I need to consume a service that sends JSON responses containing JSON-serialized nested structures, which I would like to deserialize and store in my database - my application uses Django.
Business rules are the following:
The query returns objects which always have an id property which is a unique integer, often a createdAt property and an updatedAt property, both with datetime data, and then several other properties which are primitive types (int, float, str, datetime, etc.), and several properties that can be another object or an array of objects.
In case the property value is an object, then the parent relates to it through a 'foreign key'. In case it's an array of objects, then we have two scenarios: either the objects of the array relate to the parent through a 'foreign key', or the parent and each member of the array are related through a 'many-to-many' relation.
I need to mirror each of those objects in my database, so each model has an id field which is the primary key, but it's not autogenerated, because the real ids will be provided with the imported data.
The relations between all those entities are already mirrored in my model schema. I adopted this approach (mirroring data structure) because if I flatten the received data to save it all into a single table, there will be horrendous replication, defying all data normalization rules.
For every root object, I need to do this:
check whether there is already a record in database for that id
create a new record in case there isn't
update the existing record in case there is already one (update might be skipped if updatedAt values are the same for both the record and the incoming data
recursively repeat these same steps for each nested object that is the provided value for one of its parent's properties.
Below I'm reproducing a very simplified sample of the data I receive from the service and the models I in which I want to store it. The real thing is much, much more bulky and complex than that, and that's why I'm so wanting to learn a way of letting the ORM take care of the problem, should it be able to. Hard-coding the whole thing is taking forever, aside of being pretty error-prone and creating a maintenance hell should the data schema change in the future.
EDIT: A link to a previous simplified version of the following JSON and Models*
JSON sample:
{
"id": 37125965,
"number": "029073432019403",
"idCommunication": "1843768",
"docReceivedAt": {
"date": "2019-12-20 08:46:42"
},
"createdAt": {
"date": "2019-12-20 09:01:14"
},
"updatedAt": {
"date": "2019-12-20 09:01:32"
},
"branch": {
"id": 20,
"name": "REGIONAL OFFICE #3",
"address": "457 Beau St., S\u00e3o Paulo, SP, 08547-003",
"active": true,
"createdAt": {
"date": "2013-02-14 23:12:30"
},
"updatedAt": {
"date": "2019-05-09 13:40:47"
}
},
"modality": {
"id": 1,
"valor": "CITA\u00c7\u00c3O",
"descricao": "CITA\u00c7\u00c3O",
"active": true,
"createdAt": {
"date": "2014-08-29 20:47:56"
},
"updatedAt": {
"date": "2014-08-29 20:47:56"
}
},
"operation": {
"id": 12397740,
"number": "029073432019403",
"startedAt": {
"date": "2019-11-07 22:28:25"
},
"managementType": 27,
"assessmentValue": 5000000,
"createdAt": {
"date": "2019-12-20 09:01:30"
},
"updatedAt": {
"date": "2019-12-20 09:01:30"
},
"operationClass": {
"id": 22,
"name": "A\u00c7\u00c3O RESCIS\u00d3RIA",
"createdAt": {
"date": "2014-02-28 20:24:55"
},
"updatedAt": {
"date": "2014-02-28 20:24:55"
}
},
"evaluator": {
"id": 26798,
"name": "JANE DOE",
"level": 1,
"active": true,
"createdAt": {
"date": "2017-02-22 22:54:04"
},
"updatedAt": {
"date": "2017-03-15 18:03:20"
},
"evaluatorsOffice": {
"id": 7,
"name": "ACME",
"area": 4,
"active": true,
"createdAt": {
"date": "2014-02-28 20:25:16"
},
"updatedAt": {
"date": "2014-02-28 20:25:16"
}
},
"evaluatorsOffice_id": 7
},
"operationClass_id": 22,
"evaluator_id": 26798
},
"folder": {
"id": 16901241,
"singleDocument": false,
"state": 0,
"IFN": "00409504174201972",
"closed": false,
"dataHoraAbertura": {
"date": "2019-12-20 09:01:31"
},
"dataHoraTransicao": {
"date": "2024-12-20 09:01:31"
},
"titulo": "CONTROL FOLDER REF. OP. N. 029073432019403",
"createdAt": {
"date": "2019-12-20 09:01:32"
},
"updatedAt": {
"date": "2019-12-20 09:01:32"
