Jun 26 MT-1 phase

app/tenancy/routing.py

@@ -0,0 +1,29 @@
+"""
+app/tenancy/routing.py
+----------------------
+RoutingSession — the per-request tenant-aware SQLAlchemy session.
+
+Subclasses Flask-SQLAlchemy's own Session so that all existing behaviour
+(default bind, __bind_key__ resolution) is preserved. The ONLY change: when a
+tenant engine has been selected for the current request (g.tenant_engine, set
+by the resolver middleware), every query binds to that engine instead.
+
+When no tenant engine is present — multi-tenancy disabled, an exempt path, a
+CLI invocation, or any non-request context — this falls through to the normal
+Flask-SQLAlchemy behaviour, i.e. the app's configured default database. This
+makes the routing layer completely inert until a tenant is actually resolved,
+so the existing single-tenant deployment is unaffected.
+"""
+
+from flask import g, has_app_context
+from flask_sqlalchemy.session import Session as _FlaskSQLAlchemySession
+
+
+class RoutingSession(_FlaskSQLAlchemySession):
+    def get_bind(self, mapper=None, clause=None, bind=None, **kwargs):
+        # Respect an explicitly supplied bind (engine-targeted operations).
+        if bind is None and has_app_context():
+            tenant_engine = g.get('tenant_engine', None)
+            if tenant_engine is not None:
+                return tenant_engine
+        return super().get_bind(mapper=mapper, clause=clause, bind=bind, **kwargs)