Fix #208: add migrations information for MongoDB users

Author: frankie567

docs/migration/08_to_10.md

@@ -32,6 +32,20 @@ class Model(BaseModel):
     user_id: UUID4
 ```
 
+#### MongoDB
+
+To avoid any issues, it's recommended to use the `standard` UUID representation when instantiating the MongoDB client:
+
+```py
+DATABASE_URL = "mongodb://localhost:27017"
+client = motor.motor_asyncio.AsyncIOMotorClient(
+    DATABASE_URL, uuidRepresentation="standard"
+)
+```
+
+This parameter controls how the UUID values will be encoded in the database. By default, it's set to `pythonLegacy` but new applications should consider setting this to `standard` for cross language compatibility. [Read more about this](https://pymongo.readthedocs.io/en/stable/api/pymongo/mongo_client.html#pymongo.mongo_client.MongoClient).
+
+
 ### In database
 
 Id. were before stored as strings in the database. You should make a migration to convert string data to UUID data.
@@ -72,6 +86,8 @@ ALTER TABLE "user" MODIFY id CHAR(36);
 
 #### MongoDB
 
+##### Mongo shell
+
 For MongoDB, we can use a `forEach` iterator to convert the id. for each document:
 
 ```js
@@ -81,6 +97,28 @@ db.getCollection('users').find().forEach(function(user) {
 });
 ```
 
+##### Python
+
+```py
+import uuid
+
+import motor.motor_asyncio
+
+
+async def migrate_uuid():
+    client = motor.motor_asyncio.AsyncIOMotorClient(
+        DATABASE_URL, uuidRepresentation="standard"
+    )
+    db = client["database_name"]
+    users = db["users"]
+
+    async for user in users.find({}):
+        await users.update_one(
+            {"_id": user["_id"]},
+            {"$set": {"id": uuid.UUID(user["id"])}},
+        )
+```
+
 ## Splitted routers
 
 You now have the responsibility to **wire the routers**. FastAPI Users doesn't give a bloated users router anymore.