Remove duplicates_constraint value for unique indexes

In PostgreSQL, unlike CockroachDB, the pg_constraints table doesn't
contain any entries for unique indexes, which means that the `get_indexes`
function doesn't return the `duplicates_constraint` value for unique
indexes.

The alembic PostgreSQL driver, used for both PostgreSQL and CockroachDB,
filters out any indexes that have the `duplicates_constraint` value set,
which causes Alembic to believe that the constraint has been converted
into an index, rather than recognizing that they're the same thing.

This commit fixes the problem by manually filtering out the
`duplicates_constraint` value for all unique indexes.

Signed-off-by: Jonathan Dieter <jonathan.dieter@spearline.com>

Author: jdieter

cockroachdb/sqlalchemy/dialect.py

@@ -256,7 +256,14 @@ def get_columns(self, conn, table_name, schema=None, **kw):
 
     def get_indexes(self, conn, table_name, schema=None, **kw):
         if self._is_v192plus:
-            return super().get_indexes(conn, table_name, schema, **kw)
+            indexes = super().get_indexes(conn, table_name, schema, **kw)
+            # We need to remove the `duplicates_constraints` value from unique indexes, otherwise
+            # alembic tries to delete and recreate unique indexes.  This is consistent with
+            # postgresql which doesn't set the duplicates_constraint flag on unique indexes
+            for index in indexes:
+                if index["unique"] and "duplicates_constraint" in index:
+                    del index["duplicates_constraint"]
+            return indexes
 
         # The Cockroach database creates a UNIQUE INDEX implicitly whenever the
         # UNIQUE CONSTRAINT construct is used. Currently we are just ignoring all unique indexes,