```ruby
class Spree::TaxRate < ActiveRecord::Base
  belongs_to :zone
  belongs_to :tax_category

  validates :amount, :presence => true, :numericality => true

  calculated_adjustments :default => Calculator::SalesTax
  scope :by_zone, lambda { |zone| where("zone_id = ?", zone)}

  # Searches all possible TaxRates and returns the Zone which represents the most appropriate match (if any.)
  # To be considered for a match, the Zone must include the supplied address.  If multiple matches are
  # found, the Zone with the highest rate will be returned.  This method will return +nil+ if no match is found.
  def self.match(address)
    TaxRate.all.select { |rate| rate.zone.include? address }
  end

  # For Vat the default rate is the rate that is configured for the default category
  # It is needed for every price calculation (as all customer facing prices include vat )
  # The function returns the actual amount, which may be 0 in case of wrong setup, but is never nil
  def self.default
    category = TaxCategory.includes(:tax_rates).where(:is_default => true).first
    return 0 unless category

    category.effective_amount || 0
  end
end
```

Fix reference to Spree::Calculator and Spree::TaxCategory in TaxRate model
user
```ruby
class Spree::TaxRate < ActiveRecord::Base
  belongs_to :zone
  belongs_to :tax_category

  validates :amount, :presence => true, :numericality => true

  calculated_adjustments :default => Spree::Calculator::SalesTax
  scope :by_zone, lambda { |zone| where("zone_id = ?", zone)}

  # Searches all possible TaxRates and returns the Zone which represents the most appropriate match (if any.)
  # To be considered for a match, the Zone must include the supplied address.  If multiple matches are
  # found, the Zone with the highest rate will be returned.  This method will return +nil+ if no match is found.
  def self.match(address)
    TaxRate.all.select { |rate| rate.zone.include? address }
  end

  # For Vat the default rate is the rate that is configured for the default category
  # It is needed for every price calculation (as all customer facing prices include vat )
  # The function returns the actual amount, which may be 0 in case of wrong setup, but is never nil
  def self.default
    category = Spree::TaxCategory.includes(:tax_rates).where(:is_default => true).first
    return 0 unless category

    category.effective_amount || 0
  end
end
```
assistant