Are there computed fields in MongoDB? [duplicate]

Question

Are there computed fields in MongoDB?

In SQL I can do:

SELECT A+B AS C FROM MYTABLE WHERE C>10

Can I do something similar in MongoDB?

UPDATE

I did with projection:

db.segments.aggregate(
   [
      {
         $project: {
            "_id": 1,
            numberOfRestrictions: { $size: "$Speed Restrictions" }
         }
      }
   ]
)

and it works.

Unfortunately, further pipelining does not:

db.segments.aggregate(
   [
      {
         $project: {
            "_id": 1,
            numberOfRestrictions: { $size: "$Speed Restrictions" }
         }
      },
      {
        $match: {
            "numberOfRestrictions": {
                "$gt": 1
            }
        }
      }
   ]
)

Latter causes error

The argument to $size must be an Array, but was of type: EOO

Answer 1

Yes. It is called aggregation pipelines. Specifically, you need to use a $project stage to create the C field, and then use a $match stage to find all documents which match the criterion.

Example

Let's create some documents first:

for( var i = 1; i <=10; i++){
  db.agg.insert({a:i,b:i})
}

Which results in a collection looking like this:

> db.agg.find()
{ "_id" : ObjectId("56c1b5561a3b578f37a99d4d"), "a" : 1, "b" : 1 }
{ "_id" : ObjectId("56c1b5561a3b578f37a99d4e"), "a" : 2, "b" : 2 }
{ "_id" : ObjectId("56c1b5561a3b578f37a99d4f"), "a" : 3, "b" : 3 }
{ "_id" : ObjectId("56c1b5561a3b578f37a99d50"), "a" : 4, "b" : 4 }
{ "_id" : ObjectId("56c1b5561a3b578f37a99d51"), "a" : 5, "b" : 5 }
{ "_id" : ObjectId("56c1b5561a3b578f37a99d52"), "a" : 6, "b" : 6 }
{ "_id" : ObjectId("56c1b5561a3b578f37a99d53"), "a" : 7, "b" : 7 }
{ "_id" : ObjectId("56c1b5561a3b578f37a99d54"), "a" : 8, "b" : 8 }
{ "_id" : ObjectId("56c1b5561a3b578f37a99d55"), "a" : 9, "b" : 9 }
{ "_id" : ObjectId("56c1b5561a3b578f37a99d56"), "a" : 10, "b" : 10 }

Finding all documents for which C > 10

db.agg.aggregate([
    // You need to include all fields you want to have
    // in the resulting document within the $project stage
    { "$project":{ a:1, b:1, c:{ "$add": ["$a","$b"] }}},
    { "$match":{ c:{ "$gt":10 }}}
])

Returns the following result:

{ "_id" : ObjectId("56c1b5561a3b578f37a99d52"), "a" : 6, "b" : 6, "c" : 12 }
{ "_id" : ObjectId("56c1b5561a3b578f37a99d53"), "a" : 7, "b" : 7, "c" : 14 }
{ "_id" : ObjectId("56c1b5561a3b578f37a99d54"), "a" : 8, "b" : 8, "c" : 16 }
{ "_id" : ObjectId("56c1b5561a3b578f37a99d55"), "a" : 9, "b" : 9, "c" : 18 }
{ "_id" : ObjectId("56c1b5561a3b578f37a99d56"), "a" : 10, "b" : 10, "c" : 20 }