How to check whether each item in an array exists or not

Question

I'm trying to create a watch list where users can watch items. I was trying to create it by adding a watchlist field to my users collection. The watchlist would be an array of IDs corresponding to other items.

Users Collection:

id: ObjectId
name: string
watchlist: array i.e. [9872, 342, 4545, 234, 8745]

The question I have is related to querying this structure. I want to be able to write a query where I pass in a user id and an array of ~20 IDs and check which of those IDs the user watches (i.e. which of them exists in the watchlist field for that user).

I tried this initially:

db.users.find({
  _id: 507c35dd8fada716c89d0013,
  watchlist: { $in: [342, 999, 8745, etc...] }
});

But this gives me the list of users that contain any of those watchlist items, which is not what I want. What I actually want is a response containing an array like this:

{
  id: 342,
  exists: true
},
{
  id: 999,
  exists: false
},
{
  id: 8745,
  exists: true
}

I'd even be ok just getting an array of items that match:

{
  _id: 507c35dd8fada716c89d0013,
  watching: [342, 8745]
}

Is this doable, or would I be better off moving the watchlist to a separate collection with users as an array? (My concern with the latter approach is that a user will only watch a few hundred items, but tens of thousands of users could potentially watch the same item.)

Try reversing it. Try storing list of user ids on each watchlist and query watchlist for existence of a given userId — Raja Rao

s7vr s7vr · Accepted Answer · 2017-01-16T20:16:15

You can easily achieve the second output using $setIntersection operator.

db.users.aggregate(
   [ {$match:{"_id": 507c35dd8fada716c89d0013}},
     { $project: { "watching": { $setIntersection: [ "$watchlist", [ 342, 999, 8745 ] ] } } }
   ]
)

How to check whether each item in an array exists or not

1 Answers