from sqlalchemy import event, Column, String
from sqlalchemy.orm.session import Session


class APIMixin(object):
    api_id = Column(String(255), nullable=False)

    def get_api_id(self):
        if self.api_id is None:
            self.create_api_object()

        return self.api_id

    def before_created(self):
        if self.api_id is None:
            self.create_api_object()

    def after_deleted(self):
        if self.api_id is not None:
            self.remove_api_object()
            self.api_id = None

    def create_api_object(self):
        raise NotImplementedError

    def remove_api_object(self):
        raise NotImplementedError

    def after_commit(self):
        pass


class SessionHelper(object):

    def __init__(self):
        self.new = set()
        self.dirty = set()
        self.deleted = set()

        event.listen(Session, 'before_flush', self.before_flush)
        event.listen(Session, 'after_flush', self.after_flush)
        event.listen(Session, 'after_commit', self.after_commit)
        event.listen(Session, 'after_rollback', self.after_rollback)

    def before_flush(self, session, flush_context, instances):
        for obj in session.new:
            if isinstance(obj, APIMixin):
                obj.before_created()

    def after_flush(self, session, flush_context):
        self.new.update(
            obj for obj in session.new
            if isinstance(obj, APIMixin))

        self.dirty.update(
            obj for obj in session.dirty
            if isinstance(obj, APIMixin))

        self.deleted.update(
            obj for obj in session.deleted
            if isinstance(obj, APIMixin))

    def after_commit(self, session):
        for obj in self.new:
            obj.after_commit()
        self.new.clear()

        for obj in self.dirty:
            obj.after_commit()
        self.dirty.clear()

        for obj in self.deleted:
            obj.after_deleted()
        self.deleted.clear()

    def after_rollback(self, session):
        self.dirty.clear()
        self.deleted.clear()

        for obj in self.new:
            obj.after_deleted()
        self.new.clear()


helper = SessionHelper()