Allow fetching multiple values from the cache at once

Add a simple API for fetching a list of entries from the cache, where
any missing entries are computed by a supplied block.

Author: dasch

activesupport/CHANGELOG.md

@@ -1,3 +1,15 @@
-* No changes.
+*   Add a `fetch_multi` method to the cache stores. The method provides
+    an easy to use API for fetching multiple values from the cache.
+
+    Example:
+
+        # Calculating scores is expensive, so we only do it for posts
+        # that have been updated. Cache keys are automatically extracted
+        # from objects that define a #cache_key method.
+        scores = Rails.cache.fetch_multi(*posts) do |post|
+          calculate_score(post)
+        end
+
+    *Daniel Schierbeck*
 
 Please check [4-0-stable](https://github.com/rails/rails/blob/4-0-stable/activesupport/CHANGELOG.md) for previous changes.

activesupport/lib/active_support/cache.rb

@@ -352,6 +352,34 @@ def read_multi(*names)
         results
       end
 
+      # Fetches data from the cache, using the given keys. If there is data in
+      # the cache with the given keys, then that data is returned. Otherwise,
+      # the supplied block is called for each key for which there was no data,
+      # and the result will be written to the cache and returned.
+      #
+      # Options are passed to the underlying cache implementation.
+      #
+      # Returns an array with the data for each of the names. For example:
+      #
+      #   cache.write("bim", "bam")
+      #   cache.fetch_multi("bim", "boom") {|key| key * 2 }
+      #   #=> ["bam", "boomboom"]
+      #
+      def fetch_multi(*names)
+        options = names.extract_options!
+        options = merged_options(options)
+
+        results = read_multi(*names, options)
+
+        names.map do |name|
+          results.fetch(name) do
+            value = yield name
+            write(name, value, options)
+            value
+          end
+        end
+      end
+
       # Writes the value to the cache, with the key.
       #
       # Options are passed to the underlying cache implementation.

activesupport/test/caching_test.rb

@@ -257,6 +257,26 @@ def test_read_multi_with_expires
     assert_equal({"fu" => "baz"}, @cache.read_multi('foo', 'fu'))
   end
 
+  def test_fetch_multi
+    @cache.write('foo', 'bar')
+    @cache.write('fud', 'biz')
+
+    values = @cache.fetch_multi('foo', 'fu', 'fud') {|value| value * 2 }
+
+    assert_equal(["bar", "fufu", "biz"], values)
+    assert_equal("fufu", @cache.read('fu'))
+  end
+
+  def test_multi_with_objects
+    foo = stub(:title => "FOO!", :cache_key => "foo")
+    bar = stub(:cache_key => "bar")
+
+    @cache.write('bar', "BAM!")
+
+    values = @cache.fetch_multi(foo, bar) {|object| object.title }
+    assert_equal(["FOO!", "BAM!"], values)
+  end
+
   def test_read_and_write_compressed_small_data
     @cache.write('foo', 'bar', :compress => true)
     assert_equal 'bar', @cache.read('foo')