[migrations] Fixing issue #4810 (#4815)

superset/assets/package-lock.json

@@ -0,0 +1,21 @@
+{
+  "name": "superset",
+  "version": "0.23.0dev",
+  "lockfileVersion": 1,
+  "requires": true,
+  "dependencies": {
+    "node": {
+      "version": "9.11.0",
+      "resolved": "https://registry.npmjs.org/node/-/node-9.11.0.tgz",
+      "integrity": "sha512-a4HV43+rHSgYaoGVr5PfnpTmiDeL4AmFsXKzRhBbkDLeRjBoQYENHs7mNCHlQRb9VFKRqXD5FDmadQFPznkUuQ==",
+      "requires": {
+        "node-bin-setup": "1.0.6"
+      }
+    },
+    "node-bin-setup": {
+      "version": "1.0.6",
+      "resolved": "https://registry.npmjs.org/node-bin-setup/-/node-bin-setup-1.0.6.tgz",
+      "integrity": "sha512-uPIxXNis1CRbv1DwqAxkgBk5NFV3s7cMN/Gf556jSw6jBvV7ca4F9lRL/8cALcZecRibeqU+5dFYqFFmzv5a0Q=="
+    }
+  }
+}

superset/migrations/versions/f231d82b9b26_.py

@@ -12,7 +12,6 @@ down_revision = 'e68c4473c581'
 
 from alembic import op
 import sqlalchemy as sa
-from sqlalchemy.exc import OperationalError
 
 from superset.utils import generic_find_uq_constraint_name
 
@@ -25,9 +24,6 @@ names = {
     'metrics': 'metric_name',
 }
 
-bind = op.get_bind()
-insp = sa.engine.reflection.Inspector.from_engine(bind)
-
 
 def upgrade():
 
@@ -60,13 +56,16 @@ def downgrade():
         )
 
     # Remove the previous missing uniqueness constraints.
+    bind = op.get_bind()
+    insp = sa.engine.reflection.Inspector.from_engine(bind)
+
     for table, column in names.items():
         with op.batch_alter_table(table, naming_convention=conv) as batch_op:
             batch_op.drop_constraint(
-                    generic_find_uq_constraint_name(
-                        table,
-                        {column, 'datasource_id'},
-                        insp,
-                    ) or 'uq_{}_{}'.format(table, column),
-                    type_='unique',
-                )
+                generic_find_uq_constraint_name(
+                    table,
+                    {column, 'datasource_id'},
+                    insp,
+                ) or 'uq_{}_{}'.format(table, column),
+                type_='unique',
+            )