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

)

    experimenter_name = models.CharField(
        max_length=300, null=True, blank=True, help_text='For internal record-keeping'
    )

    code = models.CharField(
        default=random_chars_8,
        max_length=16,
        # set non-nullable, until we make our CharField non-nullable
        null=False,
        unique=True,
        doc="Randomly generated unique identifier for the session.",
    )

    mturk_HITId = models.CharField(
        max_length=300,
        null=True,
        blank=True,
        help_text='Hit id for this session on MTurk',
    )
    mturk_HITGroupId = models.CharField(
        max_length=300,
        null=True,
        blank=True,
        help_text='Hit id for this session on MTurk',
    )

    # since workers can drop out number of participants on server should be
    # greater than number of participants on mturk
    # value -1 indicates that this session it not intended to run on mturk
    mturk_num_participants = models.IntegerField(

from otree.db import models
from otree.models_concrete import ParticipantToPlayerLookup
from .varsmixin import ModelWithVars

class Participant(ModelWithVars):

    class Meta:
        ordering = ['pk']
        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)

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
    )

    _current_form_page_url = models.URLField()

    _max_page_index = models.PositiveIntegerField()

    _browser_bot_finished = models.BooleanField(default=False)

    _is_bot = models.BooleanField(default=False)

code = models.CharField(
        default=random_chars_8,
        max_length=16,
        # set non-nullable, until we make our CharField non-nullable
        null=False,
        unique=True,
        doc="Randomly generated unique identifier for the session.",
    )

    mturk_HITId = models.CharField(
        max_length=300,
        null=True,
        blank=True,
        help_text='Hit id for this session on MTurk',
    )
    mturk_HITGroupId = models.CharField(
        max_length=300,
        null=True,
        blank=True,
        help_text='Hit id for this session on MTurk',
    )

    # since workers can drop out number of participants on server should be
    # greater than number of participants on mturk
    # value -1 indicates that this session it not intended to run on mturk
    mturk_num_participants = models.IntegerField(
        default=-1, help_text="Number of participants on MTurk"
    )

    mturk_use_sandbox = models.BooleanField(
        default=True, help_text="Should this session be created in mturk sandbox?"
    )

)

    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
    )

    _current_form_page_url = models.URLField()

    _max_page_index = models.PositiveIntegerField()

    _browser_bot_finished = models.BooleanField(default=False)

    _is_bot = models.BooleanField(default=False)
    # can't start with an underscore because used in template
    # can't end with underscore because it's a django field (fields.E001)

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)

    _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,

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)

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

    code = models.CharField(
        default=random_chars_8,
        max_length=16,

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 "
            "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(