Accessing from both ends of has many through relationship

Rails
1

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?

2

On 3 August 2012 15:57, Martyn W. [email protected] wrote:

class Part < ActiveRecord::Base
  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

3

For <%= work_ticket.material.number %>

I get undefined method `number’ for nil:NilClass

4

Please remember to quote the previous message so that it is easier to
follow the thread, remember this is a mailing list not a forum (though
you may be accessing it through a forum like interface).

On 3 August 2012 17:02, Martyn W. [email protected] 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

5

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.