MongoDB Shell Tips & Tricks

#MongoDBDays

MongoDB Shell Tips &
Tricks
Hannes Magnusson
Driver Engineer, 10gen
@bjori

What is the Shell?
Embedded
• vars / functions / data structs + types
Javascript • Spidermonkey / V8
Interpreter
• ObjectId("...")
Global Functions • new Date()
and Objects • Object.bsonsize()

MongoDB driver • db["collection"].find/count/update
Exposed • short-hand for collections

• Doesn't require quoted keys
JSON-like stuff • Don’t copy and paste too much

Shell Tips & Tricks, Hannes Magnusson

MongoDB Shell: Advantages
• Your goto tool
• Debugging Queries / Syntax
• Testing
• Administration
• Scripting Glue
• You don’t do rs.stepDown() from your application


MongoDB Shell: Disadvantages
• Numbers in JS are a pain
– 32/64-bit int/long  NumberInt() / NumberLong()
– Primitive numbers are all 64-bit FP doubles

• Dates can be confusing
– new Date( "1/1/1" )
– new ISODate( ... )

NOT: Date( "1/1/1" )  string


Emacs-like shell bindings
ctrl A Move cursor to start of line
ctrl E Move cursor to end of line
meta B Move cursor left by one word
meta F Move cursor right by one word
ctrl L Clear screen and redisplay line
ctrl R Reverse history search
meta < Start of history
meta > End of history


Easy to use
• Tab completion on most objects
• Built-in help on most objects (.help())
• show
– profile # 5 most recent ops of 1ms or more
– users # List all the users of the current db
– dbs # List all the databases
– logs # List all available logs
• Most examples use the shell


Speed Considerations
• Shell
– JavaScript is slow
– Always in "write-acknowledged" (safe mode) / GLE
– Data Conversions

• Server
– Applies on the server as well
– Careful with round-tripping numbers


Insert, Update, Remove

for ( i = 0; i < 1000; i++ ) {
db.test.insert( {
x: i,
ts: new Date()
} );
}


Loading Scripts
• Commandline
– --eval switch
– .js files

• Within the shell
– load()


Running Commands
• db.runCommand( { ... } )
– Runs any arbitrary command against the current DB

• db.adminCommand( { ... } )
– Run commands against the admin database


db.adminCommand
Definition
function (obj) {
if (this._name == "admin") {
return this.runCommand(obj);
}
return this.getSiblingDB("admin")
.runCommand(obj);
}


Profiling
• setProfilingLevel( lvl, <ms> )
– 0: none
– 1: time-based
– 2: all

• getProfilingLevel()
• Reading from the profile collection
– db.system.profile.find()


Cool Functions
• printjson()  tojson
• forEach on arrays, queries, and cursors


You didn’t hear it from me

db.system.js.save({ _id:"echoFunction",
value : function(x) { return x; }
})
db.loadServerScripts();
echoFunction(3);


forEach example

[{x:1},{y:1}].forEach(function(x) {
printjson(x)
})

{ "x" : 1 }
{ "y" : 1 }


Cool Functions
• printjson  tojson
• forEach on arrays, queries, and cursors
• Object.bsonsize
• load(file)
• run(file)


Print all Indexes

db.getCollectionNames().
forEach( function(x) {
print( "Collection: " + x );
printjson( db[x].getIndexes() );
})


Getting the Biggest Doc
var cursor = db.coll.find();
var biggest = 0;
var doc = {};

cursor.forEach(function (x) {
var size = Object.bsonsize(x);
if (size > biggest) {
biggest = size;
doc = x;
}
});

Administration functions
• Sharding
– sh.status()
– sh.enableSharding(dbname)

• Replicaset
– rs.status()
– cfg = rs.config(); rs.reconfig(cfg);
– db.isMaster()

• Status
– Object.bsonsize(document)
– db.collectionName.stats()


.mongorc.js
• Automagically loaded on startup
– Unless you specify --norc

• Script your command prompt
– prompt=function() { return "Hello World"; }

• Colorize output
• Move complex aggregate() queries into functions
• Can be a directory


Want to know more?
The shell is self documented, in JavaScript o/
…Except the native helper

• help
– dbs # Shows all commands you can run on a database
– connect # Connect to other nodes

• Call a function, without the brackets
– Will show you the actual code behind it

• try.mongodb.org !


#MongoDBDays

Thank You
Hannes Magnusson
Driver Engineer, 10gen
@bjori

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