How to use the runestone.db.StringField function in runestone

from flask.ext.mongoengine import MongoEngine
from flask.ext.security import Security, MongoEngineUserDatastore, \
    UserMixin, RoleMixin, login_required  

from runestone import db, app


# Define models

class Role(db.Document, RoleMixin):
    name = db.StringField(max_length=80, unique=True)
    description = db.StringField(max_length=255)

class User(db.Document, UserMixin):
    email = db.StringField(max_length=255)
    password = db.StringField(max_length=255)
    active = db.BooleanField(default=True)
    confirmed_at = db.DateTimeField()
    course = db.StringField(max_length=50)
    roles = db.ListField(db.ReferenceField(Role), default=[])

from flask.ext.mongoengine import MongoEngine
from flask.ext.security import Security, MongoEngineUserDatastore, \
    UserMixin, RoleMixin, login_required  

from runestone import db, app


# Define models

class Role(db.Document, RoleMixin):
    name = db.StringField(max_length=80, unique=True)
    description = db.StringField(max_length=255)

class User(db.Document, UserMixin):
    email = db.StringField(max_length=255)
    password = db.StringField(max_length=255)
    active = db.BooleanField(default=True)
    confirmed_at = db.DateTimeField()
    course = db.StringField(max_length=50)
    roles = db.ListField(db.ReferenceField(Role), default=[])

from runestone import db, app


# Define models

class Role(db.Document, RoleMixin):
    name = db.StringField(max_length=80, unique=True)
    description = db.StringField(max_length=255)

class User(db.Document, UserMixin):
    email = db.StringField(max_length=255)
    password = db.StringField(max_length=255)
    active = db.BooleanField(default=True)
    confirmed_at = db.DateTimeField()
    course = db.StringField(max_length=50)
    roles = db.ListField(db.ReferenceField(Role), default=[])

from flask.ext.mongoengine import MongoEngine
from flask.ext.security import Security, MongoEngineUserDatastore, \
    UserMixin, RoleMixin, login_required  

from runestone import db, app


# Define models

class Role(db.Document, RoleMixin):
    name = db.StringField(max_length=80, unique=True)
    description = db.StringField(max_length=255)

class User(db.Document, UserMixin):
    email = db.StringField(max_length=255)
    password = db.StringField(max_length=255)
    active = db.BooleanField(default=True)
    confirmed_at = db.DateTimeField()
    course = db.StringField(max_length=50)
    roles = db.ListField(db.ReferenceField(Role), default=[])

from flask.ext.mongoengine import MongoEngine
from flask.ext.security import Security, MongoEngineUserDatastore, \
    UserMixin, RoleMixin, login_required  

from runestone import db, app


# Define models

class Role(db.Document, RoleMixin):
    name = db.StringField(max_length=80, unique=True)
    description = db.StringField(max_length=255)

class User(db.Document, UserMixin):
    email = db.StringField(max_length=255)
    password = db.StringField(max_length=255)
    active = db.BooleanField(default=True)
    confirmed_at = db.DateTimeField()
    course = db.StringField(max_length=50)
    roles = db.ListField(db.ReferenceField(Role), default=[])