Suggestion: Return early from collection partial rendering if blank. (#…

…560)

* Suggestion: Return early from collection partial rendering if blank.

Co-authored-by: David Heinemeier Hansson <david@hey.com>

lib/jbuilder/jbuilder_template.rb

@@ -155,21 +155,30 @@ def _render_partial_with_options(options)
         ::Kernel.raise ::NotImplementedError, "The `:spacer_template' option is not supported in collection rendering."
       end
 
-      results = CollectionRenderer
-        .new(@context.lookup_context, options) { |&block| _scope(&block) }
-        .render_collection_with_partial(collection, partial, @context, nil)
-
-      array! if results.respond_to?(:body) && results.body.nil?
+      if collection.present?
+        results = CollectionRenderer
+          .new(@context.lookup_context, options) { |&block| _scope(&block) }
+          .render_collection_with_partial(collection, partial, @context, nil)
+
+        array! if results.respond_to?(:body) && results.body.nil?
+      else
+        array!
+      end
     elsif as && options.key?(:collection) && !CollectionRenderer.supported?
       # For Rails <= 5.2:
       as = as.to_sym
       collection = options.delete(:collection)
-      locals = options.delete(:locals)
-      array! collection do |member|
-        member_locals = locals.clone
-        member_locals.merge! collection: collection
-        member_locals.merge! as => member
-        _render_partial options.merge(locals: member_locals)
+
+      if collection.present?
+        locals = options.delete(:locals)
+        array! collection do |member|
+          member_locals = locals.clone
+          member_locals.merge! collection: collection
+          member_locals.merge! as => member
+          _render_partial options.merge(locals: member_locals)
+        end
+      else
+        array!
       end
     else
       _render_partial options

test/jbuilder_template_test.rb

@@ -98,10 +98,12 @@ class JbuilderTemplateTest < ActiveSupport::TestCase
   end
 
   test "nil partial collection by name" do
+    Jbuilder::CollectionRenderer.expects(:new).never
     assert_equal [], render('json.partial! "post", collection: @posts, as: :post', posts: nil)
   end
 
   test "nil partial collection by options" do
+    Jbuilder::CollectionRenderer.expects(:new).never
     assert_equal [], render('json.partial! partial: "post", collection: @posts, as: :post', posts: nil)
   end
 
@@ -113,7 +115,13 @@ class JbuilderTemplateTest < ActiveSupport::TestCase
     assert_equal "Pavel", result[5]["author"]["first_name"]
   end
 
+  test "empty array of partials from empty collection" do
+    Jbuilder::CollectionRenderer.expects(:new).never
+    assert_equal [], render('json.array! @posts, partial: "post", as: :post', posts: [])
+  end
+
   test "empty array of partials from nil collection" do
+    Jbuilder::CollectionRenderer.expects(:new).never
     assert_equal [], render('json.array! @posts, partial: "post", as: :post', posts: nil)
   end
 
@@ -126,10 +134,17 @@ class JbuilderTemplateTest < ActiveSupport::TestCase
   end
 
   test "empty array of partials under key from nil collection" do
+    Jbuilder::CollectionRenderer.expects(:new).never
     result = render('json.posts @posts, partial: "post", as: :post', posts: nil)
     assert_equal [], result["posts"]
   end
 
+  test "empty array of partials under key from an empy collection" do
+    Jbuilder::CollectionRenderer.expects(:new).never
+    result = render('json.posts @posts, partial: "post", as: :post', posts: [])
+    assert_equal [], result["posts"]
+  end
+
   test "object fragment caching" do
     render(<<-JBUILDER)
       json.cache! "cache-key" do
@@ -293,6 +308,7 @@ class JbuilderTemplateTest < ActiveSupport::TestCase
 
   if JbuilderTemplate::CollectionRenderer.supported?
     test "returns an empty array for an empty collection" do
+      Jbuilder::CollectionRenderer.expects(:new).never
       result = render('json.array! @posts, partial: "post", as: :post, cached: true', posts: [])
 
       # Do not use #assert_empty as it is important to ensure that the type of the JSON result is an array.