diff --git a/AUTHORS b/AUTHORS
index 21a714c..ce0aadd 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -14,3 +14,5 @@ Christina Roberts <christina@edx.org>
 Calen Pennington <calen.pennington@gmail.com>
 Ed Zarecor <ed@edx.org>
 Jay Zoldak <jzoldak@edx.org>
+Jim Abramson <jsa@edx.org>
+Greg Price <gprice@edx.org>
diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index c288568..17a6233 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -5,6 +5,9 @@ These are notable changes in cs_comments_service.  This is a rolling list of cha
 in roughly chronological order, most recent first.  Add your entries at or near
 the top.  Include a label indicating the component affected.
 
+**api:** Add the ability to filter by commentable id to more endpoints
+  (in particular, /threads).
+
 **models:** added a new sorting key and index to `Comment` documents, removing the need
 for certain hierarchical db queries.  Also added a copy of the author's username 
 to `Comment` and `CommentThread` models, to reduce the number db queries.  
diff --git a/lib/helpers.rb b/lib/helpers.rb
index 1e4d81d..70fa82f 100644
--- a/lib/helpers.rb
+++ b/lib/helpers.rb
@@ -138,6 +138,10 @@ helpers do
       end
     end
 
+    if params[:commentable_ids]
+      comment_threads = comment_threads.in(commentable_id: params[:commentable_ids].split(","))
+    end
+
     sort_key_mapper = {
       "date" => :created_at,
       "activity" => :last_activity_at,
diff --git a/models/content.rb b/models/content.rb
index 7eb7752..0cd3a40 100644
--- a/models/content.rb
+++ b/models/content.rb
@@ -14,6 +14,7 @@ class Content
 
   index({comment_thread_id: 1, sk: 1}, {sparse: true})
   index({comment_thread_id: 1, endorsed: 1}, {sparse: true})
+  index({commentable_id: 1}, {sparse: true, background: true})
 
   before_save :set_username
   def set_username
diff --git a/scripts/db/migrate-005-update-indexes.js b/scripts/db/migrate-005-update-indexes.js
new file mode 100644
index 0000000..f330959
--- /dev/null
+++ b/scripts/db/migrate-005-update-indexes.js
@@ -0,0 +1 @@
+db.contents.ensureIndex({commentable_id: 1}, {sparse: true, background: true})
diff --git a/scripts/db/revert-migrate-005-update-indexes.js b/scripts/db/revert-migrate-005-update-indexes.js
new file mode 100644
index 0000000..4201b59
--- /dev/null
+++ b/scripts/db/revert-migrate-005-update-indexes.js
@@ -0,0 +1 @@
+db.contents.dropIndex({commentable_id: 1})
diff --git a/spec/api/comment_thread_spec.rb b/spec/api/comment_thread_spec.rb
index 480b5d8..c9a7e6f 100644
--- a/spec/api/comment_thread_spec.rb
+++ b/spec/api/comment_thread_spec.rb
@@ -26,6 +26,24 @@ describe "app" do
             res["course_id"].should == "abc"
           }
         end
+        it "returns only threads where course id and commentable id match" do
+          @threads["t1"].course_id = "course1"
+          @threads["t1"].commentable_id = "commentable1"
+          @threads["t1"].save!
+          @threads["t2"].course_id = "course1"
+          @threads["t2"].commentable_id = "commentable2"
+          @threads["t2"].save!
+          @threads["t3"].course_id = "course1"
+          @threads["t3"].commentable_id = "commentable3"
+          @threads["t3"].save!
+          @threads["t4"].course_id = "course2"
+          @threads["t4"].commentable_id = "commentable1"
+          @threads["t4"].save!
+          rs = thread_result course_id: "course1", commentable_ids: "commentable1,commentable3"
+          rs.length.should == 2
+          check_thread_result(nil, @threads["t3"], rs[0])
+          check_thread_result(nil, @threads["t1"], rs[1])
+        end
         it "returns only threads where course id and group id match" do
           @threads["t1"].course_id = "omg"
           @threads["t1"].group_id = 100