Setting `reviewer_ids` when updating an MR using the API does not seem to work

Summary

When updating an MR using the API, setting the reviewer_ids field does not seem to update the reviewers of the MR.

Steps to reproduce

• Open an MR on a project using the GitLab UI without any reviewers.
• Note the project ID and merge request ID somewhere.
• Generate an API token
• Call the PUT API to projects/:id/merge_requests/:iid with a non empty reviewer_ids attribute, for example [5].

What is the current bug behavior?

The reviewers of the MR are not updated.

What is the expected correct behavior?

The reviewers of the MR are updated.

Relevant logs and/or screenshots

The minimal PUT request that made it not work is something like the following body:

{
  "reviewer_ids": [5]
}

The response in that case is:

{"error":"assignee_id, assignee_ids, description, labels, add_labels, remove_labels, milestone_id, remove_source_branch, allow_collaboration, allow_maintainer_to_push, squash, target_branch, title, state_event, discussion_locked are missing, at least one parameter must be provided"}

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:		Ubuntu 20.04
Proxy:		no
Current User:	git
Using RVM:	no
Ruby Version:	2.7.2p137
Gem Version:	3.1.4
Bundler Version:2.1.4
Rake Version:	13.0.3
Redis Version:	5.0.9
Git Version:	2.29.0
Sidekiq Version:5.2.9
Go Version:	unknown

GitLab information
Version:	13.8.0-ee
Revision:	1ae10d09692
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	PostgreSQL
DB Version:	12.4
URL:		https://[hidden]
HTTP Clone URL:	https://[hidden]/some-group/some-project.git
SSH Clone URL:	ssh://git@[hidden]:22222/some-group/some-project.git
Elasticsearch:	yes
Geo:		no
Using LDAP:	no
Using Omniauth:	yes
Omniauth Providers: google_oauth2

GitLab Shell
Version:	13.15.0
Repository storage paths:
- default: 	/var/opt/gitlab/git-data/repositories
GitLab Shell path:		/opt/gitlab/embedded/service/gitlab-shell
Git:		/opt/gitlab/embedded/bin/git

(I have hidden our URL since I'd prefer not to put it on the internet)

Results of GitLab application Check

Expand for output related to the GitLab application check

Checking GitLab subtasks ...
Checking GitLab Shell ...
GitLab Shell: ... GitLab Shell version >= 13.15.0 ? ... OK (13.15.0)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Internal API available: OK
Redis available via internal API: OK
gitlab-shell self-check successful
Checking GitLab Shell ... Finished
Checking Gitaly ...
Gitaly: ... default ... OK
Checking Gitaly ... Finished
Checking Sidekiq ...
Sidekiq: ... Running? ... yes
Number of Sidekiq processes ... 1
Checking Sidekiq ... Finished
Checking Incoming Email ...
Incoming Email: ... Reply by email is disabled in config/gitlab.yml
Checking Incoming Email ... Finished
Checking LDAP ...
LDAP: ... LDAP is disabled in config/gitlab.yml
Checking LDAP ... Finished
Checking GitLab App ...
Git configured correctly? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... yes
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ...
[a list of only yes]
Redis version >= 4.0.0? ... yes
Ruby version >= 2.7.2 ? ... yes (2.7.2)
Git version >= 2.29.0 ? ... yes (2.29.0)
Git user has default SSH configuration? ... yes
Active users: ... 14
Is authorized keys file accessible? ... yes
GitLab configured to store new projects in hashed storage? ... yes
All projects are in hashed storage? ... yes
Elasticsearch version 7.x (6.4 - 6.x deprecated to be removed in 13.8)? ... yes (7.8.0)
Checking GitLab App ... Finished
Checking GitLab subtasks ... Finished

Possible fixes

It seems that the reviewer_ids property is just not used yet when updating MR's, since the Rails attribute checker seems to already complain about missing a field.