Fix approver count reset when not editing approval

Don't reset the approval count set on an MR when it's higher than the
project default, and we're not passing an `approvals_before_merge`
param.

app/controllers/projects/merge_requests_controller.rb

@@ -745,12 +745,15 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     ]
   end
 
+  # If the number of approvals is not greater than the project default, set to
+  # nil, so that we fall back to the project default. If it's not set, we can
+  # let the normal update logic handle this.
   def clamp_approvals_before_merge(mr_params)
+    return mr_params unless mr_params[:approvals_before_merge]
+
     target_project = @project.forked_from_project if @project.id.to_s != mr_params[:target_project_id]
     target_project ||= @project
 
-    # If the number of approvals is not greater than the project default, set to nil,
-    # so that we fall back to the project default.
     if mr_params[:approvals_before_merge].to_i <= target_project.approvals_before_merge
       mr_params[:approvals_before_merge] = nil
     end

changelogs/unreleased-ee/1961-number-of-required-approvals-must-not-be-reset-when-changing-assignee.yml

+---
+title: Fix approver count reset when editing assignee or labels
+merge_request:
+author:

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -488,7 +488,24 @@ describe Projects::MergeRequestsController do
       end
 
       context 'approvals_before_merge set for the existing MR' do
-        before { merge_request.update_attribute(:approvals_before_merge, 4) }
+        before do
+          merge_request.update_attribute(:approvals_before_merge, 4)
+        end
+
+        context 'when it is not set' do
+          before do
+            update_merge_request(title: 'New title')
+          end
+
+          it 'does not change the merge request' do
+            expect(merge_request.reload.approvals_before_merge).to eq(4)
+          end
+
+          it 'updates the merge request' do
+            expect(merge_request.reload).to be_valid
+            expect(response).to redirect_to(namespace_project_merge_request_path(id: merge_request.iid, project_id: project.to_param))
+          end
+        end
 
         context 'when it is less than the one in the target project' do
           before { update_merge_request(approvals_before_merge: 1) }