},
"subjects": [
{
"id": 22255645,
"main": true,
"createdAt": {
"date": "2019-12-20 09:01:32"
},
"updatedAt": {
"date": "2019-12-20 09:01:32"
},
"subjectClass": {
"id": 20872,
"name": "SPECIAL RETIREMENT PROCESS",
"active": true,
"regulation": "8.213/91, 53.831/64, 83.080/79, 2.172/97, 1.663/98, 9.711/98, 9.528/97 AND 9.032/95",
"glossary": "SPECIAL RETIREMENT APPLICATION DUE TO HAZARDOUS LABOR CONDITION FOR 15+/20+/25+ YEARS",
"createdAt": {
"date": "2013-10-18 16:22:44"
},
"updatedAt": {
"date": "2013-10-18 16:22:44"
},
"parent": {
"id": 20866,
"name": "RETIREMENT BENEFITS",
"active": true,
"createdAt": {
"date": "2013-10-18 16:22:44"
},
"updatedAt": {
"date": "2013-10-18 16:22:44"
},
"parent": {
"id": 20126,
"name": "SOCIAL SECURITY",
"active": true,
"createdAt": {
"date": "2013-10-18 16:22:42"
},
"updatedAt": {
"date": "2013-10-18 16:22:42"
}
},
"parent_id": 20126
},
"parent_id": 20866
},
"subjectClass_id": 20872
}
],
"person": {
"id": 7318,
"isClient": true,
"isRelated": false,
"name": "SOCSEC CO.",
"createdAt": {
"date": "2013-02-14 23:11:43"
},
"updatedAt": {
"date": "2019-11-18 16:05:07"
}
},
"operation": {
"id": 12397740,
"number": "029073432019403",
"startedAt": {
"date": "2019-11-07 22:28:25"
},
"managementType": 27,
"assessmentValue": 5000000,
"createdAt": {
"date": "2019-12-20 09:01:30"
},
"updatedAt": {
"date": "2019-12-20 09:01:30"
}
},
"section": {
"id": 311,
"name": "PROTOCOL",
"address": "457 Beau St., ground floor, S\u00e3o Paulo, SP, 08547-003",
"active": true,
"management": false,
"onlyDistribution": true,
"createdAt": {
"date": "2013-02-14 23:12:31"
},
"updatedAt": {
"date": "2019-07-05 16:40:34"
},
"branch": {
"id": 20,
"name": "REGIONAL OFFICE #3",
"address": "457 Beau St., S\u00e3o Paulo, SP, 08547-003",
"active": true,
"createdAt": {
"date": "2013-02-14 23:12:30"
},
"updatedAt": {
"date": "2019-05-09 13:40:47"
}
},
"branch_id": 20
},
"person_id": 7318,
"operation_id": 12397740,
"section_id": 311
},
"branch_id": 20,
"modality_id": 1,
"operation_id": 12397740,
"folder_id": 16901241
}
Models.py sample:
from django.db import models
class Section(models.Model):
id = models.PositiveIntegerField(primary_key=True)
name = models.CharField(max_length=255, null=True)
address = models.CharField(max_length=255, null=True)
active = models.BooleanField(default=True)
management = models.BooleanField(default=False)
onlyDistribution = models.BooleanField(default=False)
createdAt = models.DateTimeField()
updatedAt = models.DateTimeField()
branch = models.ForeignKey('Branch', null=True, on_delete=models.SET_NULL)
class Person(models.Model):
id = models.PositiveIntegerField(primary_key=True)
name = models.CharField(max_length=255, null=True)
isClient = models.BooleanField(default=True)
isRelated = models.BooleanField(default=True)
createdAt = models.DateTimeField()
updatedAt = models.DateTimeField()
class SubjectClass(models.Model):
id = models.PositiveIntegerField(primary_key=True)
name = models.CharField(max_length=255, null=True)
active = models.BooleanField(default=True)
regulation = models.CharField(max_length=255, null=True)
glossary = models.CharField(max_length=255, null=True)
createdAt = models.DateTimeField()
updatedAt = models.DateTimeField()
parent = models.ForeignKey('SubjectClass', null=True, on_delete=models.SET_NULL)
class Subject(models.Model):
id = models.PositiveIntegerField(primary_key=True)
main = models.BooleanField(default=False)
createdAt = models.DateTimeField()
updatedAt = models.DateTimeField()
folder = models.ForeignKey('Folder', null=True, on_delete=models.SET_NULL)
subjectClass = models.ForeignKey(SubjectClass, null=True, on_delete=models.SET_NULL)
class Folder(models.Model):
id = models.PositiveIntegerField(primary_key=True)
singleDocument = models.BooleanField(default=False)
state = models.PositiveSmallIntegerField(null=True)
IFN = models.CharField(max_length=31, null=True)
closed = models.BooleanField(default=False)
title = models.CharField(max_length=255, null=True)
createdAt = models.DateTimeField()
updatedAt = models.DateTimeField()
subjects = models.ManyToManyField(SubjectClass, through=Subject, through_fields=('folder', 'subjectClass'))
interestedEntity = models.ForeignKey(Person, null=True, on_delete=models.SET_NULL)
