Allow nested aggregation to accept an Aggregation instance.

elasticsearch-dsl/lib/elasticsearch/dsl/search/base_aggregation_component.rb

@@ -53,7 +53,13 @@ def method_missing(name, *args, &block)
           #
           def aggregation(*args, &block)
             @aggregations ||= AggregationsCollection.new
-            @aggregations.update args.first => Aggregation.new(*args, &block)
+
+            if block
+              @aggregations.update args.first => Aggregation.new(*args, &block)
+            else
+              name = args.shift
+              @aggregations.update name => args.shift
+            end
             self
           end
 

elasticsearch-dsl/test/unit/search_base_aggregation_component_test.rb

@@ -52,6 +52,20 @@ class ::Elasticsearch::DSL::Search::Aggregations::Dummy
 
           assert_equal 'foo', subject.to_hash[:aggregations][:inner][:dummy][:field]
         end
+
+        should "add a nested aggregation instance" do
+          nested = Elasticsearch::DSL::Search::Aggregation.new do
+            dummy field: 'foo'
+          end
+          subject.aggregation :inner, nested
+
+          assert !subject.aggregations.empty?, "#{subject.aggregations.inspect} is empty"
+
+          assert_instance_of Elasticsearch::DSL::Search::Aggregation, subject.aggregations[:inner]
+          assert_equal({ dummy: { field: 'foo' } }, subject.aggregations[:inner].to_hash)
+
+          assert_equal 'foo', subject.to_hash[:aggregations][:inner][:dummy][:field]
+        end
       end
     end
   end