Inline the new checks into normal update error handling

app/controllers/projects/merge_requests_controller.rb

@@ -7,7 +7,6 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
   include RendersCommits
   include ToggleAwardEmoji
   include IssuableCollections
-  include MarkupHelper
 
   skip_before_action :merge_request, only: [:index, :bulk_update]
   before_action :whitelist_query_limiting, only: [:assign_related_issues, :update]
@@ -123,21 +122,23 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
 
     respond_to do |format|
       format.html do
-        check_branch_conflict
-
-        if @merge_request.valid?
-          redirect_to([@merge_request.target_project.namespace.becomes(Namespace), @merge_request.target_project, @merge_request])
-        else
+        if @merge_request.errors.present?
           define_edit_vars
 
           render :edit
+        else
+          redirect_to project_merge_request_path(@merge_request.target_project, @merge_request)
         end
       end
 
       format.json do
+        if merge_request.errors.present?
+          render json: @merge_request.errors, status: :bad_request
+        else
           render json: serializer.represent(@merge_request, serializer: 'basic')
         end
       end
+    end
   rescue ActiveRecord::StaleObjectError
     define_edit_vars if request.format.html?
 
@@ -258,12 +259,6 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
     @merge_request.check_if_can_be_merged
   end
 
-  def check_branch_conflict
-    if @merge_request.errors[:validate_branches]
-      flash[:alert] = markdown(@merge_request.errors[:validate_branches].to_sentence, pipeline: :single_line)
-    end
-  end
-
   def merge!
     # Disable the CI check if merge_when_pipeline_succeeds is enabled since we have
     # to wait until CI completes to know

app/models/merge_request.rb

@@ -539,15 +539,26 @@ class MergeRequest < ActiveRecord::Base
 
   def validate_branches
     if target_project == source_project && target_branch == source_branch
-      errors.add :branch_conflict, "You can not use same project/branch for source and target"
+      errors.add :branch_conflict, "You can't use same project/branch for source and target"
+      return
     end
 
     if opened?
-      similar_mrs = self.target_project.merge_requests.where(source_branch: source_branch, target_branch: target_branch, source_project_id: source_project.try(:id)).opened
-      similar_mrs = similar_mrs.where('id not in (?)', self.id) if self.id
-      if similar_mrs.any?
-        errors.add :validate_branches,
-                   "Another open Merge Request already exists for this source branch: !#{similar_mrs.first.id}"
+      similar_mrs = target_project
+        .merge_requests
+        .where(source_branch: source_branch, target_branch: target_branch)
+        .where(source_project_id: source_project&.id)
+        .opened
+
+      similar_mrs = similar_mrs.where.not(id: id) if persisted?
+
+      conflict = similar_mrs.first
+
+      if conflict.present?
+        errors.add(
+          :validate_branches,
+          "Another open merge request already exists for this source branch: #{conflict.to_reference}"
+        )
       end
     end
   end

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -291,14 +291,16 @@ describe Projects::MergeRequestsController do
       it_behaves_like 'update invalid issuable', MergeRequest
     end
 
-    context 'there are two merge requests with the same source branch' do
-      it "does not allow to reopen a closed merge request if another one is open" do
+    context 'two merge requests with the same source branch' do
+      it 'does not allow a closed merge request to be reopened if another one is open' do
         merge_request.close!
         create(:merge_request, source_project: merge_request.source_project, source_branch: merge_request.source_branch)
 
         update_merge_request(state_event: 'reopen')
 
-        expect(controller).to set_flash[:alert].to(/Another open Merge Request already exists for this source branch/)
+        errors = assigns[:merge_request].errors
+
+        expect(errors[:validate_branches]).to include(/Another open merge request already exists for this source branch/)
         expect(merge_request.reload).to be_closed
       end
     end