# Proof of Concept: Implementing MongoEngine lib with support for multi database connections.md

Proof of Concept: Implementing MongoEngine lib with support for multi database connections

Reason:

Currently MongoEngine library only support a single database connection, managing collections by Document classes in global scope.

It has a support to change database connections by switch_db but it's made by a global class attribute called ._meta['db_alias']

When you define many sub ReferenceFields in a hierarchical tree, only first Document object has its ._meta['db_alias'] atttibute changed, not working to query Documents that implement other documents.

Goal:

Implementing MongoEngine lib for multi database connections support hierarchical tree of many sub ReferenceFields

How to run:

1.) Create files bellow:

• ./execute_poc.py
• ./db/connection.py
• ./models/account.py
• ./models/phone.py
• ./models/chat.py

2.) Execute PoC script:

$ python execute_poc.py

[db connection: first-db] Account (selecting null values): {}
[db connection: first-db] Phone (selecting null values): {}
[db connection: first-db] Chat (selecting null values): {}
---
[db connection: second-db] Account (selecting null values): {}
[db connection: second-db] Phone (selecting null values): {}
[db connection: second-db] Chat (selecting null values): {}
---
[db connection: second-db] Account (created new value): {"_id": {"$oid": "6616c9862dff85db3d3c90a8"}, "name": "account 1"}
[db connection: second-db] Phone (created new value): {"_id": {"$oid": "6616c9862dff85db3d3c90a9"}, "number": "11 98765-4321", "account": {"$oid": "6616c9862dff85db3d3c90a8"}}
[db connection: second-db] Chat (created new value): {"_id": {"$oid": "6616c9862dff85db3d3c90aa"}, "name": "my first chat", "phone": {"$oid": "6616c9862dff85db3d3c90a9"}}
---
[db connection: second-db] Account (selecting all values): ['{"_id": {"$oid": "6616c9862dff85db3d3c90a8"}, "name": "account 1"}']
[db connection: second-db] Phone (selecting all values): ['{"_id": {"$oid": "6616c9862dff85db3d3c90a9"}, "number": "11 98765-4321", "account": {"$oid": "6616c9862dff85db3d3c90a8"}}']
[db connection: second-db] Chat (selecting all values): ['{"_id": {"$oid": "6616c9862dff85db3d3c90aa"}, "name": "my first chat", "phone": {"$oid": "6616c9862dff85db3d3c90a9"}}']
---
[db connection: second-db] Account (selecting cross values): {"_id": {"$oid": "6616c9862dff85db3d3c90a8"}, "name": "account 1"}
[db connection: second-db] Phone (selecting cross values): {"_id": {"$oid": "6616c9862dff85db3d3c90a9"}, "number": "11 98765-4321", "account": {"$oid": "6616c9862dff85db3d3c90a8"}}
[db connection: second-db] Chat (selecting cross values): {"_id": {"$oid": "6616c9862dff85db3d3c90aa"}, "name": "my first chat", "phone": {"$oid": "6616c9862dff85db3d3c90a9"}}
---
[db connection: first-db] Account (selecting null values again): {}
[db connection: first-db] Phone (selecting null values again): {}
[db connection: first-db] Chat (selecting null values again): {}
---

1-execute_poc.py

""" file: execute_poc.py """

from db.connection import register_db_connection, registered_models

#
# registering connections
#
register_db_connection(alias='first-db', db='my_database', host='mongodb://user:password@first-db:27017/')
register_db_connection(alias='second-db', db='my_database', host='mongodb://user:password@second-db:27017/')
register_db_connection(alias='third-db', db='my_database', host='mongodb://user:password@third-db:27017/')

#
# getting specific model classes from db connection alias: 'first-db'
#
Account = registered_models['first-db']['Account']
Phone = registered_models['first-db']['Phone']
Chat = registered_models['first-db']['Chat']

#
# cleaning first-db objects
#
Account.objects.all().delete()
Phone.objects.all().delete()
Chat.objects.all().delete()

#
# selecting null values
#
account = Account.objects.first()
print('[db connection: first-db] Account (selecting null values):', account.to_json() if account else {})

phone = Phone.objects.first()
print('[db connection: first-db] Phone (selecting null values):', phone.to_json() if phone else {})

chat = Chat.objects.first()
print('[db connection: first-db] Chat (selecting null values):', chat.to_json() if chat else {})

print('---')

#
# getting specific model classes from db connection alias: 'second-db'
#
Account = registered_models['second-db']['Account']
Phone = registered_models['second-db']['Phone']
Chat = registered_models['second-db']['Chat']

#
# cleaning database
#
Account.objects.all().delete()
Phone.objects.all().delete()
Chat.objects.all().delete()

#
# selecting null values
#
account = Account.objects.first()
print('[db connection: second-db] Account (selecting null values):', account.to_json() if account else {})

phone = Phone.objects.first()
print('[db connection: second-db] Phone (selecting null values):', phone.to_json() if phone else {})

chat = Chat.objects.first()
print('[db connection: second-db] Chat (selecting null values):', chat.to_json() if chat else {})

print('---')

