Return more consistent values for merge_status on MR API

app/models/merge_request.rb

@@ -56,6 +56,7 @@ class MergeRequest < ActiveRecord::Base
   after_create :ensure_merge_request_diff, unless: :importing?
   after_update :clear_memoized_shas
   after_update :reload_diff_if_branch_changed
+  after_update :mark_as_unchecked_if_target_branch_changed
   # When this attribute is true some MR validation is ignored
   # It allows us to close or modify broken merge requests
@@ -561,6 +562,12 @@ class MergeRequest < ActiveRecord::Base
     end
   end
+  def mark_as_unchecked_if_target_branch_changed
+    return unless target_branch_changed?
+
+    mark_as_unchecked
+  end
+
   def reload_diff(current_user = nil)
     return unless open?

changelogs/unreleased/osw-updates-merge-status-on-api-actions.yml

+---
+title: Update merge_status on MR creation/update/get on the API
+merge_request:
+author:
+type: fixed

lib/api/entities.rb

@@ -507,7 +507,16 @@ module API
       expose :work_in_progress?, as: :work_in_progress
       expose :milestone, using: Entities::Milestone
       expose :merge_when_pipeline_succeeds
-      expose :merge_status
+
+      # Ideally we should deprecate `MergeRequest#merge_status` exposure and
+      # use `MergeRequest#mergeable?` instead (boolean).
+      # See https://gitlab.com/gitlab-org/gitlab-ce/issues/42344 for more
+      # information.
+      expose :merge_status do |merge_request|
+        # In order to avoid having a breaking change for users, we keep returning the
+        # expected values from MergeRequest#merge_status state machine.
+        merge_request.mergeable? ? 'can_be_merged' : 'cannot_be_merged'
+      end
       expose :diff_head_sha, as: :sha
       expose :merge_commit_sha
       expose :user_notes_count

spec/models/merge_request_spec.rb

@@ -77,6 +77,23 @@ describe MergeRequest do
         expect(MergeRequest::Metrics.count).to eq(1)
       end
     end
+
+    describe '#mark_as_unchecked_if_target_branch_changed' do
+      let(:merge_request) { create(:merge_request, merge_status: :can_be_merged) }
+
+      it 'marks MR as unchecked if target_branch changes' do
+        expect { merge_request.update!(target_branch: 'bar') }
+          .to change(merge_request, :merge_status)
+          .from('can_be_merged')
+          .to('unchecked')
+      end
+
+      it 'does not marks MR as unchecked when target_branch does not changes' do
+        expect { merge_request.update!(title: 'foo') }
+          .not_to change(merge_request, :merge_status)
+          .from('can_be_merged')
+      end
+    end
   end
   describe 'respond to' do