I had to comment out the scope and attr_accessible as I did this sans rails, but it defines the methods correctly now on the instances of the Pump class.

h_store_properties.rb

module HStoreProperties
  extend self
  def boolean_h_store_fields_for(property_names)
    property_names.each do |key|
      attr_accessible key
      scope "has_#{key}", lambda { |value| where('properties @> (? => ?)', key, value) }

      define_method(key) do
        properties && properties[key].to_bool
      end

      define_method("#{key}?") do
        alias_method key, "#{key}?".to_s
      end

      define_method("#{key}=") do |value|
        self.properties = (properties || {}).merge(key => value)
      end
    end
  end
end

pump.rb

class Pump
  extend HStoreProperties
  
  boolean_h_store_fields_for %w[variable_speed skid_mounted speed_lock flow_lock press_lock]
end