Pardon the atrocious grammar; there is a point!

I recently had a tricky time formulating a particular query in MongoDB. As you probably know, MongoDB has a number of query operators to use. It’s got stuff like $in, $nin, $or, and others, but no $and. Normally, you don’t need something like $and, since the capability is there implicitly; you just list off all your conditions on your different document fields, and MongoDB finds all the documents that satisfy them all. However, there is a situation that I’ve come across lately where something like $and would be very helpful; declaring multiple conditions on a single field.

Let’s set up some test data to illustrate a scenario:

db.people.save({name: "Xavier", friends: ["Bob","Fred","Sam"]});
db.people.save({name: "Yorick", friends: ["Elmer","Alice"]});
db.people.save({name: "Zelda", friends: ["David","Erica","Walt"]});

Say I have one group of interesting people:

["Alice","Bob","Charlie"]

And then I have another group of interesting people:

["David","Erica","Fred"]

Now I want to find everybody in my database that is friends with at least one person from each of these groups. That is to say, I would want to find Xavier, but not Yorick or Zelda. I want to specify two conditions on my “friends” field. (This isn’t the real situation I was dealing with, but I’ll spare you the scientific background.)

You might think, “That’s easy, you just use the $in query operator!”. Well, that’s what I thought. There’s a problem with that, though. Conceptually, you want a query like this, to take advantage of MongoDB’s implicit $and for conditions:

db.people.find({
    friends: {$in: ["Alice","Bob","Charlie"]},
    friends: {$in: ["David","Erica","Fred"]}
});

However, this is going to find Xavier and Zelda!

{ "_id": ObjectId("4d11deb1a95769443d8dd7c4"),
  "name": "Xavier",
  "friends": [ "Bob", "Fred", "Sam" ] }
{ "_id": ObjectId("4d11deb2a95769443d8dd7c6"),
  "name": "Zelda",
  "friends": [ "David", "Erica", "Walt" ] }

You’ve duplicated your keys! The implicit $and only really works on different fields; you can’t combine conditions on the same field this way. Here your’re actually only looking for friends of David, Erica, or Fred instead of finding people that could bridge these two cliques. It seems that the last condition declared “wins”. Note that we didn’t find Yorick in that last query; he’s not friends with David, Erica, or Fred.

My next thought was to try $all:

db.people.find({
    friends: {
        $all: [
            {$in: ["Alice","Bob","Charlie"]},
            {$in: ["David","Erica","Fred"]}
        ]
    }
});

That doesn’t work either; it looks like $all only accepts a list of values, not additional conditions.

So how do you ask this query? You need to be a little more tricky in your formulation. After a quick fling with a truth table, here’s my solution:

db.people.find({
    $nor: [
        {friends: {$not: {$in: ["Alice","Bob","Charlie"]}}},
        {friends: {$not: {$in: ["David","Erica","Fred"]}}}
    ]
});

That gives you what you want

{ "_id": ObjectId("4d11deb1a95769443d8dd7c4"),
  "name": "Xavier",
  "friends": [ "Bob", "Fred", "Sam" ] }

It looks confusing at first, but just step through it. You are saying, “find everybody that isn’t not friends with Alice, Bob, or Charlie, and isn’t not friends with David, Erica, or Fred”. The $nor gives you the $and capabilities (albeit negated), and the $nots reverse the meanings of your tests to be compatible with $nor. It would be easier if MongoDB actually had an $and operator, but this will do in a pinch.

Since discovering this trick, I’ve actually had to use it a number of times, particularly when dealing with array fields that contain objects instead of plain values.

Double negatives can be handy, no matter what your middle school English teacher says.