Modeling pedigree properly - ruby-on-rails

Question

I'm creating an app to represent the pedigree of livestock. Each child has one dam (e.g. ewe) and one sire (e.g. ram). A dam/sire pairing can have multiple children (e.g. lambs) and a dam and sire may have many more children independent of the other. I am trying to represent this relationship so that I could do something like ewe.children and get a listing of her offspring. Similarly, I'd like to be able to do something like lamb.ewe to get her mother or lamb.ewe.ewe to get her maternal grandmother.
from schema.rb...
create_table "parent_child_relationships", force: :cascade do |t|
t.integer "parent_id"
t.integer "child_id"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.index ["child_id"], name: "index_parent_child_relationships_on_child_id"
t.index ["parent_id", "child_id"], name: "index_parent_child_relationships_on_parent_id_and_child_id", unique: true
t.index ["parent_id"], name: "index_parent_child_relationships_on_parent_id"
end
from parent_child_relationship.rb...
class ParentChildRelationship < ApplicationRecord
belongs_to :sire, class_name: "Animal"
belongs_to :dam, class_name: "Animal"
belongs_to :children, class_name: "Animal"
end
from animal.rb...
has_one :sire_relationship, class_name: "ParentChildRelationship",
foreign_key: "child_id",
dependent: :destroy
has_one :dam_relationship, class_name: "ParentChildRelationship",
foreign_key: "child_id",
dependent: :destroy
has_many :child_relationships, class_name: "ParentChildRelationship",
foreign_key: "parent_id",
dependent: :destroy
has_one :sire, through: :sire_relationship, source: :child
has_one :dam, through: :dam_relationship, source: :child
has_many :children, through: :child_relationships, source: :parent
In the console, I run the following commands to grab the animals I want to relate to each other...
s = Shepherd.first
ewe = s.animals.find_by(id: 37)
ram = s.animals.find_by(id: 133)
lamb = s.animals.find_by(id: 61)
Now, when I try to create the sire_relationship and dam_relationship I get an error since it doesn't seem to see the relationship as being unique. The sire_relationship is replaced by the dam_relationship...
>> lamb.create_sire_relationship(parent_id: ram.id)
(0.1ms) begin transaction
SQL (0.7ms) INSERT INTO "parent_child_relationships" ("parent_id", "child_id", "created_at", "updated_at") VALUES (?, ?, ?, ?) [["parent_id", 133], ["child_id", 61], ["created_at", "2018-01-15 15:33:06.649936"], ["updated_at", "2018-01-15 15:33:06.649936"]]
(2.5ms) commit transaction
ParentChildRelationship Load (0.2ms) SELECT "parent_child_relationships".* FROM "parent_child_relationships" WHERE "parent_child_relationships"."child_id" = ? LIMIT ? [["child_id", 61], ["LIMIT", 1]]
=> #<ParentChildRelationship id: 1, parent_id: 133, child_id: 61, created_at: "2018-01-15 15:33:06", updated_at: "2018-01-15 15:33:06">
>> lamb.create_dam_relationship(parent_id: ewe.id)
(0.1ms) begin transaction
SQL (0.6ms) INSERT INTO "parent_child_relationships" ("parent_id", "child_id", "created_at", "updated_at") VALUES (?, ?, ?, ?) [["parent_id", 37], ["child_id", 61], ["created_at", "2018-01-15 15:33:35.045703"], ["updated_at", "2018-01-15 15:33:35.045703"]]
(1.0ms) commit transaction
ParentChildRelationship Load (0.1ms) SELECT "parent_child_relationships".* FROM "parent_child_relationships" WHERE "parent_child_relationships"."child_id" = ? LIMIT ? [["child_id", 61], ["LIMIT", 1]]
(0.0ms) begin transaction
SQL (0.5ms) DELETE FROM "parent_child_relationships" WHERE "parent_child_relationships"."id" = ? [["id", 1]]
(1.2ms) commit transaction
=> #<ParentChildRelationship id: 2, parent_id: 37, child_id: 61, created_at: "2018-01-15 15:33:35", updated_at: "2018-01-15 15:33:35">
Creating the children_relationships, I get these errors...
>> ewe.child_relationships.create(child_id: lamb.id)
(0.1ms) begin transaction
SQL (0.9ms) INSERT INTO "parent_child_relationships" ("parent_id", "child_id", "created_at", "updated_at") VALUES (?, ?, ?, ?) [["parent_id", 37], ["child_id", 61], ["created_at", "2018-01-15 15:37:11.436086"], ["updated_at", "2018-01-15 15:37:11.436086"]]
(0.1ms) rollback transaction
ActiveRecord::RecordNotUnique: SQLite3::ConstraintException: UNIQUE constraint failed: parent_child_relationships.parent_id, parent_child_relationships.child_id: INSERT INTO "parent_child_relationships" ("parent_id", "child_id", "created_at", "updated_at") VALUES (?, ?, ?, ?)
from (irb):13
>> ram.child_relationships.create(child_id: lamb.id)
(0.1ms) begin transaction
SQL (0.6ms) INSERT INTO "parent_child_relationships" ("parent_id", "child_id", "created_at", "updated_at") VALUES (?, ?, ?, ?) [["parent_id", 133], ["child_id", 61], ["created_at", "2018-01-15 15:37:25.264947"], ["updated_at", "2018-01-15 15:37:25.264947"]]
(2.5ms) commit transaction
Finally, if I check to see whether I can access the sire of lamb, I get another error...
>> lamb.dam
ActiveRecord::HasManyThroughSourceAssociationNotFoundError: Could not find the source association(s) :child in model ParentChildRelationship. Try 'has_many :dam, :through => :dam_relationship, :source => <name>'. Is it one of sire, dam, or children?
from (irb):21
>> lamb.sire
ActiveRecord::HasManyThroughSourceAssociationNotFoundError: Could not find the source association(s) :child in model ParentChildRelationship. Try 'has_many :sire, :through => :sire_relationship, :source => <name>'. Is it one of sire, dam, or children?
from (irb):22
I get similar errors if I do ewe.children or ram.children.
I'm looking for an extra pair of eyes to tell me what I'm doing wrong or whether there's an easier way to achieve what I'm after.