class EvaluatorsOffice(models.Model):
id = models.PositiveIntegerField(primary_key=True)
name = models.CharField(max_length=255, null=True)
area = models.PositiveSmallIntegerField(null=True)
active = models.BooleanField(default=True)
createdAt = models.DateTimeField()
updatedAt = models.DateTimeField()
class Evaluator(models.Model):
id = models.PositiveIntegerField(primary_key=True)
name = models.CharField(max_length=255, null=True)
level = models.PositiveSmallIntegerField(null=True)
active = models.BooleanField(default=True)
createdAt = models.DateTimeField()
updatedAt = models.DateTimeField()
evaluatorsOffice = models.ForeignKey(EvaluatorsOffice, null=True, on_delete=models.SET_NULL)
class OperationClass(models.Model):
id = models.PositiveIntegerField(primary_key=True)
name = models.CharField(max_length=255, null=True)
active = models.BooleanField(default=True)
createdAt = models.DateTimeField()
updatedAt = models.DateTimeField()
class Operation(models.Model):
id = models.PositiveIntegerField(primary_key=True)
number = models.CharField(max_length=31, null=True)
startedAt = models.DateTimeField(null=True)
managementType = models.PositiveIntegerField(null=True)
assessmentValue = models.PositiveIntegerField(null=True)
createdAt = models.DateTimeField()
updatedAt = models.DateTimeField()
operationClass = models.ForeignKey(OperationClass, null=True, on_delete=models.SET_NULL)
evaluator = models.ForeignKey(Evaluator, null=True, on_delete=models.SET_NULL)
class Branch(models.Model):
id = models.PositiveIntegerField(primary_key=True)
name = models.CharField(max_length=255, null=True)
address = models.CharField(max_length=255, null=True)
active = models.BooleanField(default=True)
createdAt = models.DateTimeField()
updatedAt = models.DateTimeField()
class Modality(models.Model):
id = models.PositiveIntegerField(primary_key=True)
value = models.CharField(max_length=255, null=True)
createdAt = models.DateTimeField()
updatedAt = models.DateTimeField()
class CommunicationRecord(models.Model):
id = models.PositiveIntegerField(primary_key=True)
number = models.CharField(max_length=31, null=True)
idCommunication = models.CharField(max_length=31, null=True)
docReceivedAt = models.DateTimeField(null=True)
createdAt = models.DateTimeField()
updatedAt = models.DateTimeField()
branch = models.ForeignKey(Branch, null=True, on_delete=models.SET_NULL)
modality = models.ForeignKey(Modality, null=True, on_delete=models.SET_NULL)
operation = models.ForeignKey(Operation, null=True, on_delete=models.SET_NULL)
folder = models.ForeignKey(Folder, null=True, on_delete=models.SET_NULL)
EDIT (ref. DRF Serializers):
I'm trying to follow Max Malysh I Reinstate Monica's suggestion, and I started to work on a recursive serializer:
from django.db.models import Manager, Model, Field, DateTimeField, ForeignKey
from rest_framework.serializers import ModelSerializer
class RecursiveSerializer(ModelSerializer):
manager: Manager
field_dict: dict
def __init__(self, target_manager: Manager, data: dict, **kwargs):
self.manager = target_manager
self.Meta.model = self.manager.model
self.field_dict = {f.name: f for f in self.manager.model._meta.fields}
instance = None
data = self.process_data(data)
pk_name = self.manager.model._meta.pk.name
if pk_name in data:
try:
instance = target_manager.get(pk=data[pk_name])
except target_manager.model.DoesNotExist:
pass
super().__init__(instance, data, **kwargs)
def process_data(self, data: dict):
processed_data = {}
for name, value in data.items():
field: Field = self.field_dict.get(name)
if isinstance(value, dict):
if isinstance(field, ForeignKey):
processed_data[name] = self.__class__(field.related_model.objects, data=value)
continue
elif len(value) == 1 and 'date' in value and isinstance(field, DateTimeField):
processed_data[name] = value['date']
continue
processed_data[name] = value
return processed_data
class Meta:
model: Model = None
fields = '__all__'
However, it does a weird thing: when first run, against an empty database, it only creates the last and most deeply nested object. In the second run, it does nothing and returns a code='unique' validation error saying that such object already exists.
