We Keep Coding

Query by computed property in python mongoengine

I wondered if it is possible to query documents in MongoDB by computed properties using mongoengine in python.
Currently, my model looks like this:
class SnapshotIndicatorKeyValue(db.Document):
meta = {"collection": "snapshot_indicator_key_values"}
snapshot_id = db.ObjectIdField(nullable=False)
indicator_key_id = db.ObjectIdField(nullable=False)
value = db.FloatField(nullable=False)
created_at = db.DateTimeField()
updated_at = db.DateTimeField()
#property
def snapshot(self):
return Snapshot.objects(id=self.snapshot_id).first()
def indicator_key(self):
return IndicatorKey.objects(id=self.indicator_key_id).first()
When I do for example SnapshotIndicatorKeyValue .objects().first().snapshot, I can access the snapshotproperty.
But when I try to query it, it doesn't work. For example:
SnapshotIndicatorKeyValue.objects(snapshot__date_time__lte=current_date_time)
I get the error `mongoengine.errors.InvalidQueryError: Cannot resolve field "snapshot"``
Is there any way to get this working with queries?
I need to query SnapshotIndicatorKeyValue based on a property of snapshot.

In order to query the snapshot property directly through mongoengine, you can reference the related snapshot object rather than the snapshot_id in your SnapshotIndicatorKeyValue document definition.
An amended model using a Reference field would be like this:
from mongoengine import Document, ReferenceField
class Snapshot(Document)
property_abc = RelevantPropertyHere() # anything you need
class SnapshotIndicatorKeyValue(Document):
snapshot = ReferenceField(Snapshot)
You would sucessively save an instance of Snapshot and an instance of SnapshotIndicatorKeyValue like this:
sample_snapshot = Snapshot(property_abc=relevant_value_here) # anything you need
sample_snapshot.save()
sample_indicatorkeyvalue = SnapshotIndicatorKeyValue()
sample_indicatorkeyvalue.snapshot = sample_snapshot
sample_indicatorkeyvalue.save()
You can then refer to any of the snapshot's properties through:
SnapshotIndicatorKeyValue.objects.first().snapshot.property_abc

how to specify the collection in mongoengine?

Hello community,
I want to insert a mongoengine document to specific collection.
I know there is the save method on the document. However this requires to use the connect method from mongoengine which I don't want to use. I need to pass in the collection and save the document to the specified collection. Below is some example code which illustrates my problem.
Is there a way to do this?
Thanks in advance!
from mongoengine import Document, StringField
from pymongo import MongoClient
from pymongo.collection import Collection
# document which must be stored in measurements collection
class Example(Document):
value = StringField()
def insert(value: str, collection: Collection):
# create document
example = Example(value=value)
# TODO: insert document in specified collection
# conenct to db
client = MongoClient('mongodb://localhost:27017/')
db = client.test
collection: Collection = db["measurements"]
# insert document to collection
insert(value="123", collection=collection)

I would not recommend you to build a complex application on this pattern (mixing Mongoengine and pymongo) but to answer your question, you can achieve what you want with:
def insert(value: str, collection: Collection):
example = Example(value=value)
bson_data = example.to_mongo()
collection.insert_one(bson_data)

Update multiple fields on Google NDB entity

Working with Google App Engine for Python, I am trying to create and then update an ndb entity. To update a single property, you can just access the property using a dot, e.g.
post.body = body
But I would like to know if there is a simple way to update multiple fields within an ndb entity. The following code:
class Create(Handler):
def post(self):
## code to get params
post = Post(author = author,
title = title,
body = body)
post.put()
class Update(Handler):
def post(self, post_id):
post = post.get_by_id(int(post_id))
fields = ['author', 'title', 'body']
data = get_params(self.request, fields)
for field in fields:
post[field] = data[field]
post.put()
The "Create" handler works fine, but the "Update" handler results in:
TypeError: 'Post' object does not support item assignment
So it seems I would need to access the properties using a dot, but that is not going to work when I have a list of properties I want to access.
Can someone provide an alternative way to update multiple properties of an NDB entity after it has been created?

You should use setattr.
for field in fields:
setattr(post, field, data[field])
(Note that GAE objects do actually provide a hidden way of updating them via a dict, but you should use the public interface.)

