Add case for testing collection dependencies (#520)

README.md

@@ -292,6 +292,14 @@ json.array! @posts, partial: "posts/post", as: :post, cached: true
 json.comments @post.comments, partial: "comments/comment", as: :comment, cached: true
 ```
 
+If your collection cache depends on multiple sources (try to avoid this to keep things simple), you can name all these dependencies as part of a block that returns an array:
+
+```ruby
+json.array! @posts, partial: "posts/post", as: :post, cached: -> post { [post, current_user] }
+```
+
+This will include both records as part of the cache key and updating either of them will expire the cache.
+
 ## Formatting Keys
 
 Keys can be auto formatted using `key_format!`, this can be used to convert

test/jbuilder_template_test.rb

@@ -317,6 +317,33 @@ class JbuilderTemplateTest < ActiveSupport::TestCase
       assert_equal "Heinemeier Hansson", result[2]["author"]["last_name"]
       assert_equal "Pavel", result[5]["author"]["first_name"]
     end
+
+    test "supports the cached: ->() {} option" do
+      result = render('json.array! @posts, partial: "post", as: :post, cached: ->(post) { [post, "foo"] }', posts: POSTS)
+
+      assert_equal 10, result.count
+      assert_equal "Post #5", result[4]["body"]
+      assert_equal "Heinemeier Hansson", result[2]["author"]["last_name"]
+      assert_equal "Pavel", result[5]["author"]["first_name"]
+
+      expected = {
+        "id" => 1,
+        "body" => "Post #1",
+        "author" => {
+          "first_name" => "David",
+          "last_name" => "Heinemeier Hansson"
+        }
+      }
+
+      assert_equal expected, Rails.cache.read("post-1/foo")
+
+      result = render('json.array! @posts, partial: "post", as: :post, cached: ->(post) { [post, "foo"] }', posts: POSTS)
+
+      assert_equal 10, result.count
+      assert_equal "Post #5", result[4]["body"]
+      assert_equal "Heinemeier Hansson", result[2]["author"]["last_name"]
+      assert_equal "Pavel", result[5]["author"]["first_name"]
+    end
 
     test "raises an error on a render call with the :layout option" do
       error = assert_raises NotImplementedError do