Now I must say I'm quite new to Python and Django (I come from .NET development) and the difficulties I'm facing about this task begin to look very awkward for me. I've been reading docs about Django and DRF, which helped me less than I expected. Yet I refuse to believe aforementioned language and framework lack resources for performing such a trivial operation. So, If I'm missing something very obvious, as it seems, for lack of knowledge of mine, I'll be grateful if someone teaches me what I seem not to know here.
Generally why I agree that DRF is not useful for this case: DRF defines an API and is similar to a view than to a model in many aspects: It defines which part of data should be exported. It is able to support all CRUD operations over the same data structure. There could be more APIs over the same data. Therefore it normal that serializers are separated from models. It is also frequently necessary to not change anything in models if a third party package should be a part of the new API. You need only Create and Update (no Read or Delete). You confirmed that you don't need any complicated security restrictions.
EDIT Main features of my code for your updated JSON and Models would be much more similar to yours. That doesn't make sense. I will write more comments and less change the code because it could lead to endless growing of the model and JSON to explain why you e.g. ignore some errors.
The important information to your assignment is that: 1. The data of "through" entity in all many to many relations are present in JSON (while never previously) 2. The changedAt timestamp of root entity is updated with every change of a nested entity in JSON, including all intermediate entities and even including the "through" entity.
from datetime import datetime
from django.db import models
from django.utils import timezone
class UpdateableModel(models.Model):
class Meta:
abstract = True
#classmethod
def creupdate(cls, data: dict, save_main_instance: bool = True, no_optimization=False):
primary_key_name = cls._meta.pk.name
if primary_key_name not in data:
raise ValueError(f'parameter \'data\' must contain \'{primary_key_name}\' key (model\'s primary key).')
try:
instance = cls.objects.get(pk=data[primary_key_name])
at_instance = getattr(instance, 'atualizadoEm', None)
at_data = data.get('atualizadoEm', None)
operation = 'unchanged' if at_instance and at_data and at_instance >= at_data else 'updated'
if operation == 'unchanged' and not no_optimizations:
print(f'{operation} instance {primary_key_name} {instance.pk} from {instance._meta.model}')
return instance
except cls.DoesNotExist:
instance = cls()
operation = 'created'
many_to_many_instances = []
for name, value in data.items():
if isinstance(value, dict):
if len(value) == 1 and 'date' in value:
date_value = datetime.strptime(value['date'], '%Y-%m-%d %H:%M:%S')
if timezone.is_naive(date_value):
date_value = timezone.make_aware(date_value)
new_value = date_value
else:
foreign_key = cls._meta.get_field(name)
foreign_model = foreign_key.related_model
foreign_data: dict = value
foreign_instance = foreign_model.creupdate(foreign_data)
new_value = foreign_instance
elif isinstance(value, list):
remote_field = getattr(instance, name)
obj_ids = []
for remote_data in value:
assert isinstance(remote_data, dict) and remote_field.model._meta.pk.name in remote_data
obj_ids.append(remote_field.model.creupdate(remote_data, False).pk)
many_to_many_instances.append((remote_field, obj_ids))
else:
new_value = value
if operation != 'unchanged':
setattr(instance, name, new_value)
if save_main_instance and operation != 'unchanged':
instance.save()
print(f'{operation} instance {primary_key_name} {instance.pk} from {instance._meta.model}')
for remote_field, obj_ids in many_to_many_instances:
remote_field.add(*obj_ids)
return instance
Notes:
The many to many relationship is optimized to add all objects by one request to minimize save count if nothing is is changed. (It was written for the previous JSON structure without any explicit "through" data)
Added assert instead of try ... except ValueError: pass (or FieldDoesNotExist).
