new = cls()

        new.user_id = user_id

        new.permission = perm

        try:

            Session.add(new)

            Session.commit()

        except:

            Session.rollback()

    @classmethod

    def revoke_perm(cls, user_id, perm):

        if not isinstance(perm, Permission):

            raise Exception('perm needs to be an instance of Permission class')

        try:

            Session.query(cls).filter(cls.user_id == user_id)\

                .filter(cls.permission == perm).delete()

            Session.commit()

        except:

            Session.rollback()

class UsersGroupRepoToPerm(Base):

    __tablename__ = 'users_group_repo_to_perm'

    users_group_to_perm_id = Column("users_group_to_perm_id", Integer(), nullable=False, unique=True, default=None, primary_key=True)

    users_group_id = Column("users_group_id", Integer(), ForeignKey('users_groups.users_group_id'), nullable=False, unique=None, default=None)

    permission_id = Column("permission_id", Integer(), ForeignKey('permissions.permission_id'), nullable=False, unique=None, default=None)

    repository_id = Column("repository_id", Integer(), ForeignKey('repositories.repo_id'), nullable=False, unique=None, default=None)

    users_group = relationship('UsersGroup')

    permission = relationship('Permission')

    repository = relationship('Repository')

class UsersGroupToPerm(Base):

    __tablename__ = 'users_group_to_perm'

    users_group_to_perm_id = Column("users_group_to_perm_id", Integer(), nullable=False, unique=True, default=None, primary_key=True)

    users_group_id = Column("users_group_id", Integer(), ForeignKey('users_groups.users_group_id'), nullable=False, unique=None, default=None)

    permission_id = Column("permission_id", Integer(), ForeignKey('permissions.permission_id'), nullable=False, unique=None, default=None)

    users_group = relationship('UsersGroup')

    permission = relationship('Permission')

    @classmethod

    def has_perm(cls, users_group_id, perm):

        if not isinstance(perm, Permission):

            raise Exception('perm needs to be an instance of Permission class')