Writing tests with RSpec for Redis with Rails

Question

I have a model class that caches data in redis. The first time I call a method on the model, it computes a JSON/Hash value and stores it in Redis. Under certain circumstances I 'flush' that data and it gets recomputed on the next call.

Here's the code snippet similar to the one I use to store the data in Redis:

def cache_data
  self.data_values = data_to_cache
  REDIS.set(redis_key,ActiveSupport::JSON.encode(self.data_values))
  REDIS.get(redis_key) 
end

def data_to_cache
  # generate a hash of values to return
end

How should I unit test this code? I use RSpec and Capybara. I also use Cucumber and Capabara for integration testing if that helps.

Grummet

Answer 1

I like to have redis running while the tests are running. Redis, unlike e.g. postgres, is extremely fast and doesn't slow down test run time noticeably.

Just make sure you call REDIS.flush in a before(:each) block, or the corresponding cucumber hook.

You can test data_to_cache independently of redis, but unless you can fully trust the redis driver you're using and the contract it provides, it's safer to actually test cache_data (and the corresponding cache fetch method) live. That also allows you to switch to a different redis driver (or to a different fast KV store) without a wholesale rewrite of your tests.

Renown

I believe this is now REDIS.flushdb – Trephine

Answer 2

First of all add the below code in the spec_helper.rb so you'll be sure that the tests will run on any machine even if the redis server is not installed (make sure to add to your gemfile mock_redis under test scopes:

redis_instance = MockRedis.new
Redis.stub(:new).returns(redis_instance)
Redis::Store.stub(:new).returns(redis_instance)

After that I would test:

1. The data written to REDIS is the expected data
2. A sequence of cache_data, flush_data, cache_data calls the data_to_cache twice

Specialty

So you'd recommend having redis running and actually testing the interaction of the data with redis. This puts a dependency of having redis running while tests are being run (which I'm not against in general). But it adds a dependency that I can see some might recommend handling with stubs. – Grummet

Nope. That's MockRedis is doing ... it stores the data in memory so you don't need a redis server. I said "so you'll be sure that the tests will run on any machine even if the redis server is not installed". – Specialty

One thing you might want to add to the top of this answer, which was very helpful is: add gem 'mock_redis' to your Gemfile and add this line to your spec_helper as well require 'mock_redis' – Swagman

I have "undefined method `stubs' for Redis:Class (NoMethodError)". :( – Ellingson

@TiSir - Try calling .stub(:new).and_return(redis_instance) instead. – Preceptive

I think this is a bad choice to mock redis, which is super fast. A better option is to spin up a test redis server and use it instead. I created a gem that facilitate that: rubygems.org/gems/redis_test – Stuckey

I agree that running the tests without needing a redis server is valuable. I'd suggest setting the Resque redis instance to a MockRedis instance instead of stubbing it out though. Resque.redis = MockRedis.new – Banking

There are bugs in mock_redis even in 2019. It's a bad choice to use it :( – Krisha

Answer 3

Both the answers are outdated in May 2022. Following is the updated version with a sample test.

add this to Gemfile

gem 'mock_redis'

Install the mock_redis gem using

bundle install

In your spec_helper.rb or rails_helper.rb add this

require 'mock_redis'

RSpec.configure do |config|
  '''
  Creating a stub/mock Redis for running the tests without requiring an actual Redis server. This will store the Redis data in memory instead of the Redis server. 
  '''
  config.before(:each) do
    mock_redis = MockRedis.new
    allow(Redis).to receive(:new).and_return(mock_redis)
  end
end

This would make sure to mock Redis for all your test.

Let's say you have a module (module_in_my_app.rb) that uses Redis like so

module ModuleInMyApp
    extend self
    @@redis = Redis.new(url: "redis://#{ENV["REDIS_URL"]}")

    def get_or_create_my_key(keys, default_value)
       return @@redis.set(key, value)
    end

    def get_my_key(key)
       return @@redis.get(key)
    end
end

You can use it like so in your spec (module_in_my_app_spec.rb)

describe "test my redis module" do
    it "should set the key" do
        ModuleInMyApp.set_my_key("US", "United States of America")

        redis_var = class_variable_get(:@@redis)
        expect(redis_var.get("US")).to eq("United States of America")
    end

    it "should get the key" do
        mock_redis = Redis.new
        mock_redis.set("CA", "Canada")
        ModuleInMyApp.class_variable_set(:@@redis, mock_redis)

        actual_result = ModuleInMyApp.get_my_key("CA")
        expect(actual_result).to eq("Canada")
    end
end

Badenpowell