elder.rb

class Elder < ActiveRecord::Base
  def self.increment_distribution_count(user_id)
    self.find_by_user_id(user_id).increment!(:distribution_count)
  end
end

elder_spec.rb

# -*- coding: utf-8 -*-
require File.dirname(__FILE__) + '/../spec_helper'

describe Elder do
  before(:all) do
    Elder.destroy_all
    @elder = Factory(:elder)
  end
  
  it "should increment distribution count" do
    lambda { Elder.increment_distribution_count(@elder.user_id) }.should change(@elder, :distribution_count).by(1)
  end
  
end

error

Elder
should increment distribution count
distribution_count should have been changed by 1, but was changed by 0
./spec/models/elder_spec.rb:11 
9  
10  it "should increment distribution count" do
11    lambda { Elder.increment_distribution_count(@elder.user_id) }.should change(@elder, :distribution_count).by(1)
12  end
13