Rails - Aug 2012 - accessing from both ends of has many through relationship

I have a ''has many through'' relationship in my models. I am
trying to
access objects from either side of this relationship, with mixed
results.Here are my models:

    class Material < ActiveRecord::Base
      attr_accessible :description, :number
      has_many :parts
      has_many :work_tickets, :through => :parts
    end

    class Part < ActiveRecord::Base
      belongs_to :material
      attr_accessible :description, :number, :yield, :material_id
      has_many :work_tickets
      has_many :operations, :dependent => :destroy
    end

    class WorkTicket < ActiveRecord::Base
      belongs_to :part
      belongs_to :material
      attr_accessible :number, :yield, :opened, :closed, :part_id
      has_many :works, :dependent => :destroy
    end

I can access the work_tickets from the material with:

    @work_tickets = @material.work_tickets

But, cannot access material from work_ticket:

    <%= work_ticket.material.number %>

Forcing me to use:

    <%= work_ticket.part.material.number %>

Am I expecting the wrong behaviour, or am I using the wrong relationship
pattern?

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On 3 August 2012 15:57, Martyn W.
<lists-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org>
wrote:
> I have a ''has many through'' relationship in my models. I
am trying to
> access objects from either side of this relationship, with mixed
> results.Here are my models:
>
>     class Material < ActiveRecord::Base
>       attr_accessible :description, :number
>       has_many :parts
>       has_many :work_tickets, :through => :parts
>     end
>
>     class Part < ActiveRecord::Base
>       belongs_to :material
>       attr_accessible :description, :number, :yield, :material_id
>       has_many :work_tickets
>       has_many :operations, :dependent => :destroy
>     end
>
>     class WorkTicket < ActiveRecord::Base
>       belongs_to :part
>       belongs_to :material
>       attr_accessible :number, :yield, :opened, :closed, :part_id
>       has_many :works, :dependent => :destroy
>     end
>
> I can access the work_tickets from the material with:
>
>     @work_tickets = @material.work_tickets
>
> But, cannot access material from work_ticket:
>
>     <%= work_ticket.material.number %>
What happens when you do this?  You have specified a simple WorkTicket
belongs_to :material so there is nothing complex about this.
>
> Forcing me to use:
>
>     <%= work_ticket.part.material.number %>
>
> Am I expecting the wrong behaviour, or am I using the wrong relationship
> pattern?
It appears you have two routes for getting to material, that is bad as
there is redundant information somewhere and there is a danger of the
two routes getting mismatched.

Colin

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For <%= work_ticket.material.number %>

I get undefined method `number'' for nil:NilClass

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On 3 August 2012 17:02, Martyn W.
<lists-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org>
wrote:
> For <%= work_ticket.material.number %>
>
> I get undefined method `number'' for nil:NilClass
Try displaying work_ticket.material_id, I think you will find that it
is not set.

Colin

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Martyn W. wrote in post #1071177:
> I have a ''has many through'' relationship in my models. I
am trying to
> access objects from either side of this relationship, with mixed
> results.Here are my models:
>
>     class Material < ActiveRecord::Base
>       attr_accessible :description, :number
>       has_many :parts
>       has_many :work_tickets, :through => :parts
>     end
>
>     class Part < ActiveRecord::Base
>       belongs_to :material
>       attr_accessible :description, :number, :yield, :material_id
>       has_many :work_tickets
>       has_many :operations, :dependent => :destroy
>     end
>
>     class WorkTicket < ActiveRecord::Base
>       belongs_to :part
>       belongs_to :material
>       attr_accessible :number, :yield, :opened, :closed, :part_id
>       has_many :works, :dependent => :destroy
>     end
>
> I can access the work_tickets from the material with:
>
>     @work_tickets = @material.work_tickets
>
> But, cannot access material from work_ticket:
>
>     <%= work_ticket.material.number %>
>
> Forcing me to use:
>
>     <%= work_ticket.part.material.number %>
>
> Am I expecting the wrong behaviour, or am I using the wrong relationship
> pattern?
The error you get is correct as your relationships are mismatched.
Let''s try to analyze:
>     class Material < ActiveRecord::Base
>       attr_accessible :description, :number
>       has_many :parts
>       has_many :work_tickets, :through => :parts
>     end
that means when you do material.work_tickets, it will go to the parts
table and look for field matierial_id = self.id, then from those results
get the work_tickets_id and get the WorkTicket objects that match.
Sounds ok, but might not neccessarily return you the results you want.
The mySQL query will probably be something like
select work_tickets.* from work_tickets
inner join parts on parts.work_tickets_id = work_tickets.id
inner join materials on parts.material_id = [#id]

anyway, now for the other side:
>     class WorkTicket < ActiveRecord::Base
>       belongs_to :part
>       belongs_to :material
>       attr_accessible :number, :yield, :opened, :closed, :part_id
>       has_many :works, :dependent => :destroy
>     end
when you do work_ticket.material, rails will go to the Material model
and look for the other side of the relationship. Here it will fail,
because though you have has_many :work_tickets, this relationship goes
through the :parts table, and NOT directly to the WorkTicket model.
There can be no match.

There are a number of ways to fix this. Let''s make sure we got the
relationships right.

There''s a one-many from materials to tickets.
There''s a one-many from materials to parts.
There''s a one-many from parts to tickets.

No need for :through table

     class Material < ActiveRecord::Base
       attr_accessible :description, :number
       has_many :parts
       has_many :work_tickets
     end

     class Part < ActiveRecord::Base
       belongs_to :material
       attr_accessible :description, :number, :yield
       has_many :work_tickets
       has_many :operations, :dependent => :destroy
     end

     class WorkTicket < ActiveRecord::Base
       belongs_to :part
       belongs_to :material
       attr_accessible :number, :yield, :opened, :closed
       has_many :works, :dependent => :destroy
     end

of course changing your models means your table columns will have to
match.

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