@db_session
def store_member(member: Member) -> DBMember:
    db_member = DBMember.get(
        user=member.id,
        guild=member.guild.id
    )

    if not db_member:
        guild_repository.create(member.guild)
        store_user(member.user)

        db_member = DBMember(
            user=member.id,
            guild=member.guild.id
        )

        if member.nick:
            db_member.nick = member.nick

        commit()  # This gives pony.orm.core.TransactionIntegrityError: Object Member cannot be stored in the database. IntegrityError: FOREIGN KEY constraint failed


@db_session
def store_user(user: User):
    if not DBUser.exists(id=user.id):
        DBUser(
            id=user.id,
            name=user.name,
            discriminator=user.discriminator
        )

        commit()

##### TABLES
class User(db.Entity):
    id = PrimaryKey(int, size=64)
    name = Required(str, max_len=64)
    # ....
    members = Set('Member')


class Member(db.Entity):
    user = Required(User)
    guild = Required(Guild)