Mongo Berlin - Mastering the Shell

Mastering the Shell — MongoBerlin Richard M Kreuter 10gen Inc. richard@10gen.com March 25, 2011Mastering the Shell — MongoBerlin

The Mongo Shell What is the shell? The mongo shell is a JavaScript interpreter with built-in support for MongoDB connectivity. Mastering the Shell — MongoBerlin

What’s the shell good for? Interactive development/prototyping. Test scripting (cf. MongoDB’s own regression framework). Administrative operations, lightweight scripting. Learning MongoDB (and teaching it, too). Mastering the Shell — MongoBerlin

Running the Shell $ mongo MongoDB shell version: 1.6.4 connecting to: test > db.people.save({name:"Washington", no: 1}); > db.people.save({name:"Adams", no: 2}); > db.people.save({name:"Jefferson", no: 3}); > for (i=0; i<1024; i++) db.numbers.save({ num: i }); Mastering the Shell — MongoBerlin

Running the shell (continued) You can execute a ﬁle of code either by specifying command-line argument or with the built-in load extension function. $ mongo foo.js # executes foo.js and exits $ mongo MongoDB shell version: 1.6.4 connecting to: test > load("foo.js") // executes foo.js and returns to prompt Mastering the Shell — MongoBerlin

Running the shell (continued continued) As of 1.8, you can also use mongo as a “shebang” interpreter. $ cat ~/foo.js #!/Users/kreuter/10gen/mongo/mongo ... $ ./foo.js Mastering the Shell — MongoBerlin

Shell helpers The shell has some built-in helpers that resemble the MySQL shell’s. // Change database to "foo" use foo // List the collections in "foo" show collections Note that these helpers aren’t strictly JavaScript; they’re sort of preprocessors available only when the shell is run interactively. There are proper JavaScript methods for doing these things programmatically (e.g., db=db.getSisterDB("foo")). Mastering the Shell — MongoBerlin

Completion in interactive mode The shell supports completion (improved notably in 1.8). Completion can introspect on JavaScript objects to ﬁnd attributes/methods: db.<TAB> // print methods on the db object db.people.<TAB> // print methods on the people collection // etc... Mastering the Shell — MongoBerlin

Command-line editing The shell uses readline for command completion, history, editing, etc. (for now, anyway). So it has similar keybindings by default to those in bash, et. al. Mastering the Shell — MongoBerlin

Getting help > help // help is a helper... ... > db.help() // db.help() is a method on the db object ... > db.collection.help() // a method on the collection Mastering the Shell — MongoBerlin

Working with the shell The shell runs all queries in “SafeMode”, i.e., it executes getLastError and prints any error message after data manipulations. So, for example, > db.foo.insert({_id:1}); > db.foo.insert({_id:1}); E11000 duplicate key error index: test.foo.$_id_ dup key: { : 1.0 } Mastering the Shell — MongoBerlin

Working with the shell (continued) Because the shell uses JavaScript, a little care is called for when handling types MongoDB supports that JavaScript doesn’t: JavaScript’s only number type is double-ﬂoats. (Use NumberLong to construct 64-bit integers.) Documents having multiple values for the same key aren’t supported in JavaScript (but you shouldn’t really use these anyway). Binary data is represented by the BinData type. Also, note that in JavaScript, Date(string) returns a string; you almost always want new Date(string), which returns an object. (In 1.8, see the ISODate() function.) Mastering the Shell — MongoBerlin

Cursors in the shell By default, cursors in the shell are printed by iterating the cursor some number of times, and assigning the variable it to an iterator: > db.numbers.find() { "_id" : ObjectId("4cf91b32e3f85d1561593dfc"), "num" : 0 } ... has more > it { "_id" : ObjectId("4cf91b32e3f85d1561593e10"), "num" : 20 ... has more Mastering the Shell — MongoBerlin

Cursors in the shell (continued) The shell supports cursors as ﬁrst-class objects: > var cur=db.people.find() > cur.hasNext() true > while (cur.hasNext()) { printjson(cur.next().name); } "Washington" "Adams" "Jefferson" > var cur2=db.people.find({}, {name:1}) > cur2.forEach(printjson) { "_id" : ..., "name" : "Washington" } { "_id" : ..., "name" : "Adams" } { "_id" : ..., "name" : "Jefferson" } Mastering the Shell — MongoBerlin

Examining JavaScript code Most of the shell’s functionality is implemented in JavaScript itself, with the consequence that you can examine (and so cargo-cult) it yourself: > db.people.findOne function (query, fields) { var cursor = this._mongo.find(this._fullName, this._massageObject(query) || {}, fields, -1, 0, 0); if (!cursor.hasNext()) return null; var ret = cursor.next(); if (cursor.hasNext()) throw "findOne has more than 1 result!"; if (ret.$err) throw "error " + tojson(ret); return ret; } Mastering the Shell — MongoBerlin

A nifty function Here’s a nifty administrative function (stolen from Scott Hernandez’s talk on the shell): 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; } }); Mastering the Shell — MongoBerlin

Gotchas JavaScript isn’t the fastest language around. JavaScript lacks features for “programming in the large” (modules/packages/namespaces/etc.) Iterating arrays (in index order) is slow. Some data types require special care. Mastering the Shell — MongoBerlin

So give the shell another spin! www.mongodb.org — downloads, docs, community mongodb-user@googlegroups.com — mailing list #mongodb on irc.freenode.net try.mongodb.org — web-based shell 10gen is hiring. Email jobs@10gen.com. 10gen oﬀers support, training, and advising services for mongodb Mastering the Shell — MongoBerlin

Mongo Berlin - Mastering the Shell

by Dwight Merriman on Apr 05, 2011

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Mongo Berlin - Mastering the Shell — Presentation Transcript