How to get the user who accept the merge request in gitlab?

i want to get user who accept the merge request, and search from the database of gitlab of table: merge_requests.and found the following info:

1,the author_id which one create the merge request.

2,the assignee_id which one be assigned when the MR creating.

3,the updated_by_id which one update the MR.

4,the merge_user_id which one accept the MR. but the MR has been merged, the merge_user_is still shows NULL.

how can i get the user which one accept the MR in gitlab? the version is GitLab Community Edition 8.10.4