Question

I need to model up to 5 or 6 generations horse pedigrees using rails/activerecord. I did my research here on stack and on the web and ultimately utilized this article as the basis of my approach. Here’s what I’ve come up with.

Two models:

Horse has the following attributes id and horse_name
Pedigree has: id, parent_id and horse_id.

And the following associations:

has_many  :parent_horse_relationships, :class_name => "Pedigree", :foreign_key => :horse_id, :dependent => :destroy

has_one  :sire_horse_relationship, :class_name => "Pedigree", :foreign_key => :horse_id, :conditions => "horse_gender = 'Male'

has_one  :dam_horse_relationship, :class_name => "Pedigree", :foreign_key => :horse_id, :conditions => "horse_gender = 'Female'

has_many  :parents, :through => :parent_horse_relationships, :source => :parent 

has_one  :sire, :through => :sire_horse_relationship,:source => :parent

has_one  :dam, :through => :dam_horse_relationship,:source => :parent

has_many  :horse_parent_relationships, :class_name => "Pedigree", :foreign_key => :parent_id, :dependent => :destroy

has_many  :progenies, :through => :horse_parent_relationships, :source =>  :horse

This approach is close, however it appears my condition to determine the dam or sire is being applied to the Horse and not the parent. Therefore if the particular horse is Male, the horse.sire will work, but the horse.dam will not and vice versa. Once I get basic functionality working I’d like to add additional methods to get the whole pedigree, grandparents, siblings, descendants, etc.

Questions:

1. How can I apply the gender condition to the parents and not the horse so that both sire and dam work.

2. Is the approach that I have take viable or is there a more elegant, efficient way of accomplishing this.

3. Any other suggestions or guidance would be appreciated.

Apologies for the long question and thanks for your help.

Answer 1

I ended up spending a great deal of time on this one, but finally came up with a solution that met my requirements. The associations that ultimately worked follow:

  has_many :parent_horse_relationships, :class_name => "Pedigree", :foreign_key => :horse_id, :dependent => :destroy

  has_many :parents, :through => :parent_horse_relationships, :source => :parent do 
    def dam_relationship
      owner = self.proxy_association.owner
      owner = owner.parents.where(:horse_gender => "Female")
      where('pedigrees.parent_id = ?', owner)
    end

    def sire_relationship
      owner = self.proxy_association.owner
      owner = owner.parents.where(:horse_gender => "Male")
      where('pedigrees.parent_id = ?', owner)
    end
  end

  def dam
    parents.dam_relationship
  end

  def sire
    parents.sire_relationship
  end

Question responses:

I applied the gender condition through use of an association_proxy and a simple wrapper. I created a dam_relationship and corresponding sire_relationship and then wrapped those methods in a couple of dam and sire wrapper methods.

def dam_relationship
owner = self.proxy_association.owner
owner = owner.parents.where(:horse_gender => "Female")
where('pedigrees.parent_id = ?', owner)
end

def dam
parents.dam_relationship
end

This allows me to do:

@horse.parents, @horse.dam, @horse.sire (not displayed)

as well as most of the methods included in the ancestry gem mentioned below. With a little bit of recursion it’s fairly straight forward to display the entire pedigree or the number of generations that interest you.

I decided that the approach of having two models (Horse and Pedigree) provide som additional flexibility compared to having the sire_id and dam_id directly in the Horse model. This approach will enable me to more easily create methods like @horse.uncle, @horse.aunt. I believe these would be more difficult with the sire_id and dam_id directly in the Horse model.

The most popular gem for accomplishing this seems to be ancestry. The author accomplishes this and a lot more simply by adding an ancestry column to the model of interest. Its a very nice solution a definitely worth checking out.