Rails - Aug 2006 - More than one has_many :through association between the same 2 models

I wonder if you can have more than one has_many :through association between
2 models.

 

For example... 

 

I have a model Teacher and a model Class

 

Now, 1 Teacher works in many Classes, right?. So I need a join model like 

 

class Work < ActiveRecord::Base

 

  belongs_to :teacher

  belongs_to :class

 

end

 

But I also would like to know if a teacher CAN teach a class before I assign
him or her to one class. So I would like to have a new join model like
"CanTeach" :

 

class CanTeach< ActiveRecord::Base

 

  belongs_to :teacher

  belongs_to :class

 

end

 

The question is... can I do that? Because now the Teacher and Class Models
would look like...

 

 

class Teacher< ActiveRecord::Base

 

  has_many :works

  has_many :classes, :through => :works

 

  has_many :works

  has_many :classes, :through => :can_teach

 

end

 

class Class < ActiveRecord::Base

 

  has_many :works

  has_many :teachers, :through => :works

 

  has_many :works

  has_many :teachers, :through => :can_teach

 

end

 

but this doesn''t work.. 

 

Anyone knows how to do this?

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I would be very interested in an answer, too...

But I suggest you read this:
http://rubyonrails.org/api/classes/ActiveRecord/Associations/ClassMethods.html#M000530

And try to work around your problem with the the following option:

:source: Specifies the source association name used by has_many :through 
queries. Only use it if the name cannot be inferred from the 
association. has_many :subscribers, :through => :subscriptions will look 
for either +:subscribers+ or +:subscriber+ on Subscription, unless a 
+:source+ is given.

So you can have aclass.capable_teachers and 
aclass.actually_teaching_teachers or something like that. Can you tell 
me if it works?

Nauhaie

-- 
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I first suggest you find a better model name thatn class, that could
muck things up, alot. Perhaps Lesson.

j`ey
http://www.eachmapinject.com
Federico Fern?dez wrote:
> I have a model Teacher and a model Class
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Nauhaie wrote:
> I would be very interested in an answer, too...
> 
> But I suggest you read this:
>
http://rubyonrails.org/api/classes/ActiveRecord/Associations/ClassMethods.html#M000530
> 
> And try to work around your problem with the the following option:
> 
> :source: Specifies the source association name used by has_many :through 
> queries. Only use it if the name cannot be inferred from the 
> association. has_many :subscribers, :through => :subscriptions will look
> for either +:subscribers+ or +:subscriber+ on Subscription, unless a 
> +:source+ is given.
> 
> So you can have aclass.capable_teachers and 
> aclass.actually_teaching_teachers or something like that. Can you tell 
> me if it works?
> 
> Nauhaie

If I understood the documentation right, I could do this:

class Teacher< ActiveRecord::Base

  has_many :lessons, :through => :works, :source => :teacher

  has_many :classes, :through => :can_teach, :source => teacher

end



class Lesson< ActiveRecord::Base

  has_many :teachers, :through => :works, :source => :lesson

  has_many :teachers, :through => :can_teach, , :source => :lesson

end


Keeping the classes "CanTeach and Work" without changes.

I will try this and let you know.





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Federico Fernandez wrote:
> Nauhaie wrote:
>> I would be very interested in an answer, too...
>> 
>> But I suggest you read this:
>>
http://rubyonrails.org/api/classes/ActiveRecord/Associations/ClassMethods.html#M000530
>> 
>> And try to work around your problem with the the following option:
>> 
>> :source: Specifies the source association name used by has_many
:through
>> queries. Only use it if the name cannot be inferred from the 
>> association. has_many :subscribers, :through => :subscriptions will
look
>> for either +:subscribers+ or +:subscriber+ on Subscription, unless a 
>> +:source+ is given.
>> 
>> So you can have aclass.capable_teachers and 
>> aclass.actually_teaching_teachers or something like that. Can you tell 
>> me if it works?
>> 
>> Nauhaie
> 
> 
> If I understood the documentation right, I could do this:
> 
> class Teacher< ActiveRecord::Base
> 
>   has_many :lessons, :through => :works, :source => :teacher
> 
>   has_many :classes, :through => :can_teach, :source => teacher
> 
> end
> 
> 
> 
> class Lesson< ActiveRecord::Base
> 
>   has_many :teachers, :through => :works, :source => :lesson
> 
>   has_many :teachers, :through => :can_teach, , :source => :lesson
> 
> end
> 
> 
> Keeping the classes "CanTeach and Work" without changes.
> 
> I will try this and let you know.

It didn''t work, at all. Now when I do:

t = Teacher.find(:first).contracts I get a 
HasManyThroughAssociationNotFoundError

and if I do:

l = Lesson.find(:all) I get another error.

