Rails - Apr 2008 - Validation dependent on unsaved parent

I''m having trouble with a validation that depends on an attribute of a
belongs_to parent. If the child is added to an unsaved parent
(parent.children << new child), the has_many collection
parent.children includes the unsaved child. However the belongs_to
attribute child.parent appears to be nil until the parent has been
saved. Without access to the parent attributes, the validation can''t
be done in the child model. I have a sort-of workaround using
validation code in the parent, but that violates DRY. The validation
still has to be in the child because the child can be updated without
the parent validation coming into play.

Here''s the code:

============
class Item < ActiveRecord::Base
  has_many :parlevels
  has_many :locations, :through => :parlevels
  has_many :line_items
  has_many :orders, :through => :line_items
end

class Location < ActiveRecord::Base
  has_many :orders
  has_many :parlevels
  has_many :items, :through => :parlevels
end

class Parlevel < ActiveRecord::Base
  belongs_to :item
  belongs_to :location

  #Attribute quantity is the max quantity of an item which may be
ordered for a location
end

class Order < ActiveRecord::Base
  has_many :line_items, :dependent => :destroy
  belongs_to :location
end

class LineItem < ActiveRecord::Base
  belongs_to :order
  belongs_to :item

  def parlevel
    if self.order
      Parlevel.find(:first, :conditions => {:location_id =>
self.order.location_id, :item_id => self.item_id})
    else
      nil
    end
  end

  def validate
    if self.quantity_requested

      if self.parlevel
        limit = self.parlevel.quantity
      else
        limit = 0
      end

      errors.add(:quantity_requested,"above maximum of " + limit.to_s)
if self.quantity_requested > limit
    end
  end
end

============
The current result of this code in the limit is always zero when the
parent is unsaved. Not very useful.

Any suggestions?

-Mack
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Here''s the solution:

http://macksmind.net/2008/04/26/using-belongs_to-in-rails-model-validations-when-the-parent-is-unsaved/

On Wed, Apr 23, 2008 at 10:54 AM, IndyCodeWarrior
<mack.earnhardt-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
>
>  I''m having trouble with a validation that depends on an attribute
of a
>  belongs_to parent. If the child is added to an unsaved parent
>  (parent.children << new child), the has_many collection
>  parent.children includes the unsaved child. However the belongs_to
>  attribute child.parent appears to be nil until the parent has been
>  saved. Without access to the parent attributes, the validation
can''t
>  be done in the child model. I have a sort-of workaround using
>  validation code in the parent, but that violates DRY. The validation
>  still has to be in the child because the child can be updated without
>  the parent validation coming into play.
>
>  Here''s the code:
>
>  ============>
>  class Item < ActiveRecord::Base
>   has_many :parlevels
>   has_many :locations, :through => :parlevels
>   has_many :line_items
>   has_many :orders, :through => :line_items
>  end
>
>  class Location < ActiveRecord::Base
>   has_many :orders
>   has_many :parlevels
>   has_many :items, :through => :parlevels
>  end
>
>  class Parlevel < ActiveRecord::Base
>   belongs_to :item
>   belongs_to :location
>
>   #Attribute quantity is the max quantity of an item which may be
>  ordered for a location
>  end
>
>  class Order < ActiveRecord::Base
>   has_many :line_items, :dependent => :destroy
>   belongs_to :location
>  end
>
>  class LineItem < ActiveRecord::Base
>   belongs_to :order
>   belongs_to :item
>
>   def parlevel
>     if self.order
>       Parlevel.find(:first, :conditions => {:location_id =>
>  self.order.location_id, :item_id => self.item_id})
>     else
>       nil
>     end
>   end
>
>   def validate
>     if self.quantity_requested
>
>       if self.parlevel
>         limit = self.parlevel.quantity
>       else
>         limit = 0
>       end
>
>       errors.add(:quantity_requested,"above maximum of " +
limit.to_s)
>  if self.quantity_requested > limit
>     end
>   end
>  end
>
>  ============>
>  The current result of this code in the limit is always zero when the
>  parent is unsaved. Not very useful.
>
>  Any suggestions?
>
>  -Mack
>  >
>
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