Foo.joins(:bar).includes(:bar) should result in a single query with :bar as a join. Related: #1873.

activerecord/lib/active_record/relation.rb

@@ -408,7 +408,17 @@ def scope_for_create
     end
 
     def eager_loading?
-      @should_eager_load ||= (@eager_load_values.any? || (@includes_values.any? && references_eager_loaded_tables?))
+      @should_eager_load ||=
+        @eager_load_values.any? ||
+        @includes_values.any? && (joined_includes_values.any? || references_eager_loaded_tables?)
+    end
+
+    # Joins that are also marked for preloading. In which case we should just eager load them.
+    # Note that this is a naive implementation because we could have strings and symbols which
+    # represent the same association, but that aren't matched by this. Also, we could have
+    # nested hashes which partially match, e.g. { :a => :b } & { :a => [:b, :c] }
+    def joined_includes_values
+      @includes_values & @joins_values
     end
 
     def ==(other)

activerecord/test/cases/associations/eager_test.rb

@@ -1040,4 +1040,12 @@ def test_preloading_polymorphic_with_custom_foreign_type
     }
     assert_no_queries { assert_equal groucho, sponsor.thing }
   end
+
+  def test_joins_with_includes_should_preload_via_joins
+    post = assert_queries(1) { Post.includes(:comments).joins(:comments).order('posts.id desc').to_a.first }
+
+    assert_queries(0) do
+      assert_not_equal 0, post.comments.to_a.count
+    end
+  end
 end