Identifying Model Instance by Two Relations - ruby-on-rails

Question

I'm trying to pick out an instance of a model ("Package") by its relation to two other models to which it belongs ("Cut" and "Animal"). While there are, say many packages with :cut_id 3 and many with :animal_id 4, there should only be one with both, and I want to pick that one out and display its contents in a table.
I've tried the following DIY mess, and it's not really working. (cutfind is a method I created that I know works for calling out all of the cuts associated with the given animal.)
<% #animal.cutfind.each do |cut| %>
<tr>
<td><%= cut.name %></td>
<td><%= number_to_currency(cut.price) %></td>
<td><%= cut.package_weight %> lb</td>
<% #a = Package.where(:animal_id => #animal.id) %>
<% #pset = #a.where(:cut_id => cut.id) %>
<% #pset.each do |p| %>
<td><%= p.original %></td>
<td><%= p.left %></td>
<% end %>
</tr>
<%end%>
Any idea how to do this [better]? Thanks.
Update: I tried this other DIY mess and am getting the same problem (the cells aren't even being created, which leads me to believe that #pset is empty).
This is in my animal model:
def packagefind
Package.where(:animal_id => self.id)
end
And then I changed the above like so:
<td><%= cut.package_weight %> lb</td>
<% #pset = #animal.packagefind.where(:cut_id => cut.id) %>
<% #pset.each do |p| %>
<td><%= p.original %></td>
<td><%= p.left %></td>
<% end %>

Answer 1

Rails will automatically generate methods to help you find the associated records if you define the following relations:
class Animal
has_many :cuts
has_many :packages, :through => :cuts
end
class Cut
belongs_to :animal
belongs_to :package
end
class Package
has_many :cuts
has_many :animals, :through => :cuts
end
In your controller, the following line will eager load all the records you will need in your view:
#animal = Animal.includes(:cuts => :package)
Your view can then be shortened to:
<% #animal.cuts.each do |cut| %>
<tr>
<td><%= cut.name %></td>
<td><%= number_to_currency(cut.price) %></td>
<td><%= cut.package_weight %> lb</td>
<td><%= cut.package.original %></td>
<td><%= cut.package.left %></td>
</tr>
<%end%>

Answer 2

As I'm not able to comment on your post, I take a guess:
You have the folllowing architecture:
Cut -> Package <- Animal
In this, "->" and "<-" are one-to-many relationships so that
class Package < ActiveRecord::Base
has_many :cuts
has_many :animals
end
So, you want "the" package, that has Cut with id 3 and Animal id 4.
Did you try:
x = Product.select { |product| product.cuts.include?(Cut.find(3)) }.select{ |product| product.animals.include?(Animal.find(4)) }
?
EDIT: I first suggested to you use
Product.find_by_product_id_and_animal_id()
which didn't work but showed the OP the way to do it