Dynamic limit in MongoDB query

Question

I want to build a query for a very dynamic collection.

An example:

I have a collection like

{
  _id: ObjectId(),
  value: x
  // some other data
}

The example dataset has the values

{
  value: 1
},
{
  value: 1
},
{
  value: 2
},
{
  value: 3
},
{
  value: 3
}

As you can see the same value can be there multiple times.

But if I run the following query it only returns the first with value: 3

db.collection.aggregate([
  {
    $sort: "$value"
  },
  {
    $limit: 4
  }
])

But what I want is at least 4 documents which include all occurrences of the values in them. So I want all where value: 3.

Edit

Sorry, the question might be a bit misleading. I want to have a complete result. So all with value: 3. It is for a public transport database and the value is the departure time. So I want at least the next 30 departures, but if 30 and 31 depart at the same time, I want the 31 also.

Community · Accepted Answer · 2017-05-23 12:04:01Z

1

I now use a small python function which extends the limit as I want. Since the query returns a cursor I do not waste resources. I do not specify a limit in the query.

def extend_limit(cursor, original_limit):
    result = []

    try:
        while original_limit > 0:
            result.append(cursor.next())
            original_limit -= 1

        last_element = result[-1]

        while True:
            next_element = next(cursor)

            if last_element['value'] != next_element['value']:
                break

            result.append(next_element)

    except StopIteration:
        pass

    return result

Thanks to Adam Comerford

edited May 23, 2017 at 12:04

CommunityBot

11 silver badge

answered Jan 2, 2016 at 10:08

Jeremy Stucki

1892 silver badges13 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Adam Comerford · Accepted Answer · 2016-01-01 02:12:18Z

0

There is no need to use aggregation here, just do a normal find with a projection, a sort and a limit:

db.collection.find({}, {_id : 0, value : 1}).sort({value : 1}).limit(4)

I'd recommend that you actually query on some criteria (rather than empty in my example) and that the criteria have an appropriate index that includes the sorted field if possible (for performance reasons).

answered Jan 1, 2016 at 2:12

Adam Comerford

21.7k4 gold badges68 silver badges87 bronze badges

2 Comments

Jeremy Stucki Over a year ago

Sorry, the question might be a bit misleading. I want to have a complete result. So all with value: 3. It is for a public transport database and the value is the departure time. So I want at least the next 30 departures, but if 30 and 31 depart at the same time, I want the 31 also.

Adam Comerford Over a year ago

Then, just don't specify the limit, you will get all results (at least up to the total that fit in a batch), you just adjust the criteria for a given query to exclude results before/after a certain point in time

Collectives™ on Stack Overflow

Dynamic limit in MongoDB query

Edit

2 Answers 2

Comments

2 Comments

Hot Network Questions

Collectives™ on Stack Overflow

Edit

2 Answers 2

Comments

2 Comments

Related