# Beginner Problems with many-to-many Relations

**URL:** <https://www.ruby-forum.com/t/beginner-problems-with-many-to-many-relations/86981>\
**Category:** Rails\
**Created:** [February 27, 2007, 11:04am UTC](https://www.ruby-forum.com/t/beginner-problems-with-many-to-many-relations/86981 "2007-02-27T11:04:22Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Pietropizzi](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/35a633/32.png) [@Pietropizzi](https://www.ruby-forum.com/u/Pietropizzi)\
**Post date:** [February 27, 2007, 11:04am UTC](https://www.ruby-forum.com/t/beginner-problems-with-many-to-many-relations/86981/1 "2007-02-27T11:04:22Z")

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Hello people!

I’m quite new to Rails and I’ve got a project to do that requires some  
many-to-many relations right at the beginning. I’ll describe what I  
want to do, maybe you could tell me how to realize that.

I have the tables »people« and »courses«. One course can have multiple  
people (I’m talking of teachers here) and one person can be in  
multiple courses. As I started creating my application in the RESTful  
way and I learned that the »has\_and\_belongs\_to\_many« association  
doesn’t work very well with the REST-approach, I created a third table  
and model called »teachers« that is the join model between those two,  
so the models look like this:

class Course \< ActiveRecord::Base  
has\_many :teachers  
has\_many :people, :through =\> :teachers  
end

class Person \< ActiveRecord::Base  
has\_many :teachers  
has\_many :courses, :through =\> :teachers  
end

class Teacher \< ActiveRecord::Base  
belongs\_to :course  
belongs\_to :person  
end

Now following the RESTful way I should add teachers only through my  
»TeachersController«, right? Now what I’d like to do is the following.

When you are creating a new course, then on the same page with the  
course form (so /courses/new ) there should be the possibility to add  
people as teachers to the course. Then if you edit this course  
afterwords it should show you the teachers of this course and make it  
possible to get them out of this course and add new ones to the  
course. But how can I have that on the same page as the courses? How  
do I create a new teacher on the same page where the new or edit form  
of a specific course is?

I have really no clue. I mean I don’t insist on having the »Add  
Teacher« thing on the new course page, but what I don’t want, is to  
put it on an own page only for adding a teacher.

Would be very happy about some answers. 🙂

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<div class="post-metadata">

**Author:** ![Pietropizzi](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/35a633/32.png) [@Pietropizzi](https://www.ruby-forum.com/u/Pietropizzi)\
**Post date:** [February 27, 2007, 4:22pm UTC](https://www.ruby-forum.com/t/beginner-problems-with-many-to-many-relations/86981/2 "2007-02-27T16:22:09Z")

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It is very simple: REST is simply a guide to a good way to structure  
your  
program (especially useful for machine to machine communication)… I  
would  
suggest you go ahead and create the TeachersController and allow  
teachers to  
be added/edited/etc from that controller, because at some point you may  
want the ability to modify the Teachers stuff directly: if you add any  
extra  
properties to the Teachers association beyond just being a cross  
reference  
for the course - person relationship.

However, I agree that adding a teacher spot to your course controller  
seems  
like a reasonable idea, and it is very easy… although, you should  
realize,  
what you really want to do is add a Person to your Course, which you’re  
doing through your Teacher association. So what you might do is add a  
couple of text fields to your form where you could enter a Person’s  
name,  
then do something like this in the action you are submitting to:

c = Course.create(params[:course])  
c.people \<\< Person.find\_by\_name(params[:teacher\_name])  
c.save

This will automatically create the Teacher association for you. 🙂

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<div class="post-metadata">

**Author:** ![Pietropizzi](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/35a633/32.png) [@Pietropizzi](https://www.ruby-forum.com/u/Pietropizzi)\
**Post date:** [February 27, 2007, 4:50pm UTC](https://www.ruby-forum.com/t/beginner-problems-with-many-to-many-relations/86981/3 "2007-02-27T16:50:16Z")

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Your model has not captured the concepts in the domain properly.  
Student can have many teachers. Teacher can have many students.  
Each link that connects the Student to Teacher is through a Course.  
Course breaks up the many to many relationship.

You will know that your design is right when you have two entities  
that is  
connected by an association descriptive table. For instance, you will  
have Subscription  
for the entities People and Magazine, InsurancePolicy for People and  
Insurance,  
Marriage for Man and Woman and so on. This is due to the fact that we  
are  
promoting an association class to a regular class. An association  
class describes  
the relationship between two entities. It holds information that does  
not belong to  
either of the entity that it connects. So InsurancePolicy will have  
renewal date and  
this date does not logically belong to Person or Insurance. The data  
comes alive only  
when the connection between the entitities is established.

Remember that habm relationship is sufficient if your domain requires  
only a simple  
join table. You have to pick the right solution for the problem. If  
you need to capture more  
than just the foreign keys in the join table then you need to  
use :through style declaration.

Yes, there is almost one-to-one correspondence between your model  
classes and controllers.

Now you got a better idea of how to design your object model, you can  
go back and think  
about to represent them in your views.