Discussion Forums Data

Discussions in edX are stored in a MongoDB database as collections of JSON documents.

The primary collection holding all posts and comments written by users is contents. There are two types of objects stored here, though they share much of the same structure. A CommentThread represents a comment that opens a new thread – usually a student question of some sort. A Comment is a reply in the conversation started by a CommentThread.

Shared Attributes

The attributes that Comment and CommentThread objects share are listed below.

_id

The 12-byte MongoDB unique ID for this collection. Like all MongoDB IDs, they are monotonically increasing and the first four bytes are a timestamp.

_type

CommentThread or Comment depending on the type of object.

anonymous

If true, this Comment or CommentThread will show up as written by anonymous, even to those who have moderator privileges in the forums.

anonymous_to_peers

The idea behind this field was that anonymous_to_peers = true would make the the comment appear anonymous to your fellow students, but would allow the course staff to see who you were. However, that was never implemented in the UI, and only anonymous is actually used. The anonymous_to_peers field is always false.

at_position_list

No longer used. Child comments (replies) are just sorted by their created_at timestamp instead.

author_id

The user who wrote this. Corresponds to the user IDs we store in our MySQL database as auth_user.id

body

Text of the comment in Markdown. UTF-8 encoded.

course_id

The full course_id of the course that this comment was made in, including org and run. This value can be seen in the URL when browsing the courseware section. Example: BerkeleyX/Stat2.1x/2013_Spring

created_at

Timestamp in UTC. Example: ISODate(“2013-02-21T03:03:04.587Z”)

updated_at

Timestamp in UTC. Example: ISODate(“2013-02-21T03:03:04.587Z”)

votes

Both CommentThread and Comment objects support voting. Comment objects that are replies to other comments still have this attribute, even though there is no way to actually vote on them in the UI. This attribute is a dictionary that has the following inside:

• up = list of User IDs that up-voted this comment or thread.
• down = list of User IDs that down-voted this comment or thread (no longer used).
• up_count = total upvotes received.
• down_count = total downvotes received (no longer used).
• count = total votes cast.
• point = net vote, now always equal to up_count.

A user only has one vote per Comment or CommentThread. Though it’s still written to the database, the UI no longer displays an option to downvote anything.

CommentThread

The following fields are specific to CommentThread objects. Each thread in the forums is represented by one CommentThread.

closed

If true, this thread was closed by a forum moderator/admin.

comment_count

The number of comment replies in this thread. This includes all replies to replies, but does not include the original comment that started the thread. So if we had:

CommentThread: "What's a good breakfast?"
  * Comment: "Just eat cereal!"
  * Comment: "Try a Loco Moco, it's amazing!"
    * Comment: "A Loco Moco? Only if you want a heart attack!"
    * Comment: "But it's worth it! Just get a spam musubi on the side."

In that exchange, the comment_count for the CommentThread is 4.

commentable_id

We can attach a discussion to any piece of content in the course, or to top level categories like “General” and “Troubleshooting”. When the commentable_id is a high level category, it’s specified in the course’s policy file. When it’s a specific content piece (e.g. 600x_l5_p8, meaning 6.00x, Lecture Sequence 5, Problem 8), it’s taken from a discussion module in the course.

last_activity_at

Timestamp in UTC indicating the last time there was activity in the thread (new posts, edits, etc). Closing the thread does not affect the value in this field.

tags_array

Meant to be a list of tags that were user definable, but no longer used.

title

Title of the thread, UTF-8 string.

Comment

The following fields are specific to Comment objects. A Comment is a reply to a CommentThread (so an answer to the question), or a reply to another Comment (a comment about somebody’s answer). It used to be the case that Comment replies could nest much more deeply, but we later capped it at just these three levels (question, answer, comment) much in the way that StackOverflow does.

endorsed

Boolean value, true if a forum moderator or instructor has marked that this Comment is a correct answer for whatever question the thread was asking. Exists for Comments that are replies to other Comments, but in that case endorsed is always false because there’s no way to endorse such comments through the UI.

comment_thread_id

What CommentThread are we a part of? All Comment objects have this.

parent_id

The parent_id is the _id of the Comment that this comment was made in reply to. Note that this only occurs in a Comment that is a reply to another Comment; it does not appear in a Comment that is a reply to a CommentThread.

parent_ids

The parent_ids attribute appears in all Comment objects, and contains the _id of all ancestor comments. Since the UI now prevents comments from being nested more than one layer deep, it will only ever have at most one element in it. If a Comment has no parent, it’s an empty list.