Not a Valid Choice for Dynamic Select Field WTFORMS

My guess is that Area.id is a int – when data comes back from the client it is treated as a string by WTForms unless a callable is passed to the coerce keyword argument of the wtforms.fields.SelectField constructor:

area = SelectField(coerce=int)

Alternately, if you are using SQLAlchemy you could use wtforms.ext.sqlalchemy.fields.QuerySelectField (wtforms_sqlalchemy if you are using WTForms 3+):

area = QuerySelectField(query_factory=Area.objects.all,
                            get_pk=lambda a: a.id,
                            get_label=lambda a: a.name)