Batch commit and fix json load error

superset/migrations/versions/b56500de1855_add_uuid_column_to_import_mixin.py

@@ -24,6 +24,7 @@
 import json
 import os
 import time
+from json.decoder import JSONDecodeError
 from uuid import uuid4
 
 import sqlalchemy as sa
@@ -120,8 +121,12 @@ def add_uuids(table_name, session, batch_size=default_batch_size):
     print(f"Done. Assigned {count} uuids in {time.time() - start_time:.3f}s.")
 
 
-def update_position_json(dashboard, session, uuid_map={}):
-    layout = json.loads(dashboard.position_json or "{}")
+def update_position_json(dashboard, session, uuid_map):
+    try:
+        layout = json.loads(dashboard.position_json or "{}")
+    except JSONDecodeError:
+        layout = {}
+
     for object_ in layout.values():
         if (
             isinstance(object_, dict)
@@ -136,7 +141,27 @@ def update_position_json(dashboard, session, uuid_map={}):
 
     dashboard.position_json = json.dumps(layout, indent=4)
     session.merge(dashboard)
+
+
+def update_dashboards(session, uuid_map):
+    message = (
+        "Updating dasboard position json with slice uuid.."
+        if uuid_map
+        else "Cleaning up slice uuid from dashboard position json.."
+    )
+    print(f"\n{message}\r", end="")
+
+    query = session.query(models["dashboards"])
+    dashboard_count = query.count()
+    for i, dashboard in enumerate(query.all()):
+        update_position_json(dashboard, session, uuid_map)
+        if i and i % default_batch_size == 0:
+            session.commit()
+        print(f"{message} {i+1}/{dashboard_count}\r", end="")
+
     session.commit()
+    # Extra whitespace to override very long numbers, e.g. 99999/99999.
+    print(f"{message} Done.      \n")
 
 
 def upgrade():
@@ -165,31 +190,22 @@ def upgrade():
         except OperationalError:
             pass
 
-    message = "Updating dashboard position json with slice uuid.."
-    print(f"\n{message}\r", end="")
-
     # add UUID to Dashboard.position_json
-    Dashboard = models["dashboards"]
-    Slice = models["slices"]
     slice_uuid_map = {
         slc.id: slc.uuid
-        for slc in session.query(Slice).options(load_only("id", "uuid")).all()
+        for slc in session.query(models["slices"])
+        .options(load_only("id", "uuid"))
+        .all()
     }
-    dashboard_count = session.query(Dashboard).count()
-    for i, dashboard in enumerate(session.query(Dashboard).all()):
-        update_position_json(dashboard, session, slice_uuid_map)
-        print(f"{message} {i+1}/{dashboard_count}\r", end="")
-    print(f"{message} Done.")
+    update_dashboards(session, slice_uuid_map)
 
 
 def downgrade():
     bind = op.get_bind()
     session = db.Session(bind=bind)
 
     # remove uuid from position_json
-    Dashboard = models["dashboards"]
-    for dashboard in session.query(Dashboard).all():
-        update_position_json(dashboard, session, {})
+    update_dashboards(session, {})
 
     # remove uuid column
     for table_name, model in models.items():