"Errors should never pass silently." Zen of Python - Especially in development. (An unknown through name is a similar error to an unknown normal attribute.)
Added a parameter "no_optimization" and leave my logic to use "modifiedAt" only for the same entity, not to skip a check on the related entities. If an error occurs or if an update is ignored incorrectly by FieldDoesNotExist than the state of the database can be updated later by replaying the data with no_optimization=True. If all entities use a timestamp then it is even idempotent and the data can be processed in any random order e.g. by repeating data from a period of time with some bugs. It is also useful for a check of your optimizations that you get the same state of the database by processing with and without optimizations - e.g. by comparing the exported sql dump. My experience is that an optimization depending on timestamp too much is a problem much later if no alternative way is prepared.
Ok, so I gave up using DRF and just created an extended abstract model for my other models to inherit from, and gave it the feature I needed, implemented as shown below.
from datetime import datetime
from django.db import models
from django.db.models import FieldDoesNotExist
from django.utils import timezone
class UpdateableModel(models.Model):
class Meta:
abstract = True
#classmethod
def creupdate(cls, data: dict, save_main_instance: bool = True):
primary_key_name = cls._meta.pk.name
if primary_key_name not in data:
raise ValueError(f'parameter \'data\' must contain \'{primary_key_name}\' key (model\'s primary key).')
try:
instance = cls.objects.get(pk=data[primary_key_name])
at_instance = getattr(instance, 'atualizadoEm', None)
at_data = data.get('atualizadoEm', None)
if at_instance and at_data and at_instance >= at_data:
print(f'unchanged instance {primary_key_name} {instance.pk} from {instance._meta.model}')
return instance
operation = 'updated'
except cls.DoesNotExist:
instance = cls()
operation = 'created'
many_to_many_instances = []
for name, value in data.items():
if isinstance(value, dict):
if len(value) == 1 and 'date' in value:
date_value = datetime.strptime(value['date'], '%Y-%m-%d %H:%M:%S')
if timezone.is_naive(date_value):
date_value = timezone.make_aware(date_value)
setattr(instance, name, date_value)
else:
foreign_key = cls._meta.get_field(name)
foreign_model = foreign_key.related_model
foreign_data: dict = value
foreign_instance = foreign_model.creupdate(foreign_data)
setattr(instance, name, foreign_instance)
elif isinstance(value, list):
try:
relation_field = cls._meta.get_field(name)
except FieldDoesNotExist:
relation_field = None
if relation_field:
for through_data in value:
try:
through_model = getattr(instance, name).through
if isinstance(through_data,dict) and through_model._meta.pk.name in through_data:
many_to_many_instances.append(through_model.creupdate(through_data, False))
except ValueError:
pass
else:
setattr(instance, name, value)
if save_main_instance:
instance.save()
print(f'{operation} instance {primary_key_name} {instance.pk} from {instance._meta.model}')
for many_to_many_instance in many_to_many_instances:
many_to_many_instance.save()
return instance
Now, although it worked (I have just imported a ton of data using it), I won't mark this as an answer right now, for two reasons:
I'm willing to hear criticism to my implementation, that will point out flaws and ways to make it more robust and optimized.
I'm still hoping there are better solutions than mine. Only in case months go by and nothing shows up, then I'll assume there is none and will accept my own answer.
According to the docs, this is the way.
