Create unique constraints at the db_dbgroup_dbnodes table in SQLA

[szoupanos]

There is a ManyToMany relationship between AiiDA groups and nodes.
This is translated at database level with an intermediate table correlating nodes (dbnodes) and groups (dbgroups).

aiidadb_sqla1=# \d db_dbgroup_dbnodes
                           Table "public.db_dbgroup_dbnodes"
   Column   |  Type   |                            Modifiers
------------+---------+-----------------------------------------------------------------
 id         | integer | not null default nextval('db_dbgroup_dbnodes_id_seq'::regclass)
 dbnode_id  | integer |
 dbgroup_id | integer |
Indexes:
    "db_dbgroup_dbnodes_pkey" PRIMARY KEY, btree (id)
Foreign-key constraints:
    "db_dbgroup_dbnodes_dbgroup_id_fkey" FOREIGN KEY (dbgroup_id) REFERENCES db_dbgroup(id) DEFERRABLE INITIALLY DEFERRED
    "db_dbgroup_dbnodes_dbnode_id_fkey" FOREIGN KEY (dbnode_id) REFERENCES db_dbnode(id) DEFERRABLE INITIALLY DEFERRED

In this table, it is normal to have only one occurrence of group-node.
Therefore a uniqueness constraint should be added.

Moreover, for uniformity reasons, an index should be added on the group and on the node column to match the Django schema and increase the speed of any kind of containment checks.

E.g.

aiidadb_sqla_issue_1319=# \d db_dbgroup_dbnodes
                           Table "public.db_dbgroup_dbnodes"
   Column   |  Type   |                            Modifiers
------------+---------+-----------------------------------------------------------------
 id         | integer | not null default nextval('db_dbgroup_dbnodes_id_seq'::regclass)
 dbnode_id  | integer |
 dbgroup_id | integer |
Indexes:
    "db_dbgroup_dbnodes_pkey" PRIMARY KEY, btree (id)
    "db_dbgroup_dbnodes_dbgroup_id_dbnode_id_key" UNIQUE, btree (dbgroup_id, dbnode_id)
    "db_dbgroup_dbnodes_7a672316" btree (dbnode_id)
    "db_dbgroup_dbnodes_a0b4eda0" btree (dbgroup_id)
Foreign-key constraints:
    "db_dbgroup_dbnodes_dbgroup_id_fkey" FOREIGN KEY (dbgroup_id) REFERENCES db_dbgroup(id) DEFERRABLE INITIALLY DEFERRED
    "db_dbgroup_dbnodes_dbnode_id_fkey" FOREIGN KEY (dbnode_id) REFERENCES db_dbnode(id) DEFERRABLE INITIALLY DEFERRED

This is related to the comment of Martin at #1319
#1319 (comment)
and to the pull request #1677

[sphuber]

Fixed in PR #1680