Revert "FlaskPlugin: use get_id() instead of id attr"

This reverts commit 7eda527.

This was actually a misinterpretation of the API:

The API expects get_id() to be a unique string, that may or may not be
an integer. This feature is only used to uniquely identify the user, but
its not the same as the user's primary key.
https://flask-login.readthedocs.io/en/latest/#your-user-class

Please take a note on how get_id() is expected to behave on this feature
of flask-login:
https://flask-login.readthedocs.io/en/latest/#alternative-tokens

This means it can return any arbitrary string to identify the user which
can be swapped to any other value to invalidate all sessions. This
explicitly states that it is not the same as the user's primary id which
is also used as foreign key on the table.

sqlalchemy-continuum's transaction tables reference the user with a
foreign key of the actual id. If a downstream application uses the
alternative-tokens feature as described by the flask-login
documentation, this breaks horribly as sqlalchemy-continuum will then
try to insert an arbitrary unique string of the user as the foreign key
to reference the user in the user table.

Fixes kvesteri#316
Caused by kvesteri#149

sqlalchemy_continuum/plugins/flask.py

@@ -36,7 +36,7 @@ def fetch_current_user_id():
     if _app_ctx_stack.top is None or _request_ctx_stack.top is None:
         return
     try:
-        return current_user.get_id()
+        return current_user.id
     except AttributeError:
         return
 

tests/plugins/test_flask.py

@@ -1,7 +1,7 @@
 import os
 
 from flask import Flask, url_for
-from flask_login import LoginManager, UserMixin, login_user
+from flask_login import LoginManager, login_user
 from flask_sqlalchemy import SQLAlchemy, _SessionSignalEvents
 from flexmock import flexmock
 
@@ -63,7 +63,7 @@ def teardown_method(self, method):
     def create_models(self):
         TestCase.create_models(self)
 
-        class User(self.Model, UserMixin):
+        class User(self.Model):
             __tablename__ = 'user'
             __versioned__ = {
                 'base_classes': (self.Model, )