Add CacheMethod concern

Based on caching code from Repository. Since repository has some
specific cases and in future will be changed even more I decided to
create separate class specifically for AR model needs

* cache return value to the instance variable and cache store
* method to clear cached value
* allow access to uncached version of method

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

2 files changed, 109 insertions, 0 deletions

diff --git a/app/models/concerns/cache_method.rb b/app/models/concerns/cache_method.rb
new file mode 100644
index 00000000000..dca411bb644
--- /dev/null
+++ b/app/models/concerns/cache_method.rb
@@ -0,0 +1,65 @@
+# Concern for caching method into instance variable and redis cache
+module CacheMethod
+  extend ActiveSupport::Concern
+
+  module ClassMethods
+    # Wraps around the given method and caches its output in Redis and an instance
+    # variable.
+    #
+    # This only works for methods that do not take any arguments.
+    def cache_method(name)
+      original = :"_uncached_#{name}"
+
+      alias_method(original, name)
+
+      define_method(name) do
+        cache_method_output(name) { __send__(original) }
+      end
+    end
+  end
+
+  # Expires the caches of a specific set of methods
+  def expire_method_caches(methods)
+    methods.each do |key|
+      cache.delete(cache_key(key))
+
+      ivar = cache_instance_variable_name(key)
+
+      remove_instance_variable(ivar) if instance_variable_defined?(ivar)
+    end
+  end
+
+  private
+
+  # Caches the supplied block both in a cache and in an instance variable.
+  #
+  # The cache key and instance variable are named the same way as the value of
+  # the `key` argument.
+  #
+  # This method will return `nil` if the corresponding instance variable is also
+  # set to `nil`. This ensures we don't keep yielding the block when it returns
+  # `nil`.
+  #
+  # name - The name of the key to cache the data in.
+  def cache_method_output(name, &block)
+    ivar = cache_instance_variable_name(name)
+
+    if instance_variable_defined?(ivar)
+      instance_variable_get(ivar)
+    else
+      instance_variable_set(ivar, cache.fetch(cache_key(name), &block))
+    end
+  end
+
+  def cache_instance_variable_name(key)
+    :"@#{key.to_s.tr('?!', '')}"
+  end
+
+  def cache
+    @cache ||= Rails.cache
+  end
+
+  def cache_key(key)
+    "#{self.class.name.tableize}:#{id}:#{key}"
+  end
+end
diff --git a/spec/models/concerns/cache_method_spec.rb b/spec/models/concerns/cache_method_spec.rb
new file mode 100644
index 00000000000..e44bb146fff
--- /dev/null
+++ b/spec/models/concerns/cache_method_spec.rb
@@ -0,0 +1,44 @@
+require 'spec_helper'
+
+describe CacheMethod, caching: true do
+  class TestClassForCacheMethod
+    include CacheMethod
+
+    def id
+      73
+    end
+
+    def time
+      Time.now
+    end
+
+    cache_method :time
+  end
+
+  subject { TestClassForCacheMethod.new }
+
+  let!(:time) { subject.time }
+
+  describe 'cache_method' do
+    it 'returns cached value' do
+      expect(subject.time).to eq(time)
+    end
+
+    it 'sets instance variable' do
+      expect(subject.send(:instance_variable_get, '@time')).to eq(time)
+    end
+
+    it 'sets value in the cache store' do
+      another_instance = TestClassForCacheMethod.new
+
+      expect(another_instance.time).to eq(time)
+    end
+  end
+
+  describe 'expire_method_caches' do
+    it 'expires cache and returns new value' do
+      subject.expire_method_caches(%w(time))
+      expect(subject.time).not_to eq(time)
+    end
+  end
+end