How to use the pydal.objects.Field function in pydal

def count(self, query, distinct=None, snapshot=True):
        if not isinstance(query, Query):
            raise SyntaxError("Type '%s' not supported in count" % type(query))
        distinct_fields = []
        if distinct is True:
            distinct_fields = [x for x in query.first.table if x.name != 'id']
        elif distinct:
            if isinstance(distinct, Field):
                distinct_fields = [distinct]
            else:
                while (isinstance(distinct, Expression) and
                        isinstance(distinct.second, Field)):
                    distinct_fields += [distinct.second]
                    distinct = distinct.first
                if isinstance(distinct, Field):
                    distinct_fields += [distinct]
            distinct = True
        expanded = Expansion(
            self, 'count', query, fields=distinct_fields, distinct=distinct)
        ctable = expanded.get_collection()
        if not expanded.pipeline:
            return ctable.count(filter=expanded.query_dict)
        for record in ctable.aggregate(expanded.pipeline):
            return record['count']

Field('email', length=512, default='',
                          label=self.messages.label_email,
                          requires=is_unique_email),
                    Field('username', length=128, default='',
                          label=self.messages.label_username,
                          requires=is_unique_username),
                    Field(passfield, 'password', length=512,
                          readable=False, label=self.messages.label_password,
                          requires=[hasLength(minsize=settings.password_min_length), is_crypted]),
                    Field('registration_key', length=512,
                          writable=False, readable=False, default='',
                          label=self.messages.label_registration_key),
                    Field('reset_password_key', length=512,
                          writable=False, readable=False, default='',
                          label=self.messages.label_reset_password_key),
                    Field('registration_id', length=512,
                          writable=False, readable=False, default='',
                          label=self.messages.label_registration_id),
                    *extra_fields,
                    **dict(
                        migrate=self.__get_migrate(settings.table_user_name,
                                                   migrate),
                        fake_migrate=fake_migrate,
                        format='%(username)s'))
            else:
                db.define_table(
                    settings.table_user_name,
                    Field('first_name', length=128, default='',
                          label=self.messages.label_first_name,
                          requires=is_not_empty),
                    Field('last_name', length=128, default='',
                          label=self.messages.label_last_name,

self.get_mailboxes()

        names = self.connection.mailbox_names.keys()

        for name in names:
            self.db.define_table("%s" % name,
                Field("uid", writable=False),
                Field("created", "datetime", writable=False),
                Field("content", "text", writable=False),
                Field("to", writable=False),
                Field("cc", writable=False),
                Field("bcc", writable=False),
                Field("sender", writable=False),
                Field("size", "integer", writable=False),
                Field("subject", writable=False),
                Field("mime", writable=False),
                Field("email", "text", writable=False, readable=False),
                Field("attachments", "text", writable=False, readable=False),
                Field("encoding", writable=False),
                Field("answered", "boolean"),
                Field("deleted", "boolean"),
                Field("draft", "boolean"),
                Field("flagged", "boolean"),
                Field("recent", "boolean", writable=False),
                Field("seen", "boolean")
                )

            # Set a special _mailbox attribute for storing
            # native mailbox names
            self.db[name].mailbox = \
                self.connection.mailbox_names[name]

def many_query(ref, rid):
        if ref.cast and isinstance(rid, _Field):
            rid = rid.cast(ref.cast)
        return ref.model_instance.table[ref.field] == rid

def __call__(self, query=None, ignore_common_filters=None):
        if isinstance(query, Table):
            query = self._adapter.id_query(query)
        elif isinstance(query, Field):
            query = query!=None
        elif isinstance(query, dict):
            icf = query.get("ignore_common_filters")
            if icf: ignore_common_filters = icf
        return Set(self, query, ignore_common_filters=ignore_common_filters)

label=self.messages.label_description),
                *extra_fields,
                **dict(
                    migrate=self.__get_migrate(
                        settings.table_group_name, migrate),
                    fake_migrate=fake_migrate,
                    format='%(role)s (%(id)s)'))
        reference_table_group = 'reference %s' % settings.table_group_name
        if not settings.table_membership_name in db.tables:
            extra_fields = settings.extra_fields.get(
                settings.table_membership_name, []) + signature_list
            db.define_table(
                settings.table_membership_name,
                Field('user_id', reference_table_user,
                      label=self.messages.label_user_id),
                Field('group_id', reference_table_group,
                      label=self.messages.label_group_id),
                *extra_fields,
                **dict(
                    migrate=self.__get_migrate(
                        settings.table_membership_name, migrate),
                    fake_migrate=fake_migrate))
        if not settings.table_permission_name in db.tables:
            extra_fields = settings.extra_fields.get(
                settings.table_permission_name, []) + signature_list
            db.define_table(
                settings.table_permission_name,
                Field('group_id', reference_table_group,
                      label=self.messages.label_group_id),
                Field('name', default='default', length=512,
                      label=self.messages.label_name,
                      requires=is_not_empty),

