Adventures in MongoDB land

Pablo’s adventures in MongoDB-land™.

Primary key on steroids

MongoDB’s primary key _id by default is an ObjectId, which is automatically added to your documents while inserting them into a collection.

But you can also set your own _id if you want.

db.my_collection.insert({ _id: '' })

Have the ability of writing your own _id can be very useful. We have an internal system which uses MongoDB as a queue database. When we move messages across the queues (collections), we preserve the original _id.

// find a message to be processed
message = db.incoming.find_one({ _id: ObjectId("52f578c1ecf69b714400004a") })

// some processing

// => ObjectId("52f578c1ecf69b714400004a")

// remove the message from the incoming queue
db.incoming.remove({ _id: message["_id"] })

The message will have the same _id no matter where it is sitting on.

MongoDB adds an index and a unique constraint on _id by default.


ObjectId is a 12-byte BSON type, constructed using:

a 4-byte value representing the seconds since the Unix epoch,

a 3-byte machine identifier,

a 2-byte process id, and

a 3-byte counter, starting with a random value.

a 4-byte value representing the seconds since the Unix epoch means created_at field for free! Which we can use to sort, search and retrieve the creation date.

Sort by _id

// sort by created_at in descending order
db.my_collection.find().sort({ _id: -1 })

// sort by created_at in ascending order
db.my_collection.find().sort({ _id: 1 })

_id to timestamp

// => ISODate("2012-10-17T20:46:22Z")

Ruby example:

    <th>Created at</th>
  <% @documents.each do |document|
      <td><%= document['_id'] %></td>
      <td><%= document['foo'] %></td>
      <td><%= document['_id'].generation_time %></td>
  <% end %>
Filter by _id

If you don’t have a created_at field by your own and someone asks you to give a list of documents created between something and something else, can you do it? Yes, you can!

time_something = Time.utc(2010, 1, 1)
time_something_else = Time.utc(2010, 1, 10)

time_something_id = ObjectId.from_time(time_something)
time_something_else_id = ObjectId.from_time(time_something_else)

collection.find('_id' => { '$gt' => time_something, '$lt' =>  time_something_else })

Unfortunately ObjectId.from_time(time) isn’t available in the MongoDB Shell. But it is available in the mongo-ruby-driver. Ruby rocks!

Expensive counts

When you have collections with million of documents, you will notice the searches aren’t that fast, even when using indexes. It also applies for counts, if you use a simple count without any criteria, it will be fine, fast, because MongoDB knows the collection size, but if you need to filter the count, it might be very expensive:

Notification.where(account_id: '5347c18a69702d1ef10c0000').count
=> # Completed in 15.354520466 seconds

It took 15 seconds to return the filtered count in a collection with 6120054 documents. To be fair with MongoDB, I have to mention that there are lot of documents (4951369) that matches the criteria above, which makes the query even more expensive, but it’s a real use case in my application.

To make it faster, we limit the count total, so instead of showing “4951369” found we show “99+”.

Notification.where(account_id: '5371ec6869702d131d010000').limit(-100).only(:_id).count(true)
=> # Completed in 0.021699112 seconds

Pretty fast, hum?

For sure the count(true) is the most responsible for the performance improvement, but you will also notice that the negative limit and specifying only the _id to return helps with the performance too. Performance improvements is usually a lot of minor milliseconds improvements.

If you are using Mongoid, you can add the capped_count to your Models.

class Notification
  def self.capped_count(limit = -100)

# Notification.where(account_id: '5371ec6869702d131d010000').capped_count

mongorestore –drop

Use --drop for a full restore. By default the mongorestore utility only inserts documents. Any existing data in the target database will be left intact. The --drop option drops every collection from the target database before restoring the collection from the dumped backup.

WARNING: Collection names should not begin with underscores

If you are using one of the most advanced backup techniques which consists in renaming collections with a underscore prefix, you should beware of that.

show collections
// => _my_collections

// => Sat Feb  8 10:25:21.233 TypeError: Cannot call method 'find' of undefined

// => Sat Feb  8 10:25:21.233 TypeError: Cannot call method 'find' of undefined

MongoDB issue: Can’t reference collection names beginning with an underscore in the mongo shell

To retrieve the underscored collection you can use: db.getCollection('_my_collection').

Underscore suffixes work fine:

// => success

Capped collections

Convert to capped

When you convert a collection to a capped collection, the collection loses the indexes.

db.runCommand({ "convertToCapped": "my_collection", size: sizeInBytes, max: maxNumberOfDocuments })

Remember to recreate the indexes. Even the ones with TTL:

db. my_collection.ensureIndex({ "created_at": 1 }, { expireAfterSeconds: 604800 })

MongoDB likes your storage as much as you


Pay attention to the log files.

du -h /usr/local/var/log/mongodb/mongo.log
9.5G	/usr/local/var/log/mongodb/mongo.log

Short-term solution:

rm  /usr/local/var/log/mongodb/mongo.log

Long-term solution:

Edit your /usr/local/etc/mongod.conf and enable Syslog Rotation:

# Append logs to /usr/local/var/log/mongodb/mongo.log
# logpath = /usr/local/var/log/mongodb/mongo.log
# logappend = true
syslog = true

On OS X >= 10.9.1 we need to set up some extra rules to enable syslog, otherwise it will not fail, but we will not be able to get the log outputs.

Log all messages via syslog to a separate mongodb.log:

sudo vim /etc/asl/org.mongodb

? [= Sender mongod.27017] claim only
* file /var/log/mongodb.log mode=0640 compress format=bsd rotate=seq file_max=5M all_max=20M


Save all messages to system.log:

? [= Sender mongod.27017] file /var/log/system.log

Then reload the syslogd configuration:

pkill -HUP -f syslogd

Local databases

Check your local databases you might have some which you don’t use anymore.

du -h /usr/local/var/mongodb
13G    /usr/local/var/mongodb

show dbs
// => foo
use foo