Add "multi-engine" support for Trainwreck transaction views

tailbone/views/master.py

@@ -130,6 +130,8 @@ class MasterView(View):
     # other things also, to get a DB picker in the header for all views
     supports_multiple_engines = False
     engine_type_key = 'rattail'
+    SessionDefault = None
+    SessionExtras = {}
 
     row_attrs = {}
     cell_attrs = {}
@@ -172,9 +174,16 @@ class MasterView(View):
     @property
     def Session(self):
         """
-        SQLAlchemy scoped session to use when querying the database.  Defaults
-        to ``tailbone.db.Session``.
+        Which session we return may depend on user's "current" engine.
         """
+        if self.supports_multiple_engines:
+            dbkey = self.get_current_engine_dbkey()
+            if dbkey != 'default' and dbkey in self.SessionExtras:
+                return self.SessionExtras[dbkey]
+
+        if self.SessionDefault:
+            return self.SessionDefault
+
         from tailbone.db import Session
         return Session