Latest revision as of 19:02, 16 March 2026

Install

From: https://docs.mongodb.com/manual/tutorial/install-mongodb-on-os-x/

From: https://docs.mongodb.org/manual/tutorial/install-mongodb-on-ubuntu/

From: https://docs.mongodb.com/manual/tutorial/install-mongodb-on-red-hat/

Develop

rename a field, or paste one field to another collection-wide

Methods are: updateOne, updateMany or update

db.collection.<update method>(
    {},
    [
        {"$set": {"name": { "$concat": ["$firstName", " ", "$lastName"]}}}
    ]
)

Count Documents

db.collection.aggregate(
  [
    { $group: { _id: null, count: { $sum: 1 } } }
  ]
)

list users

use admin;
db.getUsers();

create user

 use admin;

db.createUser(
  {
    user: "myTester",
    pwd:  passwordPrompt(),   // or cleartext password
    roles: [ { role: "readWrite", db: "test" },
             { role: "read", db: "reporting" } ]
  }
)

add role to a user

db.grantRolesToUser('<username>', [
  { role: "readWrite", db: "<dbname>" }
], { w: "majority" , wtimeout: 4000 })

Performance Tuning

db.getProfilingStatus()
db.getProfilingLevel()
db.setProfilingLevel(1, 10)
db.getCollection('system.profile').find({})

profiling

Profiling. From: https://docs.mongodb.com/manual/reference/method/db.setProfilingLevel/

The profiler writes all the data it collects to the system.profile collection, a capped collection in the admin database.

db.getProfilingStatus()
# db.setProfilingLevel(level, options)
db.setProfilingLevel(1, { "slowms": 500 })

db.enableFreeMonitoring()

use creek_development
db.setProfilingLevel(1, { "slowms": 500 })
db.getCollection('system.profile').find({}).sort({"ts": -1}).limit( 5 )

db.lineitems.getIndexes()

view slow queries

In ruby:

 config.mongoid.logger       = Logger.new(Rails.root.join( 'tmp', 'mongo.log' ))
 config.mongoid.logger.level = Logger::DEBUG

Mongoid logging can also show caller location if you enable it:

 Mongo::Monitoring::Global.subscribe(Mongo::Monitoring::COMMAND) do |event|
   Rails.logger.debug("Mongo Query: #{event.command}")
 end

Use .explain on Suspicious Queries:

 User.where(email: "test@example.com").explain

In mongosh (old mongo):

(no need to restart mongo):

 use creek_staging ;
 db.setProfilingLevel(1, { slowms: 100 })
 db.system.profile.find().sort({ ts: -1 }).limit(5)

This means:

• 0 → profiling off
• 1 → log slow queries only (recommended)
• 2 → log all queries (very heavy, avoid in production)