django orm源碼解析(其一)

一 創建數據庫:

class UserApiKey(Model):
    api_key = CharField(max_length=50, null=True)
    create_time = DateTimeField(default=timezone.now)

關於　Model 的源碼解析(元類創建一個新的類,來替代原來的類):
    1, Model 繼承於　ModelBase
    2, ModelBase　繼承與元類(type) 主要是 創建模型類的元類
        1),從原始模型類的定義中獲取Meta元信息, Django會嘗試從中獲取一些模型類的相關屬性, 例如: abstract,ordering by等;
        2),模型的元信息類Meta會別包裝成一個特殊Options對象, 並設置在新創建的模型類對象的屬性_meta上
        3),為新創建模型類對象添加兩個模型相關的異常類DoesNotExist和MultipleObjectsReturned
        4),添加原始模型類中定義的所有屬性和字段到新創建的模型類對象上
        5),如果模型類沒有提供一個對象管理器, 那么就設置一個名為objects的Manager默認對象管理器;
        6),返回新創建的模型類, 供以后創建類的實例使用
        7),綜上所述,創建一個新的類,增加了若干屬性

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
        new_class = super_new(cls, name, bases, new_attrs, **kwargs)
        attr_meta = attrs.pop('Meta', None)
        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

        # If the model is a proxy, ensure that the base class
        # hasn't been swapped out.
        if is_proxy and base_meta and base_meta.swapped:
            raise TypeError("%s cannot proxy the swapped model '%s'." % (name, base_meta.swapped))

        # Add all attributes to the class.
        for obj_name, obj in attrs.items():
            new_class.add_to_class(obj_name, obj)

        # All the fields of any type declared on this model
        new_fields = chain(
            new_class._meta.local_fields,
            new_class._meta.local_many_to_many,
            new_class._meta.private_fields
        )
        field_names = {f.name for f in new_fields}

        # Basic setup for proxy models.
        # 設置代理模型類
        if is_proxy:
            base = None
            for parent in [kls for kls in parents if hasattr(kls, '_meta')]:
                if parent._meta.abstract:
                    if parent._meta.fields:
                        raise TypeError(
                            "Abstract base class containing model fields not "
                            "permitted for proxy model '%s'." % name
                        )
                    else:
                        continue
                if base is None:
                    base = parent
                elif parent._meta.concrete_model is not base._meta.concrete_model:
                    raise TypeError("Proxy model '%s' has more than one non-abstract model base class." % name)
            if base is None:
                raise TypeError("Proxy model '%s' has no non-abstract model base class." % name)
            new_class._meta.setup_proxy(base)
            new_class._meta.concrete_model = base._meta.concrete_model
        else:
            new_class._meta.concrete_model = new_class

        # Collect the parent links for multi-table inheritance.
        parent_links = {}
        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

        # Track fields inherited from base models.
        inherited_attributes = set()
        # Do the appropriate setup for any model parents.
        for base in new_class.mro():
            if base not in parents or not hasattr(base, '_meta'):
                # Things without _meta aren't functional models, so they're
                # uninteresting parents.
                inherited_attributes.update(base.__dict__)
                continue

            parent_fields = base._meta.local_fields + base._meta.local_many_to_many
            if not base._meta.abstract:
                # Check for clashes between locally declared fields and those
                # on the base classes.
                for field in parent_fields:
                    if field.name in field_names:
                        raise FieldError(
                            'Local field %r in class %r clashes with field of '
                            'the same name from base class %r.' % (
                                field.name,
                                name,
                                base.__name__,
                            )
                        )
                    else:
                        inherited_attributes.add(field.name)

                # Concrete classes...
                base = base._meta.concrete_model
                base_key = make_model_tuple(base)
                if base_key in parent_links:
                    field = parent_links[base_key]
                elif not is_proxy:
                    attr_name = '%s_ptr' % base._meta.model_name
                    field = OneToOneField(
                        base,
                        on_delete=CASCADE,
                        name=attr_name,
                        auto_created=True,
                        parent_link=True,
                    )

                    if attr_name in field_names:
                        raise FieldError(
                            "Auto-generated field '%s' in class %r for "
                            "parent_link to base class %r clashes with "
                            "declared field of the same name." % (
                                attr_name,
                                name,
                                base.__name__,
                            )
                        )

