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I never Meta model I didn't like Dr Russell Keith-Magee DjangoCon US 2015

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I never Meta model I didn't like Dr Russell Keith-Magee DjangoCon US 2015

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What is meta programming?

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An example: Python

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Metaprogramming Python class class Point Point: def def __init__ __init__(self, x, y): self.x = x self.y = y class class Point3D Point3D(Point): def def __init__ __init__(self, x, y, z): super().__init__(x, y) self.z = z >>> p1 = Point(3, 4) >>> p2 = Point3D(5, 6, 7)

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Metaprogramming Python def def output_point output_point(p): print("x = %s" % p.x) print("y = %s" % p.y) def def output_point3D output_point3D(p): output_point(p) print("z = %s" % p.z)

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Metaprogramming Python >>> output_point(p1) x = 3 y = 4 >>> output_point3D(p2) x = 5 y = 6 z = 7

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Metaprogramming Python def def output_point2 output_point2(p): print("x = %s" % p.x) print("y = %s" % p.y) if if isinstance(p, Point3d): print("z = %s" % p.z)

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Metaprogramming Python >>> output_point2(p1) x = 3 y = 4 >>> output_point2(p2) x = 5 y = 6 z = 7

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Metaprogramming Python def def output_point3 output_point3(p): print("x = %s" % p.x) print("y = %s" % p.y) if if hasattr(p, 'z'): print("z = %s" % p.z)

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Metaprogramming Python class class Point4D Point4D(Point3D): def def __init__ __init__(self, x, y, z, t): super().__init__(x, y, z) self.t = t

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Metaprogramming Python def def output_point4 output_point4(p): for for attr in in sorted(p.__dict__): print('%s = %s' % ( attr, getattr(p, attr) ))

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Metaprogramming Python def def output_point4 output_point4(p): for for attr in in sorted(p.__dict__): print('%s = %s' % ( attr, getattr(p, attr) ))

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Metaprogramming Python

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Metaprogramming Django

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Modelforms

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django.contrib.admin

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django.contrib.admin

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django.contrib.admin

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History of Django's Meta

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The Old Meta API meta = MyObject._meta meta.get_field(name) meta.get_field_by_name(name) meta.get_fields_with_model() meta.get_concrete_fields_with_model() meta.get_m2m_with_model() meta.get_all_related_objects() meta.get_all_related_objects_with_model() meta.get_all_related_many_to_many_objects() meta.get_all_related_m2m_objects_with_model() meta.get_all_field_names()

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Unofficially stable

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Daniel Pyrathon

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The New Meta API

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The New Meta API meta = MyObject._meta meta.get_field(field_name) meta.get_fields( include_parents=True True, include_hidden=False False )

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The New Meta API field.is_relation field.one_to_one field.many_to_one field.one_to_many field.many_to_many field.related_model field.auto_created field.concrete

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The New Meta API

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The New Meta API

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So why bother?

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High level frameworks and tools

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A practical example

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A practical example

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A practical example

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Merging records class class Customer Customer(Model): name = CharField(max_length=100) ... class class WorkOrder WorkOrder(Model): customer = ForeignKey(Customer) order_id = CharField(max_length=100) ...

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Merging records class class Invoice Invoice(Model): customer = ForeignKey(Customer) invoice_number = CharField(max_length=100) ... class class Payment Payment(Model): customer = ForeignKey(Customer) received = DateField() ...

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Merging records def def merge_customer merge_customer(orig, dup): dup.workorder_set.update(customer=orig) dup.invoice_set.update(customer=orig) dup.payment_set.update(customer=orig) dup.delete()

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Time passes...

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Adding quotes class class Quote Quote(Model): customer = ForeignKey(Customer) quote_number = CharField(max_length=100) ...