In summary, that''s not the way to do it (you need the "has
many"
:JoinTable statement, and/or I didn''t understand the
":source" option.

-- 
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Federico Fernandez wrote:
> Nauhaie wrote:
>> I would be very interested in an answer, too...
>> 
>> But I suggest you read this:
>>
http://rubyonrails.org/api/classes/ActiveRecord/Associations/ClassMethods.html#M000530
>> 
>> And try to work around your problem with the the following option:
>> 
>> :source: Specifies the source association name used by has_many
:through
>> queries. Only use it if the name cannot be inferred from the 
>> association. has_many :subscribers, :through => :subscriptions will
look
>> for either +:subscribers+ or +:subscriber+ on Subscription, unless a 
>> +:source+ is given.
>> 
>> So you can have aclass.capable_teachers and 
>> aclass.actually_teaching_teachers or something like that. Can you tell 
>> me if it works?
>> 
>> Nauhaie
> 
> 
> If I understood the documentation right, I could do this:
> 
> class Teacher< ActiveRecord::Base
>   has_many :lessons, :through => :works, :source => :teacher
>   has_many :classes, :through => :can_teach, :source => teacher
> end
> 
> class Lesson< ActiveRecord::Base
>   has_many :teachers, :through => :works, :source => :lesson
>   has_many :teachers, :through => :can_teach, , :source => :lesson
> end
> 
> Keeping the classes "CanTeach and Work" without changes.
> 
> I will try this and let you know.
I responded to your email about this yesterday but I guess my response 
was lost. You must have unique names for associations within a class. If 
you reuse the same name, only the last has_many will be available. Just 
name them differently. And DON''T use :class or :classes - that will
just
bork everything up as "class" is both a keyword and a method on Object
in ruby. You can spell it klass or clazz if you want.

You can also take the approach of having only one join model with an 
attribute that indicates the state of the relationship, such as whether 
the teacher is qualified to teach the class vs if he is currently 
teaching it. I like this way best.

class Teaching < ActiveRecord::Base
  belongs_to :teachers
  belongs_to :courses
end

class Course < ActiveRecord::Base
  has_many :teachings
  has_many :teachers, :through => :teachings
end

class Teacher < ActiveRecord::Base
  has_many :teachings
  has_many :courses, :through => :teachings
  has_many :courses_can_teach, :through => :teachings,
              :class_name => "Course", :conditions =>
"status = ''can''"
  has_many :courses_does_teach, :through => :teachings,
              :class_name => "Course", :conditions =>
"status = ''does''"
end

--
Josh Susser
http://blog.hasmanythrough.com

-- 
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Uhh... I''m not sure I get the difference between :source and 
:class_name...

:class_name - specify the class name of the association. Use it only if 
that name can?t be inferred from the association name. So has_many 
:products will by default be linked to the Product class, but if the 
real class name is SpecialProduct, you?ll have to specify it with this 
option.

:source: Specifies the source association name used by has_many :through 
queries. Only use it if the name cannot be inferred from the 
association. has_many :subscribers, :through => :subscriptions will look 
for either +:subscribers+ or +:subscriber+ on Subscription, unless a 
+:source+ is given.

Huh?

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In fact, Rails Recipes says:

class Magazine < ActiveRecord::Base
   has_many :subscriptions
   has_many :readers, :through => :subscriptions
   has_many :semiannual_subscribers,
      :through => :subscriptions,
      :source => :reader,
      :conditions => [''length_in_issues = 6'']
end

They say that :source should be used instead of :class_name... 
Frederico, can you make :source work in your case?

