django之orm

每个model有个manager，通过manager就可以对数据库的表进行select，delete，insert等，manager是对queryset进行操作，当需要求值时才会与数据库连接，queryset要调用相应的sql compiler转换成网站所用数据库的select语句，查询结果出来后又要通过orm转换成对象。

class ConnectionHandler(object):#connections = ConnectionHandler()数据库连接处理器
    def __init__(self, databases=None):
        """
        databases is an optional dictionary of database definitions (structured
        like settings.DATABASES).
        """
        self._databases = databases#词典
        self._connections = local()#线程tls

    @cached_property
    def databases(self):
        if self._databases is None:
            self._databases = settings.DATABASES
        if self._databases == {}:
            self._databases = {
                DEFAULT_DB_ALIAS: {
                    'ENGINE': 'django.db.backends.dummy',
                },
            }
        if self._databases[DEFAULT_DB_ALIAS] == {}:
            self._databases[DEFAULT_DB_ALIAS]['ENGINE'] = 'django.db.backends.dummy'

        if DEFAULT_DB_ALIAS not in self._databases:
            raise ImproperlyConfigured("You must define a '%s' database" % DEFAULT_DB_ALIAS)
        return self._databases

    def ensure_defaults(self, alias):
        """
        Puts the defaults into the settings dictionary for a given connection
        where no settings is provided.
        """
        try:
            conn = self.databases[alias]
        except KeyError:
            raise ConnectionDoesNotExist("The connection %s doesn't exist" % alias)

        conn.setdefault('ATOMIC_REQUESTS', False)
        conn.setdefault('AUTOCOMMIT', True)
        conn.setdefault('ENGINE', 'django.db.backends.dummy')
        if conn['ENGINE'] == 'django.db.backends.' or not conn['ENGINE']:
            conn['ENGINE'] = 'django.db.backends.dummy'
        conn.setdefault('CONN_MAX_AGE', 0)
        conn.setdefault('OPTIONS', {})
        conn.setdefault('TIME_ZONE', None)
        for setting in ['NAME', 'USER', 'PASSWORD', 'HOST', 'PORT']:
            conn.setdefault(setting, '')

    def prepare_test_settings(self, alias):
        """
        Makes sure the test settings are available in the 'TEST' sub-dictionary.
        """
        try:
            conn = self.databases[alias]
        except KeyError:
            raise ConnectionDoesNotExist("The connection %s doesn't exist" % alias)

        test_settings = conn.setdefault('TEST', {})
        for key in ['CHARSET', 'COLLATION', 'NAME', 'MIRROR']:
            test_settings.setdefault(key, None)

    def __getitem__(self, alias):
        if hasattr(self._connections, alias):
            return getattr(self._connections, alias)

        self.ensure_defaults(alias)
        self.prepare_test_settings(alias)
        db = self.databases[alias]
        backend = load_backend(db['ENGINE'])#装入后台
        conn = backend.DatabaseWrapper(db, alias)#生成数据库包装器
        setattr(self._connections, alias, conn)
        return conn

    def __setitem__(self, key, value):
        setattr(self._connections, key, value)

    def __delitem__(self, key):
        delattr(self._connections, key)

    def __iter__(self):
        return iter(self.databases)

    def all(self):
        return [self[alias] for alias in self]

    def close_all(self):
        for alias in self:
            try:
                connection = getattr(self._connections, alias)
            except AttributeError:
                continue
            connection.close()