MongoDB query - Filter array by odd size

Question

Is it possible to build a query filter for documents with an array with odd size instead of this?

db.records.find({'$or': [{'my_array': {'$size': 1}},
                         {'my_array': {'$size': 3}},
                         {'my_array': {'$size': 5}},
                                     ...
                         {'my_array': {'$size': 15}}]}))

The filter may not catch all the items if you stop at a certain number.

What's your MongoDB server version?

chridam
– chridam

2017-03-20 14:24:40 +00:00
Commented Mar 20, 2017 at 14:24 — chridam
– chridam, Commented Mar 20, 2017 at 14:24
The version is 3.0.7 @chridam .

gcw
– gcw

2017-03-20 14:29:26 +00:00
Commented Mar 20, 2017 at 14:29 — gcw
– gcw, Commented Mar 20, 2017 at 14:29

s7vr · Accepted Answer · 2017-03-20 16:39:47Z

5

You can use below aggregation query to $project the size and data field and use $mod function to keep documents with odd-sized array.

db.records.aggregate(
  { $project: { size:{ $size: "$my_array" }, data:"$$ROOT"} },
  { $match: { size: { $mod: [ 2, 1 ] } } }
)

answered Mar 20, 2017 at 16:39

s7vr

76.2k12 gold badges118 silver badges135 bronze badges

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

3 Comments

gcw Over a year ago

Yes, this works. I would like to understand more this $match filter using $mod. Seems that $mod[2,1] returns 2 always. How does this filter even sizes?

s7vr Over a year ago

You have to use mod as a filter. Take a look here docs.mongodb.com/manual/reference/operator/query/mod/#examples. For even size document change the remainder from 1 to 0.

gcw Over a year ago

Sorry I meant odd sizes in my previous comment. The docs link clarifies everything.

sheilak · Accepted Answer · 2017-03-20 22:57:29Z

4

You could use $where to supply a Javascript expression to query:

db.records.find({ $where: "this.my_array.length % 2 == 1" })

The JavaScript expression will be processed for each document matched by the rest of the query so for performance you should try and ensure that the rest of your query is selective.

edited Mar 20, 2017 at 22:57

answered Mar 20, 2017 at 16:34

sheilak

5,8737 gold badges36 silver badges46 bronze badges

Collectives™ on Stack Overflow

MongoDB query - Filter array by odd size

2 Answers 2

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related