-- 
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Josh Susser wrote:
> Federico Fernandez wrote:
>> Nauhaie wrote:
>>> I would be very interested in an answer, too...
>>> 
>>> But I suggest you read this:
>>>
http://rubyonrails.org/api/classes/ActiveRecord/Associations/ClassMethods.html#M000530
>>> 
>>> And try to work around your problem with the the following option:
>>> 
>>> :source: Specifies the source association name used by has_many
:through
>>> queries. Only use it if the name cannot be inferred from the 
>>> association. has_many :subscribers, :through => :subscriptions
will look
>>> for either +:subscribers+ or +:subscriber+ on Subscription, unless
a
>>> +:source+ is given.
>>> 
>>> So you can have aclass.capable_teachers and 
>>> aclass.actually_teaching_teachers or something like that. Can you
tell
>>> me if it works?
>>> 
>>> Nauhaie
>> 
>> 
>> If I understood the documentation right, I could do this:
>> 
>> class Teacher< ActiveRecord::Base
>>   has_many :lessons, :through => :works, :source => :teacher
>>   has_many :classes, :through => :can_teach, :source => teacher
>> end
>> 
>> class Lesson< ActiveRecord::Base
>>   has_many :teachers, :through => :works, :source => :lesson
>>   has_many :teachers, :through => :can_teach, , :source =>
:lesson
>> end
>> 
>> Keeping the classes "CanTeach and Work" without changes.
>> 
>> I will try this and let you know.
> 
> I responded to your email about this yesterday but I guess my response 
> was lost. You must have unique names for associations within a class. If 
> you reuse the same name, only the last has_many will be available. Just 
> name them differently. And DON''T use :class or :classes - that
will just
> bork everything up as "class" is both a keyword and a method on
Object
> in ruby. You can spell it klass or clazz if you want.
> 
> You can also take the approach of having only one join model with an 
> attribute that indicates the state of the relationship, such as whether 
> the teacher is qualified to teach the class vs if he is currently 
> teaching it. I like this way best.
> 
> class Teaching < ActiveRecord::Base
>   belongs_to :teachers
>   belongs_to :courses
> end
> 
> class Course < ActiveRecord::Base
>   has_many :teachings
>   has_many :teachers, :through => :teachings
> end
> 
> class Teacher < ActiveRecord::Base
>   has_many :teachings
>   has_many :courses, :through => :teachings
>   has_many :courses_can_teach, :through => :teachings,
>               :class_name => "Course", :conditions =>
"status = ''can''"
>   has_many :courses_does_teach, :through => :teachings,
>               :class_name => "Course", :conditions =>
"status = ''does''"
> end
> 
> --
> Josh Susser
> http://blog.hasmanythrough.com
Hello Josh! wow.. thanks for the help, but let me see if I got it:
>   has_many :courses_can_teach, :through => :teachings,
>               :class_name => "Course", :conditions =>
"status = ''can''"
This means that I need a table called "courses_can_teach" with the
same
structure of the table "courses", right? The database will have 2 
different tables, but as you use :class_name => "Course" they will
be
represented with only one model class called "Course", am I right?

Then you use :conditions => "status = ..." so you can use 1 Join
Table
instead of two.

However, even though your suggestion is good, I don''t want one join 
table, but two, becase I want the Join Table "teachings" to use a 
different set of attributes than the Join Table "capabilities", and it
would be strange to mix both sets in one Join Table. If I want to use 
more than one Join Table I don''t know what to do.

In addition, there is something strange in your solution, because I''ll 
be able to do this:

t1 = Teacher.find(:first).courses_can_teach
t2 = Teacher.find(:first).courses_does_teach

and t1 << course, etc..

But what if, given a course, I want to do operations over the teachers 
associated to it.

If I do this:

c = Course.find(:first).teacher

what am I going to get? A teacher who can teach that course, or a 
teacher who does teach that course?

I will probably have to make the searching methods by hand. Then, I 
would say that in rails you have to decide between freedom to do your 
database models pretty and efficient, and the benefits of using 
ActiveRecords Associations.

-- 
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Nauhaie wrote:
> In fact, Rails Recipes says:
> 
> class Magazine < ActiveRecord::Base
>    has_many :subscriptions
>    has_many :readers, :through => :subscriptions
>    has_many :semiannual_subscribers,
>       :through => :subscriptions,
>       :source => :reader,
>       :conditions => [''length_in_issues = 6'']
> end
> 
> They say that :source should be used instead of :class_name... 
> Frederico, can you make :source work in your case?
I haven''t tried Josh''s reply yet, because my real model is a
bit more
complicated than the example. I was doing the migrations now, I''m late 
for college, so I will complete it tomorrow, and let you know if it 
worked with "source" and/or "class_name".

-- 
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Federico Fernandez wrote:
> Nauhaie wrote:
>> In fact, Rails Recipes says:
>> 
>> class Magazine < ActiveRecord::Base
>>    has_many :subscriptions
>>    has_many :readers, :through => :subscriptions
>>    has_many :semiannual_subscribers,
>>       :through => :subscriptions,
>>       :source => :reader,
>>       :conditions => [''length_in_issues = 6'']
>> end
>> 
>> They say that :source should be used instead of :class_name... 
>> Frederico, can you make :source work in your case?
> 
> I haven''t tried Josh''s reply yet, because my real model
is a bit more
> complicated than the example. I was doing the migrations now, I''m
late
> for college, so I will complete it tomorrow, and let you know if it 
> worked with "source" and/or "class_name".
Ok... I could make it work with 2 join tables. It was a little pain on 
the ass to get it set, but then I felt good... I have what I wanted. 
Below is the explanation of what I did. Notice that I changed some class 
names for the sake of understandability. Also please note that my real 
application is more complex, which is the reason why using 2 join tables 
may look like a strange decision in this example, but it is not (or not 
that much) in my application.