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Adding quotes def def merge_customer merge_customer(orig, dup): dup.workorder_set.update(customer=orig) dup.invoice_set.update(customer=orig) dup.payment_set.update(customer=orig) dup.delete()

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Adding quotes def def merge_customer merge_customer(orig, dup): dup.workorder_set.update(customer=orig) dup.invoice_set.update(customer=orig) dup.payment_set.update(customer=orig) dup.quote_set.update(customer=orig) dup.delete()

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Metaprograming merge def def merge_customer merge_customer(orig, dup): for for f in in orig._meta.get_fields(): if if f.one_to_many and and f.auto_created: getattr( dup, f.get_accessor_name() ).update(customer=orig) dup.delete()

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Metaprograming merge def def merge_anything merge_anything(orig, dup): for for f in in orig._meta.get_fields(): if if f.one_to_many and and f.auto_created: getattr( dup, f.get_accessor_name() ).update(**{ f.field.name: orig }) dup.delete()

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Killer app

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Wrapping non-relational data stores

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What does "How do I use X with Django?" actually mean?

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What does "How do I use X with Django?" actually mean?

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Can I get a Django form for my model? and Can I query and edit my data in Django's Admin?

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Can I get a Django form for my model?

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Can I query and edit my MongoDB data in Django's Admin?

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Can I query and edit my MongoDB data in Django's Admin?

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Dream the impossible dream

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None of this needs to be in core

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django-mailer https://github.com/PirosB3/django-mailer

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django-mailer https://github.com/PirosB3/django-mailer

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Managers class class GmailManager GmailManager(object): def def __init__ __init__(self, model): storage = Storage(settings.CREDENTIALS_PATH) self.creds = storage.get() self.model = model ...

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Managers class class ThreadManager ThreadManager(GmailManager): def def get_queryset get_queryset(self): ... return return ThreadQuerySet(self.creds, ...) class class MessageManager MessageManager(GmailManager): def def get_queryset get_queryset(self): ... return return MessageQuerySet(self.creds, ...)

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Queries and QuerySets class class GmailQuery GmailQuery(object): ... class class GmailQuerySet GmailQuerySet(list): def def __init__ __init__(self, *args, **kwargs): self.model = kwargs.pop('model') self.creds = kwargs.pop('creds') super().__init__(*args, **kwargs) self.query = GmailQuery()

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Queries and QuerySets class class ThreadQuerySet ThreadQuerySet(GmailQuerySet): def def get get(self, *args, **kwargs): thread_id = kwargs['id'] thread = get_thread_by_id( self.creds, thread_id ) ... return return thread

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Queries and QuerySets class class MessageQuerySet MessageQuerySet(GmailQuerySet): def def get get(self, *args, **kwargs): ... def def filter filter(self, *args, **kwargs): ... return return MessageQuerySet( model=self.model, credentials=self.credentials, selected_thread=selected_thread ) def def count count(self, *args, **kwargs): ... ...

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Meta class class GmailOptions GmailOptions(Options): ... def def _get_fields _get_fields(self, reverse=True, forward=True): ... def def get_field get_field(self, field_name): ... class class ThreadOptions ThreadOptions(GmailOptions): ... class class MessageOptions MessageOptions(GmailOptions): ...

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Models class class GmailModel GmailModel(object): _deferred = False False _state = ModelState() ... class class constructor constructor(type): def def __new__ __new__(cls, name, bases, attrs): dm = attrs.pop('_default_manager') klass = super().__new__( cls, name, bases, attrs) klass._default_manager = dm(klass) return return klass

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Models class class Message Message(GmailModel): __metaclass__ = constructor _default_manager = MessageManager _meta = MessageOptions() class class Thread Thread(GmailModel): __metaclass__ = constructor _default_manager = ThreadManager _meta = ThreadOptions()

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Admin class class MessageInline MessageInline(admin.TabularInline): model = Message class class ThreadAdmin ThreadAdmin(admin.ModelAdmin): inlines = [MessageInline] fields = ('number_of_messages',) list_display = ('id',) ordering = ('id',) admin.site.register(Thread, ThreadAdmin)

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Admin class class MessageAdmin MessageAdmin(admin.ModelAdmin): fields = ('sender', 'receiver', 'body', 'snippet') list_display = ('id',) ordering = ('id',) admin.site.register(Message, MessageAdmin)

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(... and breathe ....)

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Call to action

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Why not meta duck at the sprints?

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MongoDB Google App Engine Datastore Cassandra Riak CouchDB (or any other non-relational data store)

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LDAP Local mailbox directory File system AWS S3/EC2 resources

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SQLAlchemy

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Caveat emptor

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Thank you! russell@keith-magee.com @freakboy3742