SQLAlchemy-作为子查询结果的列

我有两个由SQLAlchemy管理的表,如下所示:

class Contact(db.Model):
    __tablename__      = 'contacts'

    id                  = db.Column(db.Integer, primary_key=True, autoincrement=True)

    name                = db.Column(db.String(250), nullable=True, default=None)
    email               = db.Column(db.String(250), nullable=False)

    last_posted_message = db.SubQuery('SELECT created FROM messages WHERE contact_id = {current_alias}.id ORDER BY created DESC LIMIT 1')


class Message(db.Model):
    __tablename__      = 'messages'

    id                  = db.Column(db.Integer, primary_key=True, autoincrement=True)

    message             = db.Column(db.String(250), nullable=False)
    created             = db.Column(db.DateTime, nullable=False, default=datetime.datetime.utcnow)

    contact_id          = db.Column(db.Integer, db.ForeignKey('contacts.id'))

last_posted_message ,要求SQLAlchemy为该特定列执行子查询。这在SQL查询中很容易实现,如下所示:

'SELECT c.id, c.name, c.email, (SELECT created FROM messages WHERE contact_id = c.id ORDER BY created DESC LIMIT 1) AS last_posted_message FROM contacts c'

所以我希望它可以通过SQLAlchemy实现,不会有很多麻烦。

我试着在网上搜索,但没有成功。