Django Model源码解析（一）

from django.db import models

class Student(models.Model):
    name = models.CharField(max_length=20)
    age = models.SmallIntegerField()

class Model(metaclass=ModelBase):

    def __init__(self, *args, **kwargs):
        # Alias some things as locals to avoid repeat global lookups
        cls = self.__class__
        opts = self._meta
        _setattr = setattr
        _DEFERRED = DEFERRED

        pre_init.send(sender=cls, args=args, kwargs=kwargs)

        # Set up the storage for instance state
        self._state = ModelState()

        # There is a rather weird disparity here; if kwargs, it's set, then args
        # overrides it. It should be one or the other; don't duplicate the work
        # The reason for the kwargs check is that standard iterator passes in by
        # args, and instantiation for iteration is 33% faster.
        if len(args) > len(opts.concrete_fields):
            # Daft, but matches old exception sans the err msg.
            raise IndexError("Number of args exceeds number of fields")

        if not kwargs:
            fields_iter = iter(opts.concrete_fields)
            # The ordering of the zip calls matter - zip throws StopIteration
            # when an iter throws it. So if the first iter throws it, the second
            # is *not* consumed. We rely on this, so don't change the order
            # without changing the logic.
            for val, field in zip(args, fields_iter):
                if val is _DEFERRED:
                    continue
                _setattr(self, field.attname, val)
        else:
            # Slower, kwargs-ready version.  fields_iter = iter(opts.fields)
            for val, field in zip(args, fields_iter):
                if val is _DEFERRED:
                    continue
                _setattr(self, field.attname, val)
                kwargs.pop(field.name, None)

    def save_base(self, raw=False, force_insert=False,
                  force_update=False, using=None, update_fields=None):
        """
        Handle the parts of saving which should be done only once per save,
        yet need to be done in raw saves, too. This includes some sanity
        checks and signal sending.

        The 'raw' argument is telling save_base not to save any parent
        models and not to do any changes to the values before save. This
        is used by fixture loading.
        """
        using = using or router.db_for_write(self.__class__, instance=self)
        assert not (force_insert and (force_update or update_fields))
        assert update_fields is None or update_fields
        cls = origin = self.__class__
        # Skip proxies, but keep the origin as the proxy model.
        if cls._meta.proxy:
            cls = cls._meta.concrete_model
        meta = cls._meta
        if not meta.auto_created:
            pre_save.send(
                sender=origin, instance=self, raw=raw, using=using,
                update_fields=update_fields,
            )
        # A transaction isn't needed if one query is issued.
        if meta.parents:
            context_manager = transaction.atomic(using=using, savepoint=False)
        else:
            context_manager = transaction.mark_for_rollback_on_error(using=using)
        with context_manager:
            parent_inserted = False
            if not raw:
                parent_inserted = self._save_parents(cls, using, update_fields)
            updated = self._save_table(
                raw, cls, force_insert or parent_inserted,
                force_update, using, update_fields,
            )
        # Store the database on which the object was saved
        self._state.db = using
        # Once saved, this is no longer a to-be-added instance.
        self._state.adding = False

        # Signal that the save is complete
        if not meta.auto_created:
            post_save.send(
                sender=origin, instance=self, created=(not updated),
                update_fields=update_fields, raw=raw, using=using,
            )

class ModelBase(type):
    """Metaclass for all models."""
    def __new__(cls, name, bases, attrs, **kwargs):
        super_new = super().__new__

        # Also ensure initialization is only performed for subclasses of Model
        # (excluding Model class itself).
        parents = [b for b in bases if isinstance(b, ModelBase)]
        if not parents:
            return super_new(cls, name, bases, attrs)

        # Create the class.
        module = attrs.pop('__module__')
        new_attrs = {'__module__': module}
        classcell = attrs.pop('__classcell__', None)
        if classcell is not None:
            new_attrs['__classcell__'] = classcell
        attr_meta = attrs.pop('Meta', None)
        # Pass all attrs without a (Django-specific) contribute_to_class()
        # method to type.__new__() so that they're properly initialized
        # (i.e. __set_name__()).
        contributable_attrs = {}
        for obj_name, obj in list(attrs.items()):
            if _has_contribute_to_class(obj):
                contributable_attrs[obj_name] = obj
            else:
                new_attrs[obj_name] = obj
        new_class = super_new(cls, name, bases, new_attrs, **kwargs)

        abstract = getattr(attr_meta, 'abstract', False)
        meta = attr_meta or getattr(new_class, 'Meta', None)
        base_meta = getattr(new_class, '_meta', None)

        app_label = None

        # Look for an application configuration to attach the model to.
        app_config = apps.get_containing_app_config(module)

        if getattr(meta, 'app_label', None) is None:
            if app_config is None:
                if not abstract:
                    raise RuntimeError(
                        "Model class %s.%s doesn't declare an explicit "
                        "app_label and isn't in an application in "
                        "INSTALLED_APPS." % (module, name)
                    )

            else:
                app_label = app_config.label

        new_class.add_to_class('_meta', Options(meta, app_label))
        if not abstract:
            new_class.add_to_class(
                'DoesNotExist',
                subclass_exception(
                    'DoesNotExist',
                    tuple(
                        x.DoesNotExist for x in parents if hasattr(x, '_meta') and not x._meta.abstract
                    ) or (ObjectDoesNotExist,),
                    module,
                    attached_to=new_class))
            new_class.add_to_class(
                'MultipleObjectsReturned',
                subclass_exception(
                    'MultipleObjectsReturned',
                    tuple(
                        x.MultipleObjectsReturned for x in parents if hasattr(x, '_meta') and not x._meta.abstract
                    ) or (MultipleObjectsReturned,),
                    module,
                    attached_to=new_class))
            if base_meta and not base_meta.abstract:
                # Non-abstract child classes inherit some attributes from their
                # non-abstract parent (unless an ABC comes before it in the
                # method resolution order).
                if not hasattr(meta, 'ordering'):
                    new_class._meta.ordering = base_meta.ordering
                if not hasattr(meta, 'get_latest_by'):
                    new_class._meta.get_latest_by = base_meta.get_latest_by

        is_proxy = new_class._meta.proxy

# Meta属性会再赋值给__meta
attr_meta = attrs.pop('Meta', None)
# 调用父类的__new__创建类对象，也就是Model对象
new_class = super_new(cls, name, bases, new_attrs, **kwargs)
# 给Model增加_meta属性, 转化成了Options对象
new_class.add_to_class('_meta', Options(meta, app_label))

# 处理抽象父Model对应的域
for base in reversed([new_class] + parents):
    # Conceptually equivalent to `if base is Model`.
    if not hasattr(base, '_meta'):
        continue
    # Skip concrete parent classes.
    if base != new_class and not base._meta.abstract:
        continue
    # Locate OneToOneField instances.
    for field in base._meta.local_fields:
        if isinstance(field, OneToOneField):
            related = resolve_relation(new_class, field.remote_field.model)
            parent_links[make_model_tuple(related)] = field

# 给Model对象追加objects作为manager
if not opts.managers:
    if any(f.name == 'objects' for f in opts.fields):
        raise ValueError(
            "Model %s must specify a custom Manager, because it has a "
            "field named 'objects'." % cls.__name__
        )
    manager = Manager()
    manager.auto_created = True
    cls.add_to_class('objects', manager)