> class Scheduling< ActiveRecord::Base  
# the teachers work in some courses through schedulings
>   belongs_to :teachers
>   belongs_to :courses
> end
> class Screening< ActiveRecord::Base  
# the teachers can work in some courses through
screenings
>   belongs_to :teachers
>   belongs_to :courses
>
> end
> 
> class Course < ActiveRecord::Base
>   has_many :schedulings
>   has_many :screenings
>   has_many :teachers_does_teach, :through => :schedulings, :source =>
:teacher
>   has_many :teachers_can_teach, :through => :screenings, :source =>
:teacher
> end
> 
> class Teacher < ActiveRecord::Base
>   has_many :schedulings
>   has_many :screenings
>   has_many :courses_does_teach, :through => :schedulings, :source =>
:course
>   has_many :courses_can_teach, :through => :screenings, :source =>
:course
> end
And I have the following tables:

- teachers, courses, screenings, scheduling AND...
- teachers_can_teach, teachers_does_teach with the same structure as 
teachers
- courses_can_teach, courses_does_teach with the same structure as 
courses

So now I can do this:

screen = Screening.new
t = Tutor.find(11)
c = Course.find(3)
t.screenings << screen
c.screenings << screen
s.year = 1974
s.college = "University of Minesota"
s.comments = "Graduated with honors"
s.save

sched = Scheduling.new
t = Tutor.find(11)
c = Course.find(3)
t.schedulings << sched
c.schedulings << sched
s.day = "monday"
s.save

Then I can do things like:

t.courses_can_teach.find(:first).name
t.courses_does_teach_count

AND I don''t have the following methods:

t.courses
c.teachers

which is fine, since that wouldn''t make sense in my example.

Now... I don''t know if using two has-many-through associations is
better
than using one (as Josh suggested) but I''m happy to know that you can
do
it in rails. I really don''t like having to change my ER diagrams
because
of framework restrictions, do you?






-- 
Posted via http://www.ruby-forum.com/.

Federico Fernandez wrote:
> Ok... I could make it work with 2 join tables. It was a little pain on 
> the ass to get it set, but then I felt good... I have what I wanted. 
> Below is the explanation of what I did. Notice that I changed some class 
> names for the sake of understandability. Also please note that my real 
> application is more complex, which is the reason why using 2 join tables 
> may look like a strange decision in this example, but it is not (or not 
> that much) in my application.
> 
> ...
>
> Then I can do things like:
> 
> t.courses_can_teach.find(:first).name
> t.courses_does_teach_count
> 
> AND I don''t have the following methods:
> 
> t.courses
> c.teachers
> 
> which is fine, since that wouldn''t make sense in my example.
> 
> Now... I don''t know if using two has-many-through associations is
better
> than using one (as Josh suggested) but I''m happy to know that you
can do
> it in rails. I really don''t like having to change my ER diagrams
because
> of framework restrictions, do you?
There''s no problem with having multiple has_many :through associations 
in a model. If that''s the best fit to model your problem, then
that''s
what you should do. Looks like you''ve worked out a good solution.
Nicely
done.

--
Josh Susser
http://blog.hasmanythrough.com

-- 
Posted via http://www.ruby-forum.com/.

Josh Susser wrote:
> Federico Fernandez wrote:
>> Ok... I could make it work with 2 join tables. It was a little pain on 
>> the ass to get it set, but then I felt good... I have what I wanted. 
>> Below is the explanation of what I did. Notice that I changed some
class
>> names for the sake of understandability. Also please note that my real 
>> application is more complex, which is the reason why using 2 join
tables
>> may look like a strange decision in this example, but it is not (or not
>> that much) in my application.
>> 
>> ...
>>
>> Then I can do things like:
>> 
>> t.courses_can_teach.find(:first).name
>> t.courses_does_teach_count
>> 
>> AND I don''t have the following methods:
>> 
>> t.courses
>> c.teachers
>> 
>> which is fine, since that wouldn''t make sense in my example.
>> 
>> Now... I don''t know if using two has-many-through associations
is better
>> than using one (as Josh suggested) but I''m happy to know that
you can do
>> it in rails. I really don''t like having to change my ER
diagrams because
>> of framework restrictions, do you?
> 
> There''s no problem with having multiple has_many :through
associations
> in a model. If that''s the best fit to model your problem, then
that''s
> what you should do. Looks like you''ve worked out a good solution.
Nicely
> done.
> 
> --
> Josh Susser
> http://blog.hasmanythrough.com
Yes, that''s what I think too... and this issue made me think about the 
philosophy behing ActiveRecords Associations. In an initial stage you 
have 1 Model - 1 Table, but then, as the ER Diagramm gets more complex 
you will end with more tables than models, if you want Rails to generate 
methods for you. I''m not sure, but perhaps this is something to improve
in the framework (or maybe it is something normal and good... I''m just 
thinking loud)

Thanks for your help. I''m a newbie in ror and I''m starting to
love it in
part due to the nice community.

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