Is it possible to rspec this properly without hitting the database?

order_spec.rb

  create_table "line_items", :force => true do |t|
    t.integer  "cost"
    t.boolean  "paid"
    t.integer  "order_id"
  end

  create_table "orders", :force => true do |t|
  end

# -----

class LineItem < ActiveRecord::Base
  scope :paid, where(:paid => true)
end

# -----

class Order < ActiveRecord::Base
  has_many :line_items

  def total_cost
    line_items.paid.sum(:cost)
  end
end

# -----

require 'spec_helper'

describe Order do
  let(:order) { Order.new(:line_items => line_items) }

  describe '#total_cost' do
    subject { order.total_cost }

    context 'with line items costing 2 and 3 and 4 (unpaid)' do
      before { order.save! }

      let(:line_items) {
        [
          LineItem.new(:cost => 2, :paid => true),
          LineItem.new(:cost => 3, :paid => true),
          LineItem.new(:cost => 4, :paid => false),
        ] 
      }
      it { should == 5 }
    end

    context 'with no line items' do
      let(:line_items) { [] }
      it { should == 0 }
    end
  end
end

I like the idea of creating a gem that allows you to do in memory AREL queries. If it could also allow you to test ActiveRecord callbacks without saving that would be nice as well.

That sounds pretty challenging but if it passed the AREL test suite (I haven't looked into it but I assume it has one and that it is reasonably complicated), then it would seem reasonable enough to run your unit tests against that instead of the database.