refactor(admin): replace dynamic subquery with explicit join (#772)

The `reana-admin retention-rules-apply` command selected
candidate retention rules either from `workflow.retention_rules`
(a query object, under the old `lazy="dynamic"` setting on
`Workflow.retention_rules`) or by joining onto
`user.workflows.subquery()` (a subquery built from the dynamic
`User.workflows` query).

After dropping `lazy="dynamic"` in reana-db so these
relationships return regular lists, the previous code no longer
works: lists have neither a `.subquery()` method nor the
follow-up `.filter(...)` chain that the rest of the function
relies on.

Replace both branches with explicit queries on
`WorkspaceRetentionRule`: filter by `workflow_id` for the
single-workflow case, and join `Workflow` filtered by
`owner_id` for the per-user case. Result is the same set of
rows, but expressed through standard `Session.query(...)`
calls.

Author: tiborsimko

reana_server/reana_admin/cli.py

@@ -764,10 +764,10 @@ def retention_rules_apply(
 
     candidate_rules = Session.query(WorkspaceRetentionRule)
     if workflow:
-        candidate_rules = workflow.retention_rules
+        candidate_rules = candidate_rules.filter_by(workflow_id=workflow.id_)
     elif user:
-        candidate_rules = Session.query(WorkspaceRetentionRule).join(
-            user.workflows.subquery()
+        candidate_rules = candidate_rules.join(Workflow).filter(
+            Workflow.owner_id == user.id_
         )
 
     click.echo("Setting the status of all the rules that will be applied to `pending`")