Answer 1

The problem is that you only have one parent_id in your animals table which can only store the ID of a single parent. This works for bacteria but not for animals which have two parents. Your parent_id is getting written to when you set the dam and the sire.
There are multiple ways to do this but I think the simplest is to have a dam_id and sire_id in your animals table.
This is the migration to create the table:
class CreateAnimals < ActiveRecord::Migration[5.1]
def change
create_table :animals do |t|
t.integer :dam_id, index: true
t.integer :sire_id, index: true
t.timestamps
end
end
end
This is what your model will look like this. Notice that you need two belongs_to/has_many relationships:
class Animal < ApplicationRecord
belongs_to :dam, class_name: 'Animal'
belongs_to :sire, class_name: 'Animal'
has_many :children_as_sire, class_name: 'Animal', foreign_key: :sire_id
has_many :children_as_dam, class_name: 'Animal', foreign_key: :dam_id
def children
children_as_dam + children_as_sire
end
end
Notice the getter method children that grabs both children_as_dam and children_as_sire. This will result in two SQL queries which is not ideal. If you're tracking the sec of the Animal, you could do something like:
def children?
case sex
when 'male'
children_as_sire
when 'female'
children_as_dam
end
end
I wrote some specs to demonstrate:
require 'rails_helper'
RSpec.describe Animal, type: :model, focus: true do
it 'can be created' do
expect { Animal.create }.to_not raise_error
end
it 'can have a dam' do
animal = Animal.new
animal.update! dam: Animal.create
expect(animal.dam).to be_a(Animal)
expect(animal.sire).to be_nil
end
it 'can have a sire' do
animal = Animal.new
animal.update! sire: Animal.create
expect(animal.sire).to be_a(Animal)
expect(animal.dam).to be_nil
end
it 'can have both a dam and a sire and tell the difference' do
dam = Animal.create
sire = Animal.create
child = Animal.create dam: dam, sire: sire
expect(child.reload.dam).to eq(dam)
expect(child.reload.sire).to eq(sire)
end
it 'grandma' do
grandma = Animal.create
dam = Animal.create dam: grandma
child = Animal.create dam: dam
expect(child.reload.dam.dam).to eq(grandma)
end
it 'has children' do
sire = Animal.create
animal = Animal.create sire: sire
expect(sire.reload.children).to include(animal)
end
end
Notice that you can't add children to a model:
animal = Animal.create
animal.children << Animal.create # will raise an error
Instead, you have to manually set the sire and dam (which is probably what you want to do since you're keeping track).