MongoDB ReferenceField to JSON

Question

I am writing an API with Flask. I use a ReferenceField for the user who has written a Post. The API should return a JSON object with the data about the post and the data about the user.

Flask endpoint:

@app.route("/api/posts")
def posts_index():
    posts = Post.objects()
    return jsonify({
        "posts": json.loads(posts.to_json())
    })

Database model:

class User(Document):
    email = EmailField(required=True, unique=True)
    username = StringField(max_length=50, required=True, unique=True)
    password = StringField(required=True)
    created = DateTimeField(required=True, default=datetime.datetime.now())


class Comment(EmbeddedDocument):
    content = StringField(max_length=5000)
    user = ReferenceField(User)
    created = DateTimeField(required=True, default=datetime.datetime.now())


class Post(Document):
    title = StringField(max_length=120, required=True)
    user = ReferenceField(User, reverse_delete_rule=CASCADE)
    content = StringField(max_length=5000)
    comments = ListField(EmbeddedDocumentField(Comment))
    created = DateTimeField(required=True, default=datetime.datetime.now())

JSON response:

{
  "posts": [
    {
      "_id": {
        "$oid": "5a5efd1f9ef3161fc64cb95a"
      }, 
      "comments": [], 
      "content": "Lorem Ipsum", 
      "created": {
        "$date": 1516178223018
      }, 
      "title": "Hello", 
      "user": {
        "$oid": "5a5d0b9b9ef316220b6d9881"
      }
    }, 
    {
      "_id": {
        "$oid": "5a5efd1f9ef3161fc64cb95b"
      }, 
      "comments": [], 
      "content": "Lorem Ipsum Dolor", 
      "created": {
        "$date": 1516178223018
      }, 
      "title": "Hello Again", 
      "user": {
        "$oid": "5a5d0b9b9ef316220b6d9881"
      }
    }
  ]
}

I'd like to get the json response, but the ReferenceField should contain the information about the user.

MongoEngine Reference Field doesn't store the document, instead it only takes the id of the document, hence the name Reference. You can do this two ways, either have the `User` as [Embedded Document](http://docs.mongoengine.org/apireference.html#mongoengine.EmbeddedDocument) or take user reference of each `Post object` from query `cursor` of `Post` and update the final response using a loop. — Sreenadh T C, Jan 19 '18 at 03:57
Possible duplicate of [How to get ReferenceField data in mongoengine?](https://stackoverflow.com/questions/23977951/how-to-get-referencefield-data-in-mongoengine) — Sreenadh T C, Jan 19 '18 at 04:02

score 0 · Answer 1 · answered Jan 26 '18 at 08:12

First, you should recursively iterate through all fields end try to get its value. I wrote a small function which does that.

from mongoengine import Document, EmbeddedDocument
from mongoengine.queryset import QuerySet


def to_dict(obj):
    if isinstance(obj, (QuerySet, list)):
        return map(to_dict, obj)
    elif isinstance(obj, (Document, EmbeddedDocument)):
        doc = {}
        for field_name, field_type in obj._fields.iteritems():
            field_value = getattr(obj, field_name)
            doc[field_name] = to_dict(field_value)
        return doc
    else:
        return obj

Call this function like to_dict(Posts.objects) or to_dict(Post.objects.first()) and you should receive deep dictionary.

Next, create json response with custom json encoder.

import json, bson


def json_response(obj, cls=None):
    response = make_response(json.dumps(obj, cls=cls))
    response.content_type = 'application/json'

    return response

class MongoJsonEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, bson.ObjectId):
            return str(obj)

        if isinstance(obj, bson.DBRef):
            return str(obj.id)

        return json.JSONEncoder.default(self, obj)

@app.route('/')
def index():
    posts_json = to_dict(Post.objects)
    return json_response(dict(data=posts_json), cls=MongoJsonEncoder)

`obj._fields.iteritems()`changed to `obj._fields.items()` when using Python 3. You also need to write`return list(map(to_dict, obj))` instead if `return map(to_dict, obj)`. — Kruspe, Jan 15 '21 at 03:37

Brian Chojnowski · Answer 2 · 2020-04-03T17:18:10.153

0

hasattr is not working for the attribute name $oid example:

value = {'$oid': '5e8764ed70f190b802bdc008'}
if hasattr(value, '$oid'):
    print('this never prints')

I had to access it this way:

try:
    oid = value['$oid']
    print('this works')
except:
    print('no oid')

edited Apr 03 '20 at 17:18

answered Apr 03 '20 at 17:07

Brian Chojnowski

21
4

`value` (bad name) is a dict, so to get a key you use `get`, not `hasattr`. – Patrick Mevzek Apr 03 '20 at 18:05

MongoDB ReferenceField to JSON

2 Answers2