python - デフォルトの接続が定義されていません

Question

このエラーを解決するにはどうすればよいですか?

File "/usr/local/lib/python2.7/dist-packages/mongoengine/connection.py", line 95, in get_connection
    raise ConnectionError(msg)
mongoengine.connection.ConnectionError: You have not defined a default connection

model.py:

from flask.ext.mongoengine import MongoEngine

db = MongoEngine()

class Currencies(db.Document):
    # meta = {"db_alias": "currencies"}
    name = db.StringField(max_length=40, min_length=3)
    def __unicode__(self):
        return self.name

app.py の MongoEngine 設定:

from models import db

app = Flask(__name__)
app.config['MONGODB_SETTINGS'] = {'DB':'testing'}
db.init_app(app)

ビュー.py:

from models import Currencies

print Currencies.objects.find()

Flask-Admin は正常に動作します。