Lessons learnt building @RossC0 http://github.com/rozza 

WHAT IS MONGODB? A document database Highly scalable Developer friendly http://mongodb.org In BSON { _id : ObjectId("..."), author : "Ross", date : ISODate("2012-07-05..."), text : "About MongoDB...", tags : [ "tech", "databases" ], comments : [{ author : "Tim", date : ISODate("2012-07-05..."), text : "Best Post Ever!" }], comment_count : 1 } 

WHAT IS MONGODB? In Python In BSON { "_id" : ObjectId("..."), "author" : "Ross", "date" : datetime(2012,7,5,10,0), "text" : "About MongoDB...", "tags" : ["tech", "databases"], "comments" : [{ "author" : "Tim", "date" : datetime(2012,7,5,11,35), "text" : "Best Post Ever!" }], "comment_count" : 1 } { _id : ObjectId("..."), author : "Ross", date : ISODate("2012-07-05..."), text : "About MongoDB...", tags : [ "tech", "databases" ], comments : [{ author : "Tim", date : ISODate("2012-07-05..."), text : "Best Post Ever!" }], comment_count : 1 } 

http://education.10gen.com Want to know more? 

http://www.flickr.com/photos/51838104@N02/5841690990 WH Y DO YO U EVE N NE ED AN ODM? 

MongoDB a good fit Documents schema in code Enforces schema Data validation Speeds up development Build tooling off it Can DRY up code... SCHEMA LESS != CHAOS 

Inspired by Django's ORM Supports Python 2.5 - Python 3.3 Originally authored by Harry Marr 2010 I took over development in May 2011 Current release 0.7.5 http://github.com/MongoEngine/mongoengine 

INTRODUCING MONGOENGINE class Post(Document): title = StringField(max_length=120, required=True) author = ReferenceField('User') tags = ListField(StringField(max_length=30)) comments = ListField(EmbeddedDocumentField('Comment')) class Comment(EmbeddedDocument): content = StringField() name = StringField(max_length=120) class User(Document): email = StringField(required=True) first_name = StringField(max_length=50) last_name = StringField(max_length=50) 

CREATING A MODEL class Post(Document): title = StringField(max_length=120, required=True) author = ReferenceField('User') tags = ListField(StringField(max_length=30)) comments = ListField(EmbeddedDocumentField('Comment')) Define a class inheriting from Document Map a field to a defined data type strings, ints, binary, files, lists etc.. By default all declared fields aren't required Pass keyword arguments to apply constraints eg set if unique, max_length, default values. 

INSERTING DATA # Pass data into the constructor user = User(email="[email protected]", name="Ross").save() # Create instance and edit / update in place post = Post() post.title = "mongoengine" post.author = user post.tags = ['odm', 'mongodb', 'python'] post.save() Create instance of the object Update its attributes Call save, insert, update to persist the data 

QUERYING DATA # An `objects` manager is added to every `Document` class users = User.objects(email='[email protected]') # Pass kwargs to commands are lazy and be extended as needed users.filter(auth=True) # Iterating evaluates the queryset print [u for u in users] Documents have a queryset manager (objects) for querying You can continually extend it Queryset evaluated on iteration 

6 LESSONS LEARNT 

LE S SO N 1: DI VE IN ! http://www.flickr.com/photos/jackace/565857899/ 

In May 2011 >200 forks >100 issues ~50 pull requests I needed it Volunteered to help Started reviewing issues Supported Harry and community PROJECT STALLED 

LE S SO N 2 : METACL ASSES http://www.flickr.com/photos/ubique/135848053 

WHATS NEEDED TO MAKE AN ORM? Instance methods validation data manipulate data convert data to and from mongodb Queryset methods Finding data Bulk changes 

METACLASSES class Document(object): __metaclass__ = TopLevelDocumentMetaclass ... class EmbeddedDocument(object): __metaclass__ = DocumentMetaclass ... Needed for: 1. inspect the object inheritance 2. inject functionality to the class Its surprisingly simple - all we need is: __new__ 

METACLASSES 101 TopLevelDocument Document python's type cls, name, bases, attrs IN new class Out 

METACLASSES TopLevelDocument Document python's type Creates default meta data inheritance rules, id_field, index information, default ordering. Merges in parents meta Validation abstract flag on an inherited class collection set on a subclass Manipulates the attrs going in. IN 

METACLASSES TopLevelDocument Document python's type Merges all fields from parents Adds in own field definitions Creates lookups _db_field_map _reverse_db_field_map Determine superclasses (for model inheritance) IN 

