Look up a product's shipping method though a particular distributor

app/models/spree/product_decorator.rb

@@ -14,6 +14,13 @@ Spree::Product.class_eval do
   scope :in_distributor, lambda { |distributor| joins(:product_distributions).where('product_distributions.distributor_id = ?', (distributor.respond_to?(:id) ? distributor.id : distributor.to_i)) }
 
 
+  def shipping_method_for_distributor(distributor)
+    distribution = self.product_distributions.find_by_distributor_id(distributor)
+    raise ArgumentError, "This product is not available through that distributor" unless distribution
+    distribution.shipping_method
+  end
+
+
   # Build a product distribution for each distributor
   def build_product_distributions
     Spree::Distributor.all.each do |distributor|

spec/models/product_spec.rb

@@ -19,4 +19,21 @@ describe Spree::Product do
     end
   end
 
+  context "finders" do
+    it "finds the shipping method for a particular distributor" do
+      shipping_method = create(:shipping_method)
+      distributor = create(:distributor)
+      product = create(:product)
+      product_distribution = create(:product_distribution, :product => product, :distributor => distributor, :shipping_method => shipping_method)
+      product.shipping_method_for_distributor(distributor).should == shipping_method
+    end
+
+    it "raises an error if distributor is not found" do
+      distributor = create(:distributor)
+      product = create(:product)
+      expect do
+        product.shipping_method_for_distributor(distributor)
+      end.to raise_error "This product is not available through that distributor"
+    end
+  end
 end