Implement MongoDB database adapter (#29)

* Implement MongoDB adapter using motor

* Add mongo container to build pipeline

* Tidy up dependencies

* Update documentation for MongoDB

* Export MongoDB adapter from db package

* Pass black format

* Update README

tests/test_db_mongodb.py

@@ -0,0 +1,63 @@
+from typing import AsyncGenerator
+
+import motor.motor_asyncio
+import pytest
+import pymongo.errors
+
+from fastapi_users.db.mongodb import MongoDBUserDatabase
+from fastapi_users.models import BaseUserDB
+from fastapi_users.password import get_password_hash
+
+
+@pytest.fixture
+async def mongodb_user_db() -> AsyncGenerator[MongoDBUserDatabase, None]:
+    client = motor.motor_asyncio.AsyncIOMotorClient("mongodb://localhost:27017")
+    db = client["test_database"]
+    collection = db["users"]
+
+    yield MongoDBUserDatabase(collection)
+
+    await collection.drop()
+
+
+@pytest.mark.asyncio
+async def test_queries(mongodb_user_db):
+    user = BaseUserDB(
+        id="111",
+        email="lancelot@camelot.bt",
+        hashed_password=get_password_hash("guinevere"),
+    )
+
+    # Create
+    user_db = await mongodb_user_db.create(user)
+    assert user_db.id is not None
+    assert user_db.is_active is True
+    assert user_db.is_superuser is False
+    assert user_db.email == user.email
+
+    # Update
+    user_db.is_superuser = True
+    await mongodb_user_db.update(user_db)
+
+    # Get by id
+    id_user = await mongodb_user_db.get(user.id)
+    assert id_user.id == user_db.id
+    assert id_user.is_superuser is True
+
+    # Get by email
+    email_user = await mongodb_user_db.get_by_email(user.email)
+    assert email_user.id == user_db.id
+
+    # List
+    users = await mongodb_user_db.list()
+    assert len(users) == 1
+    first_user = users[0]
+    assert first_user.id == user_db.id
+
+    # Exception when inserting existing email
+    with pytest.raises(pymongo.errors.DuplicateKeyError):
+        await mongodb_user_db.create(user)
+
+    # Unknown user
+    unknown_user = await mongodb_user_db.get_by_email("galahad@camelot.bt")
+    assert unknown_user is None