\
**Post date:** [December 15, 2009, 6:27pm UTC](https://www.ruby-forum.com/t/linking-two-belongs-to-associations-through-a-common-class/180216/3 "2009-12-15T18:27:53Z")

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Luke P. wrote:

> Not sure if you missed something out in your code above but you’ll need  
> the has\_many :customers in the Department class to do this e.g.
> 
> class Department \< ActiveRecord::Base  
> has\_many :customers  
> end
> 
> Then you can do:  
> e = Employee.find(1)  
> p e.department.customers.size
> 
> Don’t forget to add has\_many :employees too if you want to do the  
> reverse (i.e. cust.department.employees)
> 
> Cheers  
> Luke

No, that was just a typo. The code works if you replace :comments with  
customers and of course class Customers \< ActiveRecord::Base should be  
class Customer \< ActiveRecord::Base

This does work in Rails for the following reason.class Customers \<  
ActiveRecord::Base and to my great shame, I’d actually  
read this article  
[http://blog.hasmanythrough.com/2008/2/27/count-length-size](http://blog.hasmanythrough.com/2008/2/27/count-length-size)

In rails, you need to use .count and not .size for this to work  
because the first collection has already been loaded and therefore you  
are asking for the length and not the count! The length == 0 and so  
no records are returned. Problem solved!