Encoding Custom Python Objects as BSON with PyMongo

💡 Problem Formulation: Developers working with MongoDB and Python often need to store custom objects in the database. MongoDB stores data in BSON format, so Python objects must be encoded to BSON before insertion. This article guides you on how to encode Python objects, such as a Person class instance, into BSON format using the PyMongo library so that it can be stored in MongoDB.

Method 1: Using Custom Encoding Function

Implement a custom function that translates your custom object into a BSON-compatible dictionary, using Python’s built-in types. The provided function, encode_to_bson(obj), should return a dictionary that BSON can serialize. Note that you must handle complex types manually, ensuring they are BSON-serializable.

Here’s an example:

from bson import BSON

class Person:
  def __init__(self, name, age):
    self.name = name
    self.age = age

def encode_to_bson(obj):
  if isinstance(obj, Person):
    return {'name': obj.name, 'age': obj.age}
  return None

person = Person('Alice', 30)
person_bson = BSON.encode(encode_to_bson(person))

Output: A BSON object representing the person’s data.

The code defines a Person class and a function encode_to_bson() that converts an instance of Person into a dictionary suitable for BSON serialization. The BSON.encode() method from PyMongo is then used to convert the dictionary into a BSON object.

Method 2: Extend Default Encoder

PyMongo’s default encoder can be extended by subclassing pymongo.codec_options.TypeEncoder and implementing the transform_python method. This approach is more modular, allowing you to define complex encoding logic tailored to your objects.

Here’s an example:

from bson.codec_options import TypeEncoder, TypeRegistry, CodecOptions
from pymongo import MongoClient

class PersonEncoder(TypeEncoder):
  python_type = Person
  
  def transform_python(self, value):
    return {"name": value.name, "age": value.age}

person_encoder = PersonEncoder()
type_registry = TypeRegistry([person_encoder])
codec_options = CodecOptions(type_registry=type_registry)

client = MongoClient()
db = client.test_db
collection = db.get_collection('people', codec_options=codec_options)

person = Person('Bob', 25)
collection.insert_one(person)

Output: The person object is inserted into the MongoDB collection as BSON.

This snippet creates a custom encoder PersonEncoder for the Person class and registers it with the collection using a custom CodecOptions instance. This modification allows direct insertion of Person instances into the database.

Method 3: Using Custom A BSON Encoding Handler

Another technique is to leverage the pymongo.registry module, which manages BSON encoding. Set a custom encoding function with register_type() to handle the conversion of your custom types.

Here’s an example:

from pymongo import MongoClient, collection
from bson import register_type

def person_encoder(person):
  if isinstance(person, Person):
    return {'name': person.name, 'age': person.age}

register_type(Person, person_encoder)

client = MongoClient()
db = client.sample_db
coll = collection.Collection(db, 'people', codec_options=codec_options)

person = Person('Charlie', 40)
coll.insert_one(person)

Output: The custom object is now saved in MongoDB as a BSON document.

By registering a type encoder person_encoder with PyMongo’s register_type(), custom objects of type Person can be directly inserted into MongoDB collections using the insert_one() method.

Method 4: Leveraging __bson__ Method

Add a __bson__ method to your custom class. This method should return a serializable BSON document. PyMongo will automatically call this method when encountering an instance of the class during serialization.

Here’s an example:

class Person:
  def __init__(self, name, age):
    self.name = name
    self.age = age

  def __bson__(self):
    return {'name': self.name, 'age': self.age}

person = Person('Daniel', 35)
encoded_person = BSON.encode(person)

Output: Encoded BSON representation of the Person instance.

This handy approach does not require additional functions or registry configuration. Simply including the __bson__ method in your class definition is enough for PyMongo to serialize your objects correctly.

Bonus One-Liner Method 5: In-line Dictionary Conversion

For simple objects and one-off cases, bypass custom encoders by converting the object to a dictionary on the fly while performing the database operation.

Here’s an example:

client = MongoClient()
db = client.demo_db
collection = db.people

person = Person('Eva', 28)
collection.insert_one(person.__dict__)

Output: The custom object’s attributes are inserted into the database as a BSON document.

This method requires no special setup. The __dict__ attribute of the Person class instance is used directly to get a dictionary representation, which is then passed to the insert_one method.

Summary/Discussion

• Method 1: Custom Encoding Function. Straightforward. Manual mapping of objects required.
• Method 2: Extend Default Encoder. Clean and reusable. More complex to set up.
• Method 3: Custom BSON Encoding Handler. Flexible. Requires understanding of encoding registry.
• Method 4: Leveraging __bson__ Method. Elegant and self-contained. Limited to classes you can modify.
• Bonus Method 5: In-line Dictionary Conversion. Quick and easy. Not suitable for complex object hierarchies.