How to use the otree.db.models.PositiveIntegerField function in otree

app_label = "otree"
        index_together = ['session', 'mturk_worker_id', 'mturk_assignment_id']

    session = models.ForeignKey(
        'otree.Session', on_delete=models.CASCADE
    )

    label = models.CharField(
        max_length=50, null=True, doc=(
            "Label assigned by the experimenter. Can be assigned by passing a "
            "GET param called 'participant_label' to the participant's start "
            "URL"
        )
    )

    id_in_session = models.PositiveIntegerField(null=True)

    payoff = models.CurrencyField(default=0)

    time_started = models.DateTimeField(null=True)
    user_type_in_url = constants_internal.user_type_participant
    mturk_assignment_id = models.CharField(
        max_length=50, null=True)
    mturk_worker_id = models.CharField(max_length=50, null=True)

    _index_in_subsessions = models.PositiveIntegerField(default=0, null=True)

    _index_in_pages = models.PositiveIntegerField(default=0, db_index=True)

    def _id_in_session(self):
        """the human-readable version."""
        return 'P{}'.format(self.id_in_session)

"""Base class for all Groups.
    """

    class Meta:
        abstract = True
        index_together = ['session', 'id_in_subsession']
        ordering = ['pk']

    id_in_subsession = models.PositiveIntegerField(db_index=True)

    session = models.ForeignKey(
        'otree.Session', related_name='%(app_label)s_%(class)s',
        on_delete=models.CASCADE
    )

    round_number = models.PositiveIntegerField(db_index=True)

    def __unicode__(self):
        return str(self.pk)

    def get_players(self):
        return list(self.player_set.order_by('id_in_group'))

    def get_player_by_id(self, id_in_group):
        try:
            return self.player_set.get(id_in_group=id_in_group)
        except django.core.exceptions.ObjectDoesNotExist:
            raise ValueError(
                'No player with id_in_group {}'.format(id_in_group)) from None

    def get_player_by_role(self, role):
        for p in self.get_players():

"GET param called 'participant_label' to the participant's start "
            "URL"
        )
    )

    id_in_session = models.PositiveIntegerField(null=True)

    payoff = models.CurrencyField(default=0)

    time_started = models.DateTimeField(null=True)
    user_type_in_url = constants_internal.user_type_participant
    mturk_assignment_id = models.CharField(
        max_length=50, null=True)
    mturk_worker_id = models.CharField(max_length=50, null=True)

    _index_in_subsessions = models.PositiveIntegerField(default=0, null=True)

    _index_in_pages = models.PositiveIntegerField(default=0, db_index=True)

    def _id_in_session(self):
        """the human-readable version."""
        return 'P{}'.format(self.id_in_session)

    _waiting_for_ids = models.CharField(null=True, max_length=300)

    code = models.CharField(
        default=random_chars_8,
        max_length=16,
        # set non-nullable, until we make our CharField non-nullable
        null=False,
        # unique implies DB index
        unique=True,

"main ViewList for sessions"
        ),
    )

    comment = models.TextField(blank=True)

    _anonymous_code = models.CharField(
        default=random_chars_10, max_length=10, null=False, db_index=True
    )

    is_demo = models.BooleanField(default=False)

    _admin_report_app_names = models.TextField(default='')
    _admin_report_num_rounds = models.CharField(default='', max_length=255)

    num_participants = models.PositiveIntegerField()

    def __unicode__(self):
        return self.code

    @property
    def participation_fee(self):
        '''This method is deprecated from public API,
        but still useful internally (like data export)'''
        return self.config['participation_fee']

    @property
    def real_world_currency_per_point(self):
        '''This method is deprecated from public API,
        but still useful internally (like data export)'''
        return self.config['real_world_currency_per_point']

from otree.db import models
from otree.models.fieldchecks import ensure_field
from django.db import models as djmodels


class BasePlayer(models.Model):
    """
    Base class for all players.
    """

    class Meta:
        abstract = True
        index_together = ['participant', 'round_number']
        ordering = ['pk']

    id_in_group = models.PositiveIntegerField(
        null=True,
        db_index=True,
        doc=("Index starting from 1. In multiplayer games, "
             "indicates whether this is player 1, player 2, etc.")
    )

