Rails: active-records query for entry in range & included in

I am working on a shipping implementation for a checkout process.

My app has carts, cart_items, orders and order_items.

Weight and size of all items are in the database and I calculate total_weight in the order and cart models. I also have a shipping_service model with weightmin and weightmax for each shipping service + a postzone and land (country) model.

Now I would like to show on the shopping cart page only the shipping services which are conform to the weight of the cart or order.

I suppose my carts_controller should be something like:

class CartsController < ApplicationController
def show
    @cart = Cart.find(params[:id])
    @lands = Land.find(:all)
    @shippingservices = Shippingservice.where('@cart.total_weight BETWEEN ? AND ?', :weightmin, :weightmax)
end

My cart model is:

class Cart < ActiveRecord::Base
  attr_accessor :total_weight

  has_many :cart_items
  has_many :products, :through => :cart_items
  has_many :lands
  has_many :shipping_services, :through => :postzones

  def total_weight
    cart_items.inject(0) {|sum, n| n.weight * n.amount + sum}
  end
end

My land model is

class Land < ActiveRecord::Base
  has_many :shippingservices, :through => :postzones
  has_many :postzones
  has_many :carts
end

My shipping_service model is:

class Shippingservice < ActiveRecord::Base
  has_many :lands, :through => :postzones
  has_many :postzones
  has_many :carts
  has_many :orders
end

My postzone model is:

class Postzone < ActiveRecord::Base
  belongs_to :shippingservice
  belongs_to :land
end

The postzone table has foreign keys for lands and shipping_services.

Latter I would like to implement two selector fields: one for ship_to_countries and one for shipping_services, with the second selector being populate only with entries related to the entry selected in the first selector.

I had already this working inside the carts_controller:

@shippingservices = Shippingservice.includes(:lands, :postzones).where('postzones.land_id = ?', Land.first.id) 

Which load only shipping services for a specific country into the second selector. But I do not know how to combine the two where clauses relative to weight and postzone into one query.

Any help is very much appreciated!

Thank you in advance.