currently.rb

class UserSearch < Valuable
  has_value :term
  has_value :include_inactive, :default => false
  has_value :postal_code
  
  def scope
    User.apply( conditions )
  end
  
  def term_like
    "#{term}%".sanitize
  end
  
  def conditions
    [].tap do |out|
      out << "first_name LIKE #{term_like} OR ... " unless term.blank? # fear of injection attach
      out << "active = 1" unless include_inactive? # SQL implementation specific. :(
      out << "postal_code = #{self.postal_code.sanitize}" if postal_code.not.blank?
    end
  end
end

goal.rb

class UserSearch < Valuable
  has_value :term
  has_value :include_inactive, :default => false
  has_value :postal_code
  
  def scope
    User.apply( conditions )
  end
  
  def conditions
    [].tap do |out|
      out << lambda { where('first_name LIKE ":term%" OR last_name LIKE ":term%" OR username LIKE ":term%"', term: self.term) } unless term.blank?
      out << lambda { where(active: true) } unless include_inactive?
      out << lambda { where(postal_code: self.postal_code) } if postal_code.not.blank?
    end
  end
end

why_not_this.rb

class UserSearch < Valuable
  has_value :term
  has_value :include_inactive, :default => false
  has_value :postal_code
  
  def scope
    User.apply( conditions )
  end
  
  def conditions
    scope = User.scope
    scope = scope.where('first_name LIKE ":term%" OR last_name LIKE ":term%" OR username LIKE ":term%"', term: self.term) } unless term.blank?
    scope = scope.where(active: true) } unless include_inactive?
    scope = scope.where(postal_code: self.postal_code) } if postal_code.not.blank?
  end
end