    # don't modify this directly! Set player.payoff instead
    _payoff = models.CurrencyField(
        null=True,
        doc="""The payoff the player made in this subsession""",
        default=0
    )

    participant = models.ForeignKey(
        'otree.Participant', related_name='%(app_label)s_%(class)s',
        on_delete=models.CASCADE

"""Base class for all Subsessions.
    """

    class Meta:
        abstract = True
        ordering = ['pk']
        index_together = ['session', 'round_number']

    session = models.ForeignKey(
        'otree.Session',
        related_name='%(app_label)s_%(class)s',
        null=True,
        on_delete=models.CASCADE,
    )

    round_number = models.PositiveIntegerField(
        db_index=True,
        doc='''If this subsession is repeated (i.e. has multiple rounds), this
        field stores the position of this subsession, among subsessions
        in the same app.
        ''',
    )

    def in_round(self, round_number):
        return in_round(type(self), round_number, session=self.session)

    def in_rounds(self, first, last):
        return in_rounds(type(self), first, last, session=self.session)

    def in_previous_rounds(self):
        return self.in_rounds(1, self.round_number - 1)

)
    )

    id_in_session = models.PositiveIntegerField(null=True)

    payoff = models.CurrencyField(default=0)

    time_started = models.DateTimeField(null=True)
    user_type_in_url = constants_internal.user_type_participant
    mturk_assignment_id = models.CharField(
        max_length=50, null=True)
    mturk_worker_id = models.CharField(max_length=50, null=True)

    _index_in_subsessions = models.PositiveIntegerField(default=0, null=True)

    _index_in_pages = models.PositiveIntegerField(default=0, db_index=True)

    def _id_in_session(self):
        """the human-readable version."""
        return 'P{}'.format(self.id_in_session)

    _waiting_for_ids = models.CharField(null=True, max_length=300)

    code = models.CharField(
        default=random_chars_8,
        max_length=16,
        # set non-nullable, until we make our CharField non-nullable
        null=False,
        # unique implies DB index
        unique=True,
        doc=(
            "Randomly generated unique identifier for the participant. If you "

null=True,
        doc="""The payoff the player made in this subsession""",
        default=0
    )

    participant = models.ForeignKey(
        'otree.Participant', related_name='%(app_label)s_%(class)s',
        on_delete=models.CASCADE
    )

    session = models.ForeignKey(
        'otree.Session', related_name='%(app_label)s_%(class)s',
        on_delete=models.CASCADE
    )

    round_number = models.PositiveIntegerField(db_index=True)

    _gbat_arrived = models.BooleanField(default=False)
    _gbat_grouped = models.BooleanField(default=False)

    @property
    def payoff(self):
        return self._payoff

    @payoff.setter
    def payoff(self, value):
        if value is None:
            value = 0
        delta = value - self._payoff
        self._payoff += delta
        self.participant.payoff += delta
        # should save it because it may not be obvious that modifying

"would like to merge this dataset with those from another "
            "subsession in the same session, you should join on this field, "
            "which will be the same across subsessions."
        )
    )


    visited = models.BooleanField(
        default=False, db_index=True,
        doc="""Whether this user's start URL was opened"""
    )

    ip_address = models.GenericIPAddressField(null=True)

    # stores when the page was first visited
    _last_page_timestamp = models.PositiveIntegerField(null=True)

    _last_request_timestamp = models.PositiveIntegerField(null=True)

    is_on_wait_page = models.BooleanField(default=False)

    # these are both for the admin
    # In the changelist, simply call these "page" and "app"
    _current_page_name = models.CharField(max_length=200, null=True,
                                          verbose_name='page')
    _current_app_name = models.CharField(max_length=200, null=True,
                                         verbose_name='app')

    # only to be displayed in the admin participants changelist
    _round_number = models.PositiveIntegerField(
        null=True
    )