def where(name)
      # Must define the attributes before constraining them
      raise ArgumentError, "Unknown attribute: #{name}" unless @attributes.include? name
      @constraints[name] = AttributeConstraint.new(name)
    end
def where(name)
      # Must define the attributes before constraining them
      raise ArgumentError, "Unknown attribute: #{name}" unless @attributes.include? name
      @constraints[name] = AttributeConstraint.new(name)
    end
Helpful method to define constraints