Using flask_wtf and wtforms_appengine to generate forms from an ndb.Model subclass with an ndb.KeyProperty

entities.py

from google.appengine.ext import ndb


class AuditMixin(object):
    """Audit fields for datastore objects"""
    created_at = ndb.DateTimeProperty(auto_now_add=True)
    created_user = ndb.StringProperty(indexed=False)
    modified_at = ndb.DateTimeProperty(auto_now=True)
    modified_user = ndb.StringProperty(indexed=False)


class Themes(ndb.Model, AuditMixin):
    name = ndb.StringProperty(required=True)
    template = ndb.StringProperty(required=True)


class Article(ndb.Model, AuditMixin):
    """Entity that stores the article content and redirect URL"""
    image_url = ndb.TextProperty(required=True)
    summary = ndb.TextProperty(required=True)
    redirect_url = ndb.StringProperty(required=True)
    #: The date to display for the article. Defaults to now
    story_date = ndb.DateTimeProperty(auto_now_add=True)
    theme = ndb.KeyProperty(Themes, required=True)

form.html

{% extends "base.html" %}

{% block content %}
    <form method="POST">
        {{ form.hidden_tag() }}
        <table>
        {% for field in form if field.name != 'csrf_token' %}
            <tr>
                <th>{{ field.label }}:</th>
                <td>{{ field }}</td>
            </tr>
        {% endfor %}
        </table>
    </form>
{% endblock %}

forms.py

import flask_wtf
from wtforms_appengine import ndb
import entities

ArticleForm = ndb.model_form(entities.Article,
                             base_class=flask_wtf.Form,
                             exclude=vars(entities.AuditMixin),
                             field_args={'theme': {'get_label': 'name'}})

main.py

# snip from a flask app
@app.route('/form')
def form():
    """Return a form."""
    return render_template('form.html', form=forms.ArticleForm())

rendered.html

<form method="POST">
        <div style="display:none;"><input id="csrf_token" name="csrf_token" type="hidden" value="1438627230##43addfeb49214c338bf559c4daaf5d3a76525758"></div>
        <table>
        
            <tr>
                <th><label for="image_url">Image Url</label>:</th>
                <td><textarea id="image_url" name="image_url"></textarea></td>
            </tr>
        
            <tr>
                <th><label for="summary">Summary</label>:</th>
                <td><textarea id="summary" name="summary"></textarea></td>
            </tr>
        
            <tr>
                <th><label for="redirect_url">Redirect Url</label>:</th>
                <td><input id="redirect_url" name="redirect_url" type="text" value=""></td>
            </tr>
        
            <tr>
                <th><label for="theme">Theme</label>:</th>
                <td><select id="theme" name="theme"><option value="entertainment">Entertainment Gossip</option><option value="news">News Site</option></select></td>
            </tr>
        
        </table>
    </form>

This is what it looks like in a browser