Added concern for a faster "cache_key" methodfaster-cache-keys

This concern provides an optimized/simplified version of the "cache_key"
method. This method is about 9 times faster than the default "cache_key"
method.

The produced cache keys _are_ different from the previous ones but this
is worth the performance improvement. To showcase this I set up a
benchmark (using benchmark-ips) that compares FasterCacheKeys#cache_key
with the regular cache_key. The output of this benchmark was:

    Calculating -------------------------------------
               cache_key     4.825k i/100ms
          cache_key_fast    21.723k i/100ms
    -------------------------------------------------
               cache_key     59.422k (± 7.2%) i/s -    299.150k
          cache_key_fast    543.243k (± 9.2%) i/s -      2.694M

    Comparison:
          cache_key_fast:   543243.4 i/s
               cache_key:    59422.0 i/s - 9.14x slower

To see the impact on real code I applied these changes and benchmarked
Issue#referenced_merge_requests. For an issue referencing 10 merge
requests these changes shaved off between 40 and 60 milliseconds.

3 files changed, 18 insertions, 0 deletions

diff --git a/app/models/concerns/faster_cache_keys.rb b/app/models/concerns/faster_cache_keys.rb
new file mode 100644
index 00000000000..5b14723fa2d
--- /dev/null
+++ b/app/models/concerns/faster_cache_keys.rb
@@ -0,0 +1,16 @@
+module FasterCacheKeys
+  # A faster version of Rails' "cache_key" method.
+  #
+  # Rails' default "cache_key" method uses all kind of complex logic to figure
+  # out the cache key. In many cases this complexity and overhead may not be
+  # needed.
+  #
+  # This method does not do any timestamp parsing as this process is quite
+  # expensive and not needed when generating cache keys. This method also relies
+  # on the table name instead of the cache namespace name as the latter uses
+  # complex logic to generate the exact same value (as when using the table
+  # name) in 99% of the cases.
+  def cache_key
+    "#{self.class.table_name}/#{id}-#{read_attribute_before_type_cast(:updated_at)}"
+  end
+end
diff --git a/app/models/issue.rb b/app/models/issue.rb
index 11f734cfc6d..d62ffb21467 100644
--- a/app/models/issue.rb
+++ b/app/models/issue.rb
@@ -7,6 +7,7 @@ class Issue < ActiveRecord::Base
   include Sortable
   include Taskable
   include Spammable
+  include FasterCacheKeys
 
   DueDateStruct = Struct.new(:title, :name).freeze
   NoDueDate     = DueDateStruct.new('No Due Date', '0').freeze
diff --git a/app/models/note.rb b/app/models/note.rb
index b6b2ac6aa42..ddcd7f9d034 100644
--- a/app/models/note.rb
+++ b/app/models/note.rb
@@ -5,6 +5,7 @@ class Note < ActiveRecord::Base
   include Mentionable
   include Awardable
   include Importable
+  include FasterCacheKeys
 
   # Attribute containing rendered and redacted Markdown as generated by
   # Banzai::ObjectRenderer.