python ORM 模块peewee(四): Model类源码初探

Model的初始化方法主要继承自BaseModel类的__new__方法。 下面粗略的通过源码来看看：

class BaseModel(type):
    # 定义可被继承的属性列表（全局）
    inheritable = set([
        'constraints', 'database', 'db_table_func', 'indexes', 'order_by',
        'primary_key', 'schema', 'validate_backrefs', 'only_save_dirty'])

    def __new__(cls, name, bases, attrs):
        # 如果没有父类，则之间创建BaseModel类
        if not bases:
            return super(BaseModel, cls).__new__(cls, name, bases, attrs)

        # Meta类的属性通过meta_options存储在Model类中
        meta_options = {}
        
        # 将Meta从属性中移除，将Meta中的非私有属性加入meta_options中
        meta = attrs.pop('Meta', None)
        if meta:
            for k, v in meta.__dict__.items():
                if not k.startswith('_'):
                    meta_options[k] = v

        # 从meta中获取主键信息
        model_pk = getattr(meta, 'primary_key', None)
        parent_pk = None

        ############################################################
        # 开始考虑从父类中继承的情况
        #############################################################
        for b in bases:
            if not hasattr(b, '_meta'):
                continue

            base_meta = getattr(b, '_meta')
            if parent_pk is None:
                parent_pk = deepcopy(base_meta.primary_key)
            all_inheritable = cls.inheritable | base_meta._additional_keys

            # 获取父类中的Meta内部类字段，只考虑all_inheritable中的字段
            for (k, v) in base_meta.__dict__.items():
                if k in all_inheritable and k not in meta_options:
                    meta_options[k] = v
                     
            # 获取父类中的Fields, 即表的字段
            for (k, v) in b.__dict__.items():
                if k in attrs:
                    continue
                if isinstance(v, FieldDescriptor):
                    if not v.field.primary_key:
                        attrs[k] = deepcopy(v.field)

        # initialize the new class and set the magic attributes
        cls = super(BaseModel, cls).__new__(cls, name, bases, attrs)
        ModelOptionsBase = meta_options.get('model_options_base', ModelOptions)
        cls._meta = ModelOptionsBase(cls, **meta_options)
        cls._data = None
        cls._meta.indexes = list(cls._meta.indexes)

        # 默认表名的设定，Model名的小写，然后将非数字和英文字符换成'_'
        if not cls._meta.db_table:
            cls._meta.db_table = re.sub('[^\w]+', '_', cls.__name__.lower())

        # replace fields with field descriptors, calling the add_to_class hook
        # 这里筛选attr中的Field类型字段，设置Model中的数据类型
        # 也许可以测试一下类里面的函数是怎么继承的
        fields = []
        for name, attr in cls.__dict__.items():
            if isinstance(attr, Field):
                if attr.primary_key and model_pk:
                    raise ValueError('primary key is overdetermined.')
                elif attr.primary_key:
                    model_pk, pk_name = attr, name
                else:
                    fields.append((attr, name))
        
        composite_key = False
        # 默认主键的设置，如果无法从父类继承，，则使用'id'为key, 也就是行号
        if model_pk is None:
            if parent_pk:
                model_pk, pk_name = parent_pk, parent_pk.name
            else:
                model_pk, pk_name = PrimaryKeyField(primary_key=True), 'id'
        elif isinstance(model_pk, CompositeKey):
            pk_name = '_composite_key'
            composite_key = True

        # 如果model本身有主键的情况
        if model_pk is not False:
            model_pk.add_to_class(cls, pk_name)
            cls._meta.primary_key = model_pk
            cls._meta.auto_increment = (
                isinstance(model_pk, PrimaryKeyField) or
                bool(model_pk.sequence))
            cls._meta.composite_key = composite_key

        # 设置Fields
        for field, name in fields:
            field.add_to_class(cls, name)

        # create a repr and error class before finalizing
        # __unicode__的设置
        if hasattr(cls, '__unicode__'):
            setattr(cls, '__repr__', lambda self: '<%s: %r>' % (
                cls.__name__, self.__unicode__()))

        # 错误信息
        exc_name = '%sDoesNotExist' % cls.__name__
        exc_attrs = {'__module__': cls.__module__}
        exception_class = type(exc_name, (DoesNotExist,), exc_attrs)
        cls.DoesNotExist = exception_class
        cls._meta.prepared()

        if hasattr(cls, 'validate_model'):
            cls.validate_model()

        return cls