CODEWITHSUNDEEP
mongodbmongoose
Richard Morgan
Richard Morgan

Reputation: 753

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

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:

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.)

Upvotes: 0

Views: 72

Answers (1)

s7vr
s7vr

Reputation: 75964

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

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

Upvotes: 0

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