#
# creating objects
#
account = Account(name='account 1')
account.save()
print('[db connection: second-db] Account (created new value):', account.to_json())

phone = Phone(number='11 98765-4321', account=account)
phone.save()
print('[db connection: second-db] Phone (created new value):', phone.to_json())

chat = Chat(name='my first chat', phone=phone)
chat.save()
print('[db connection: second-db] Chat (created new value):', chat.to_json())

print('---')

#
# selecting all values
#
accounts = Account.objects.all()
print('[db connection: second-db] Account (selecting all values):',
      [account.to_json() if account else {} for account in accounts])

phones = Phone.objects.all()
print('[db connection: second-db] Phone (selecting all values):',
      [phone.to_json() if phone else {} for phone in phones])

chats = Chat.objects.all()
print('[db connection: second-db] Chat (selecting all values):', [chat.to_json() if chat else {} for chat in chats])

print('---')

#
# selecting cross values
#
account = Account.objects.first()
print('[db connection: second-db] Account (selecting cross values):', account.to_json() if account else {})

phone = Phone.objects(account=account).first()
print('[db connection: second-db] Phone (selecting cross values):', phone.to_json() if phone else {})

chat = Chat.objects(phone=phone).first()
print('[db connection: second-db] Chat (selecting cross values):', chat.to_json() if chat else {})

print('---')

#
# getting specific model classes from db connection alias: 'first-db'
#
Account = registered_models['first-db']['Account']
Phone = registered_models['first-db']['Phone']
Chat = registered_models['first-db']['Chat']

#
# selecting null values
#
account = Account.objects.first()
print('[db connection: first-db] Account (selecting null values again):', account.to_json() if account else {})

phone = Phone.objects.first()
print('[db connection: first-db] Phone (selecting null values again):', phone.to_json() if phone else {})

chat = Chat.objects.first()
print('[db connection: first-db] Chat (selecting null values again):', chat.to_json() if chat else {})

print('---')

2-db_connection.py

""" file: db/connection.py """

from mongoengine.connection import register_connection
from mongoengine.document import BaseDocument

registered_models = {}
#
# Example of populated dict bellow:
#
# registered_models = {
#     'first-db': {
#         'Account': <Account class [for 'first-db' db connection]>,
#         'Phone': <Phone class [for 'first-db' db connection]>,
#         'Chat': <Chat class [for 'first-db' db connection]>
#     },
#     'second-db': {
#         'Account': <Account class [for 'second-db' db connection]>,
#         'Phone': <Phone class [for 'second-db' db connection]>,
#         'Chat': <Chat class [for 'second-db' db connection]>
#     },
#     'third-db': {
#         'Account': <Account class [for 'third-db' db connection]>,
#         'Phone': <Phone class [for 'third-db' db connection]>,
#         'Chat': <Chat class [for 'third-db' db connection]>
#     }
# }


def register_db_connection(alias, db, host):
    # regiter connection in mongoengine
    register_connection(alias=alias, db=db, host=host)

    # create db alias key in registered_models
    registered_models[alias] = {}

    # import and register models for this specific db alias
    from models import account, phone, chat

    try:
        model_class = chat.build_model_class(alias)
        register_model_class(model_class, alias)
    except ModelBuildingHasPendingReference:
        ...

    model_class = account.build_model_class(alias)
    register_model_class(alias, model_class)

    model_class = phone.build_model_class(alias)
    register_model_class(alias, model_class)

    model_class = chat.build_model_class(alias)
    register_model_class(alias, model_class)


def register_model_class(db_alias: str, model_class: BaseDocument) -> None:
    # inject db alias in created model class
    model_class._meta['db_alias'] = db_alias
    model_class._collection = None

    # get class name
    class_name = model_class.__name__

    # save new class in global object for specific db connection
    global registered_models
    registered_models[db_alias][class_name] = model_class


def get_referenced_model(class_name, db_alias):
    global registered_models
    try:
        return registered_models[db_alias][class_name]
    except KeyError:
        raise ModelBuildingHasPendingReference('please wait...')


class ModelBuildingHasPendingReference(Exception):
    ...

3-models_account.py

""" file: models/account.py """

from mongoengine.document import BaseDocument, Document
from mongoengine.fields import StringField


def build_model_class(db_alias: str) -> BaseDocument:
    class Account(Document):
        name = StringField(max_lenght=50)

        meta = {
            'collection': 'accounts'
        }

    return Account

4-models_phone.py

""" file: models/phone.py """

from mongoengine.document import BaseDocument, Document
from mongoengine.fields import StringField, ReferenceField

from db.connection import get_referenced_model

def build_model_class(db_alias: str) -> BaseDocument:
    Account = get_referenced_model('Account', db_alias)

    class Phone(Document):
        number = StringField(max_lenght=13)
        account = ReferenceField(Account)

        meta = {
            'collection': 'phones'
        }

    return Phone

5-models_chat.py

""" file: models/chat.py """

from mongoengine.document import BaseDocument, Document
from mongoengine.fields import StringField, ReferenceField

from db.connection import get_referenced_model

def build_model_class(db_alias: str) -> BaseDocument:
    Phone = get_referenced_model('Phone', db_alias)

    class Chat(Document):
        name = StringField(max_lenght=22)
        phone = ReferenceField(Phone)

        meta = {
            'collection': 'chats'
        }

    return Chat