else:
            self.static_names = None
        if not isinstance(self.connection.mailbox_names, dict):
            self.get_mailboxes()

        names = self.connection.mailbox_names.keys()

        for name in names:
            self.db.define_table("%s" % name,
                Field("uid", writable=False),
                Field("created", "datetime", writable=False),
                Field("content", "text", writable=False),
                Field("to", writable=False),
                Field("cc", writable=False),
                Field("bcc", writable=False),
                Field("sender", writable=False),
                Field("size", "integer", writable=False),
                Field("subject", writable=False),
                Field("mime", writable=False),
                Field("email", "text", writable=False, readable=False),
                Field("attachments", "text", writable=False, readable=False),
                Field("encoding", writable=False),
                Field("answered", "boolean"),
                Field("deleted", "boolean"),
                Field("draft", "boolean"),
                Field("flagged", "boolean"),
                Field("recent", "boolean", writable=False),
                Field("seen", "boolean")
                )

            # Set a special _mailbox attribute for storing
            # native mailbox names

self.signature = Table(
            self.db, 'auth_signature',
            Field('is_active', 'boolean',
                  default=True,
                  readable=False, writable=False,
                  label='Is Active'),
            Field('created_on', 'datetime',
                  default=lambda: datetime.now(),
                  writable=False, readable=False,
                  label='Created On'),
            Field('created_by',
                  reference_user,
                  default=lazy_user, represent=represent,
                  writable=False, readable=False,
                  label='Created By', ondelete=ondelete),
            Field('modified_on', 'datetime',
                  update=lambda: datetime.now(),
                  default=lambda: datetime.now(),
                  writable=False, readable=False,
                  label='Modified On'),
            Field('modified_by',
                  reference_user, represent=represent,
                  default=lazy_user, update=lazy_user,
                  writable=False, readable=False,
                  label='Modified By',  ondelete=ondelete))

for name in names:
            self.db.define_table("%s" % name,
                Field("uid", writable=False),
                Field("created", "datetime", writable=False),
                Field("content", "text", writable=False),
                Field("to", writable=False),
                Field("cc", writable=False),
                Field("bcc", writable=False),
                Field("sender", writable=False),
                Field("size", "integer", writable=False),
                Field("subject", writable=False),
                Field("mime", writable=False),
                Field("email", "text", writable=False, readable=False),
                Field("attachments", "text", writable=False, readable=False),
                Field("encoding", writable=False),
                Field("answered", "boolean"),
                Field("deleted", "boolean"),
                Field("draft", "boolean"),
                Field("flagged", "boolean"),
                Field("recent", "boolean", writable=False),
                Field("seen", "boolean")
                )

            # Set a special _mailbox attribute for storing
            # native mailbox names
            self.db[name].mailbox = \
                self.connection.mailbox_names[name]

            # decode quoted printable
            self.db[name].to.represent = self.db[name].cc.represent = \
            self.db[name].bcc.represent = self.db[name].sender.represent = \
            self.db[name].subject.represent = self.header_represent

settings.table_membership_name,
                Field('user_id', reference_table_user,
                      label=self.messages.label_user_id),
                Field('group_id', reference_table_group,
                      label=self.messages.label_group_id),
                *extra_fields,
                **dict(
                    migrate=self._get_migrate(
                        settings.table_membership_name, migrate),
                    fake_migrate=fake_migrate))
        if settings.table_permission_name not in db.tables:
            extra_fields = settings.extra_fields.get(
                settings.table_permission_name, []) + signature_list
            db.define_table(
                settings.table_permission_name,
                Field('group_id', reference_table_group,
                      label=self.messages.label_group_id),
                Field('name', default='default', length=512,
                      label=self.messages.label_name,
                      requires=is_not_empty),
                Field('table_name', length=512,
                      label=self.messages.label_table_name),
                Field('record_id', 'integer', default=0,
                      label=self.messages.label_record_id,
                      requires=IS_INT_IN_RANGE(0, 10 ** 9)),
                *extra_fields,
                **dict(
                    migrate=self._get_migrate(
                        settings.table_permission_name, migrate),
                    fake_migrate=fake_migrate))
        if settings.table_event_name not in db.tables:
            db.define_table(