Merge branch 'faster-cache-keys' into 'master'

Optimize "cache_key" using a concern

## What does this MR do?

This MR adds a concern (used by Issue and Note) that provides an optimized version of Rails' `cache_key` method. See 77c8520e2ecd70520757aed0fbdf434643b60234 for more details.

## Are there points in the code the reviewer needs to double check?

No, though a spell check would be appreciated.

## Why was this MR needed?

When loading a lot of data from Redis (e.g. an issue with lots of notes) quite a large amount of time is spent in generating cache keys. This is due to multiple reasons such as:

* Rails trying to figure out if it should use `updated_at` or `updated_on` using somewhat inefficient code
* Rails relying on pluralization logic to figure out how to generate a cache namespace using a model name
* Rails calling a whole bunch of methods in general in the process of generating cache keys

In short, Rails is trying to cater to every possible use case, at the cost of performance.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/13651 is not directly related but I ran into this `cache_key` problem when looking into said issue.

See merge request !5715

1 files changed, 17 insertions, 0 deletions

diff --git a/spec/models/concerns/faster_cache_keys_spec.rb b/spec/models/concerns/faster_cache_keys_spec.rb
new file mode 100644
index 00000000000..8d3f94267fa
--- /dev/null
+++ b/spec/models/concerns/faster_cache_keys_spec.rb
@@ -0,0 +1,17 @@
+require 'spec_helper'
+
+describe FasterCacheKeys do
+  describe '#cache_key' do
+    it 'returns a String' do
+      # We're using a fixed string here so it's easier to set an expectation for
+      # the resulting cache key.
+      time = '2016-08-08 16:39:00+02'
+      issue = build(:issue, updated_at: time)
+      issue.extend(described_class)
+
+      expect(issue).to receive(:id).and_return(1)
+
+      expect(issue.cache_key).to eq("issues/1-#{time}")
+    end
+  end
+end