class UserSerializer(serializers.Serializer):
email = serializers.EmailField()
username = serializers.CharField(max_length=100)
class CommentSerializer(serializers.Serializer):
user = UserSerializer()
content = serializers.CharField(max_length=200)
created = serializers.DateTimeField()
From Django Rest Framework: nested serializers
I would like to map object from another API and send in GET response. I'm going to change only id of received object. Let's assume I get data from another API in such format:
{
"id": "31242",
"name": "sth1",
"price": "44",
"data": "2017-06-07",
}
In my database I have table object1 with values:
{
"id": "123",
"name": "sth1",
},
{
"id": "124",
"name": "sth2",
},
{
"id": "125",
"name": "sth3",
}
Field name is unique both in data from API and in data from database. I receive an object named sth1. So now I would like to find it in my database and get his id, replace with id from API and send GET response. In this case my response would look in this way:
{
"id": "123",
"name": "sth1",
"price": "44",
"data": "2017-06-07",
}
At this moment this is my URL - url(r'^data/(?P<name>\w+)$', views.DataList),
but I would like to have such URL - localhost:8000/data?name=sth
Myview.py:
#api_view(['GET'])
def DataList(request, name=None):
if request.method == 'GET':
quote = getDataFromAPI().get(name)
return Response(quote)
serializers.py:
class Object1Serializer(serializers.ModelSerializer):
class Meta:
model = Object1
depth = 1
fields = '__all__'
models.py:
class Object1(models.Model):
name = models.CharField(max_length=200)
I have done it in this way:
#api_view(['GET'])
def DataList(request):
t = request.GET.get("t","")
quote = getDataFromAPI().get(t)
id = Object1.objects.get(t=t)
quote["id"] = id
return Response(quote)
But I get error:
TypeError: Object of type 'Object1' is not JSON serializable
I suppose, your view should look somewhat like this,
#api_view(['GET'])
def DataList(request):
t = request.GET.get("t","")
quote = getDataFromAPI().get(t)
id = Object1.objects.get(t=t).id #put the id of the object in the variable.
#not the object itself.
quote["id"] = id
return Response(quote)
If you want to change the url from
url(r'^data/(?P<name>\w+)$', views.DataList) to localhost:8000/data?name=sth you'd need to change your api endpoint from
#api_view(['GET'])
def DataList(request, name=None):
to
#api_view(['GET'])
def DataList(request):
name = request.GET.get("name","")
and then take the id of object from your database by querying
id = Object1.objects.get(name=name)
and then updating id in response to be sent
quote["id"] = id
I am somewhat new to the google app engine models system(and all database models for that matter), and I am trying to figure out how make a model have a collection of model instances as one of the properties. Some how I feel that there has to be a better way to model the data I have.
So here is the example. I'm writing a journal webapp. I want to allow users to have multible journals as defined by the model Journals
#Represents a collection of journals
class Journals(db.Model):
user = db.UserProperty()
journals = db.ListProperty(int) #there has to be a better way of keeping track of journals then by a list of their id
In the property journals I keep a list of the id's of the Journal model(shown below).
class Journal(db.Model):
user = db.UserProperty()
name = db.StringProperty()
date_created = db.DateTimeProperty(auto_now_add=True)
last_modified = db.DateTimeProperty(auto_now=True)
journal_item = db.ListProperty(int)
In the Journal model I store a list of id's of JournalItem instances as a list of id's
class JournalItem(db.Model):
user = db.UserProperty()
name = db.StringProperty()
date_created = db.DateTimeProperty(auto_now_add=True)
last_modified = db.DateTimeProperty(auto_now=True)
content = db.StringProperty(multiline=True)
So I guess the question really is how should I model data with this structure(I omitted some properties to simplify it)(shown in json).
"Journals": {
"Journal": {
"user": "GAE user object",
"name": "journal 1",
"JournalItems": {
"JournalItem": {
"name": "entry 1",
"content": "some example content"
},
"JournalItem": {
"name": "entry 2",
"content": "some example content"
},
"JournalItem": {
"name": "entry n",
"content": "some example content"
}
}
}
"Journal": {
"user": "GAE user object",
"name": "journal 2",
"JournalItems": {
"JournalItem": {
"name": "entry 1",
"content": "some example content"
},
"JournalItem": {
"name": "entry 2",
"content": "some example content"
},
"JournalItem": {
"name": "entry n",
"content": "some example content"
}
}
}
}
I hope that wasn't to long-winded to be annoying. Any help on the matter would be greatly appreciated.
That's pretty much the way to do it, except I would use a ListProperty(db.Key) instead of int. Then you can do db.get(userjournals.journals) to get all the user's journals once you have the Journals instance belonging to that user (note that to avoid confusion, you should probably call that something like UserJournals).
Remember that GAE is a non-relational datastore, and sometimes you have to think a bit differently from how you would using SQL.
You can follow this guide which describes in an elegant fashion how you can model your entities relationships on Google Appengine.
The reversed approach you can take is to create a reference for example of a Journal in the JournalItem entity and assign a collection_name to it.
class JournalItem:
Journal = db.ReferencePoperty(Journal, collection_name='journal_items')
name = db....
content = db...
The you will be able to iterate through the JournalItems of the Journal in this fashion:
for item in journal.journal_items
logging.info(item.content)