METACLASSES TopLevelDocument Document python's type Adds in handling for delete rules So we can handle deleted References Adds class to the registry So we can load the data into the correct class OUT 

METACLASSES TopLevelDocument Document python's type Builds index specifications Injects queryset manager Sets primary key (if needed) OUT 

LESSONS LEARNT Spend time learning what is being done and why Don't continually patch: Rewrote the metaclasses in 0.7 

LE S SO N 3: S TR AYI NG F ROM THE PATH http://www.flickr.com/photos/51838104@N02/5841690990 

REWRITING THE QUERY LANGUAGE # In pymongo you pass dictionaries to query uk_pages = db.page.find({"published": True}) # In mongoengine uk_pages = Page.objects(published=True) # pymongo dot syntax to query sub documents uk_pages = db.page.find({"author.country": "uk"}) # In mongoengine we use dunder instead uk_pages = Page.objects(author__country='uk') 

REWRITING THE QUERY LANGUAGE #Somethings are nicer - regular expresion search db.post.find({'title': re.compile('MongoDB', re.IGNORECASE)}) Post.objects(title__icontains='MongoDB') # In mongoengine # Chaining is nicer db.post.update({"published": False}, {"$set": {"published": True}}, multi=True) Post.objects(published=False).update(set__published=True) 

LE S SO N 4 : NOT ALL IDEAS ARE GOOD http://www.flickr.com/photos/abiding_silence/6951229015 

CHANGING SAVE # In pymongo save replaces the whole document db.post.save({'_id': 'my_id', 'title': 'MongoDB', 'published': True}) # In mongoengine we track changes post = Post.objects(_id='my_id').first() post.published = True post.save() # Results in: db.post.update({'_id': 'my_id'}, {'$set': {'published': True}}) 

CHANGING SAVE Any field changes are noted How to monitor lists and dicts? Custom List and Dict classes Observes changes and marks as dirty 

HOW IT WORKS class Post(Document): title = StringField(max_length=120, required=True) author = ReferenceField('User') tags = ListField(StringField(max_length=30)) comments = ListField(EmbeddedDocumentField('Comment')) class User(Document): email = StringField(required=True) first_name = StringField(max_length=50) last_name = StringField(max_length=50) class Comment(EmbeddedDocument): content = StringField() name = StringField(max_length=120) 

Post HOW IT WORKS - comments comment comment comment post = Post.objects.first() post.comments[1].name = 'Fred' post.save() 

Post HOW IT WORKS - comments comment 1.Convert the comments data to a BaseList BaseList Stores the instance and name / location comment comment post.comments[1].name = 'Fred' 

Post HOW IT WORKS - comments comment 2.Convert the comment data to BaseDict sets name as: 'comments.1' comment comment post.comments[1].name = 'Fred' 

Post HOW IT WORKS - comments comment 3.Change name to "Fred" 4. Tell Post 'comments.1.name' has changed comment comment post.comments[1].name = 'Fred' 

Post HOW IT WORKS - comments comment 5.On save() Iterate all the changes on post and run $set / $unset queries comment comment post.save() db.post.update( {'_id': 'my_id'}, {'$set': { 'comments.1.name': 'Fred'}} ) 

A GOOD IDEA? + Makes it easier to use + save acts how people think it should - Its expensive - Doesn't help people understand MongoDB 

LE S SO N 5: M ANAGI NG A COMMUNIT Y http://kingscross.the-hub.net/ 

Github effect >10 django mongoengine projects None active on pypi Little cross project communication CODERS JUST WANT TO CODE * * Side effect of being stalled? 

Flask-mongoengine on pypi There were 2 different projects Now has extra maintainers from the flask-mongorest Django-mongoengine* Spoke to authors of 7 projects and merged their work together to form a single library * Coming soon! REACH OUT 

THE COMMUNITY Not all ideas are good! Vocal people don't always have great ideas Travis is great* * but you still have to read the pull request Communities have to be managed I've only just started to learn how to herding cats 

LE S SO N 6 : DON' T BE AF R A ID TO ASK http://www.flickr.com/photos/kandyjaxx/2012468692 

Website Documentation Tutorials Framework support Core mongoengine http://twitter.com/RossC0 I NEED HELP ;) http://github.com/MongoEngine 

Q UE S T I ONS ? http://www.flickr.com/photos/9550033@N04/5020799468