You can use the populate method:
post.populate(**data)

Refresh mongodb collection structure through python mongoengine

I'm writing a simple Flask app, with the sole purpose to learn Python and MongoDB.
I've managed to reach to the point where all the collections are defined, and CRUD operations work in general. Now, one thing that I really want to understand, is how to refresh the collection, after updating its structure. For example, say that I have the following model:
user.py
class User(db.Document, UserMixin):
email = db.StringField(required=True, unique=True)
password = db.StringField(required=True)
active = db.BooleanField()
first_name = db.StringField(max_length=64, required=True)
last_name = db.StringField(max_length=64, required=True)
registered_at = db.DateTimeField(default=datetime.datetime.utcnow())
confirmed = db.BooleanField()
confirmed_at = db.DateTimeField()
last_login_at = db.DateTimeField()
current_login_at = db.DateTimeField()
last_login_ip = db.StringField(max_length=45)
current_login_ip = db.StringField(max_length=45)
login_count = db.IntField()
companies = db.ListField(db.ReferenceField('Company'), default=[])
roles = db.ListField(db.ReferenceField(Role), default=[])
meta = {
'indexes': [
{'fields': ['email'], 'unique': True}
]
}
Now, I already have entries in my user collection, but I want to change companies to:
company = db.ReferenceField('Company')
How can I refresh the collection's structure, without having to bring the whole database down?
I do have a manage.py script that helps me and also provides a shell:
#!/usr/bin/python
from flask.ext.script import Manager
from flask.ext.script.commands import Shell
from app import factory
app = factory.create_app()
manager = Manager(app)
manager.add_command("shell", Shell(use_ipython=True))
# manager.add_command('run_tests', RunTests())
if __name__ == "__main__":
manager.run()
and I have tried a couple of commands, from information that I could recompile and out of my basic knowledge:
>>> from app.models import db, User
>>> import mongoengine
>>> mongoengine.Document(User)
field = iter(self._fields_ordered)
AttributeError: 'Document' object has no attribute '_fields_ordered'
>>> mongoengine.Document(User).modify() # well, same result as above
Any pointers on how to achieve this?
Update
I am asking all of this, because I have updated my user.py to match my new requests, but anytime I interact with the db its self, since the table's structure was not refreshed, I get the following error:
FieldDoesNotExist: The field 'companies' does not exist on the
document 'User', referer: http://local.faqcolab.com/company

Solution is easier then I expected:
db.getCollection('user').update(
// query
{},
// update
{
$rename: {
'companies': 'company'
}
},
// options
{
"multi" : true, // update all documents
"upsert" : false // insert a new document, if no existing document match the query
}
);
Explanation for each of the {}:
First is empty because I want to update all documents in user collection.
Second contains $rename which is the invoking action to rename the fields I want.
Last contains aditional settings for the query to be executed.

I have updated my user.py to match my new requests, but anytime I interact with the db its self, since the table's structure was not refreshed, I get the following error
MongoDB does not have a "table structure" like relational databases do. After a document has been inserted, you can't change it's schema by changing the document model.
I don't want to sound like I'm telling you that the answer is to use different tools, but seeing things like db.ListField(db.ReferenceField('Company')) makes me think you'd be much better off with a relational database (Postgres is well supported in the Flask ecosystem).
Mongo works best for storing schema-less documents (you don't know before hand how your data is structured, or it varies significantly between documents). Unless you have data like that, it's worth looking at other options. Especially since you're just getting started with Python and Flask, there's no point in making things harder than they are.

Store a mongo_id on a MongoAlchemy Document?

Is it possible to store a mongo_id as an ObjectId object in a MongoAlchemy field? I've been able to store an ObjectId inside of a document I defined manually, but it seems as though I'm restricted to storing the string value of the id in the context of the MongoAlchemy ORM.
Here's some of my code:
class Group(db.Document):
name = db.StringField()
trial_id = db.StringField(required=False)
participants = db.ListField(
db.DictField(db.AnythingField()), default_empty=True, required=False)
def add_participant(self, participant):
self.participants.append({
'participant_id': participant.mongo_id,
'start': datetime.utcnow(),
})
class Trial(db.Document):
name = db.StringField()
groups = db.ListField(
db.DocumentField(Group), default_empty=True, required=False)
def add_group(self, group):
group.trial_id = str(self.mongo_id)
group.save()
def get_group(self, group):
return Group.query.filter(
{'name': group, 'trial_id': str(self.mongo_id)}).first()
You'll see that I'm able to store a mongo_id as an ObjectId object in the Group method add_participant (since it's creating document manually, not through the MongoAlchemy ORM), but am forced to convert the mongo_id to a string in order to store it in a db.StringField.
I tried storing the original ObjectId in a db.AnythingField, but was then unable to filter by it.
Does anyone know if it's possible to store an ObjectId in a MongoAlchemy field and then filter by it in a database query?
Thank you!

You want an ObjectIdField: http://www.mongoalchemy.org/api/schema/fields.html#mongoalchemy.fields.ObjectIdField
This is the type of field which is used for mongo_id (although that one is special-cased)

try
id = db.ObjectIdField().gen()
This would automatically generate the object id for each instance of the mongo db object/document - as would id's in relational dbs