Active Record error with Association

I am learning to use ActiveRecord so bear with me. I have a script that
looks like this

class Product < ActiveRecord::Base
  set_primary_key "ProductID"
  set_table_name "Product"
  has_many :variants, :foreign_key => 'ProductID'
end

class Variant < ActiveRecord::Base
  set_primary_key "VariantID"
  set_table_name "ProductVariant"
  belongs_to :product, :foreign_key => 'ProductID'
  has_one :capacity, :foreign_key => 'CapacityID'
  has_one :material, :foreign_key => 'MaterialID'
end

class Capacity < ActiveRecord::Base
  set_primary_key "CapacityID"
  has_many :variants, :foreign_key => 'CapacityID'
end

class Material < ActiveRecord::Base
  set_primary_key "MaterialID"
  has_many :variants, :foreign_key => 'MaterialID'
end

widget = Product.find("10")
puts widget.Name
puts '-------------------------------------'

widget.variants.each do |v|
  puts v.Name.squeeze(" ").chop! + "\t\t In stock: \t" +
v.Inventory.to_s + "\tPrice: \t" + v.Price.to_s + "\tCapacity: \t" +
v.capacity.Name.to_s
  puts "\t Material: \t" + v.material.Name.to_s # this line breaks the
script

end

The "puts "\t Material: \t" + v.material.Name.to_s" line in the do
loop triggers the following error:

B2