Redis -- undefined method `SMEMBERS' for nil:NilClass - ruby-on-rails

Question

I'm building a Rails shopping cart app using Redis. Everything works fine until I try to view my cart, at which point I get this error.
NoMethodError in CartsController#show
undefined method `SMEMBERS' for nil:NilClass
Extracted source (around line #6):
5 def show
6 cart_ids = $redis.SMEMBERS current_user_cart
7 #order_items = current_order.order_items
8 end
The beginning of the full trace:
app/controllers/carts_controller.rb:6:in `show'
actionpack (4.2.0) lib/action_controller/metal/implicit_render.rb:4:in `send_action'
actionpack (4.2.0) lib/abstract_controller/base.rb:198:in `process_action'
actionpack (4.2.0) lib/action_controller/metal/rendering.rb:10:in `process_action'
actionpack (4.2.0) lib/abstract_controller/callbacks.rb:20:in `block in process_action'
activesupport (4.2.0) lib/active_support/callbacks.rb:117:in `call'
activesupport (4.2.0) lib/active_support/callbacks.rb:117:in `call'
activesupport (4.2.0) lib/active_support/callbacks.rb:169:in `block in halting'
activesupport (4.2.0) lib/active_support/callbacks.rb:234:in `call'
...and thence through a bunch more behind-the-scenes stuff I haven't touched, mostly activesupport, activerecord, and railties.
Redis-server is installed and responsive; I have tested it with redis-cli.
Config/initializers/redis.rb:
uri = URI.parse(ENV["REDISTOGO_URL"])
REDIS = Redis.new(:url => uri)
App/controllers/carts_controller.rb:
class CartsController < ApplicationController
before_action :authenticate_user!
def show
cart_ids = $redis.SMEMBERS current_user_cart
#order_items = current_order.order_items
end
def add
$redis.SADD current_user_cart, params[:product_id]
render json: current_user.cart_count, status: 200
end
def remove
$redis.SREM current_user_cart, params[:product_id]
render json: current_user.cart_count, status: 200
end
private
def current_user_cart
"cart#{current_user.id}"
end
end
The worst part is that I had this problem last week, fixed it, and neglected to write down what the fix was! Anybody have any leads?

Answer 1

You are using uninitialized global variable $redis (look at your initializers/redis.rb where you declared constant REDIS but not $redis)
Ruby is case-sensitive language, the SMEMBERS and smembers are two different methods. So, I think you should use smembers method.

Answer 2

Turned out I had two versions of redis installed. I don't know if that was causing the problem, but when I uninstalled both and reinstalled the gem, the problem disappeared!