                    # Only add the ptr field if it's not already present;
                    # e.g. migrations will already have it specified
                    if not hasattr(new_class, attr_name):
                        new_class.add_to_class(attr_name, field)
                else:
                    field = None
                new_class._meta.parents[base] = field
            else:
                base_parents = base._meta.parents.copy()

                # Add fields from abstract base class if it wasn't overridden.
                for field in parent_fields:
                    if (field.name not in field_names and
                            field.name not in new_class.__dict__ and
                            field.name not in inherited_attributes):
                        new_field = copy.deepcopy(field)
                        new_class.add_to_class(field.name, new_field)
                        # Replace parent links defined on this base by the new
                        # field. It will be appropriately resolved if required.
                        if field.one_to_one:
                            for parent, parent_link in base_parents.items():
                                if field == parent_link:
                                    base_parents[parent] = new_field

                # Pass any non-abstract parent classes onto child.
                new_class._meta.parents.update(base_parents)

            # Inherit private fields (like GenericForeignKey) from the parent
            # class
            for field in base._meta.private_fields:
                if field.name in field_names:
                    if not base._meta.abstract:
                        raise FieldError(
                            'Local field %r in class %r clashes with field of '
                            'the same name from base class %r.' % (
                                field.name,
                                name,
                                base.__name__,
                            )
                        )
                else:
                    field = copy.deepcopy(field)
                    if not base._meta.abstract:
                        field.mti_inherited = True
                    new_class.add_to_class(field.name, field)

        # Copy indexes so that index names are unique when models extend an
        # abstract model.
        new_class._meta.indexes = [copy.deepcopy(idx) for idx in new_class._meta.indexes]

        if abstract:
            # Abstract base models can't be instantiated and don't appear in
            # the list of models for an app. We do the final setup for them a
            # little differently from normal models.
            attr_meta.abstract = False
            new_class.Meta = attr_meta
            return new_class
        # 如果模型類沒有提供一個對象管理器, 那么就設置一個名為objects的Manager默認對象管理器;
        new_class._prepare()
        # 使用緩存 新的模型類會被緩存在應用程序中
        new_class._meta.apps.register_model(new_class._meta.app_label, new_class)
        # 返回新創建的模型類, 供以后創建類的實例使用;
        return new_class

二 模型類管理器:
    1,　ModelBase　利用元類為模型類增加了屬性objects 並指向模型類管理器對象

# 來自於 base.py 文件下　ModelBase　__new__()
    def _prepare(cls):
        """Create some methods once self._meta has been populated."""
        opts = cls._meta
        opts._prepare(cls)
        """......."""
        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 是模型類管理器
            manager = Manager()
            manager.auto_created = True
            # 如果沒有制定模型類管理器　那么就是　cls.objects = Manager()
            cls.add_to_class('objects', manager)

　　2, 我們可以繼承　Manager 用來自定義管理器 Manager

class Manager(BaseManager.from_queryset(QuerySet)):
        pass

　　3, from_queryset 返回的是一個又元類type定義的一個新的類，同時把QuerySet類中的多有方法等屬性也添加到了這個用type新建的類
　　4, from_queryset 就是將Manager 與　QuerySet 兩個類的屬性整合到一起造了一個新的類
　　5, 也就是說　Manager 繼承與這個新創建的類

    # 來自於 manager.py 文件下 BaseManager類
    @classmethod
    def from_queryset(cls, queryset_class, class_name=None):
        if class_name is None:
            class_name = '%sFrom%s' % (cls.__name__, queryset_class.__name__)
         # 使用　type 方法進行整合 到一個新的類　並且返回 繼承於 cls(Basemanager)
        return type(class_name, (cls,), {
            '_queryset_class': queryset_class,
            **cls._get_queryset_methods(queryset_class),
        })

　　6, 我們重寫的管理器　UserManager 繼承與新創建的類

    class UserManager(models.Manager):
    def get_queryset(self):
        # 重寫　get_queryset 方法　來自於 BaseManager　類之中的方法
        return super(UserManager, self).get_queryset().filter(isDelete=False)