1 files changed, 40 insertions, 17 deletions

diff --git a/app/helpers/compare_helper.rb b/app/helpers/compare_helper.rb
index f9d62747308..e955ad4cfda 100644
--- a/app/helpers/compare_helper.rb
+++ b/app/helpers/compare_helper.rb
@@ -1,25 +1,30 @@
 # frozen_string_literal: true
 
 module CompareHelper
-  def create_mr_button?(from: params[:from], to: params[:to], source_project: @project, target_project: @target_project)
+  def create_mr_button?(source_project:, from:, to: nil, target_project: nil)
+    target_project ||= source_project.default_merge_request_target
+    to ||= target_project.default_branch
+
     from.present? &&
       to.present? &&
       from != to &&
       can?(current_user, :create_merge_request_from, source_project) &&
       can?(current_user, :create_merge_request_in, target_project) &&
-      target_project.repository.branch_exists?(from) &&
-      source_project.repository.branch_exists?(to)
+      target_project.repository.branch_exists?(to) &&
+      source_project.repository.branch_exists?(from)
   end
 
-  def create_mr_path(from: params[:from], to: params[:to], source_project: @project, target_project: @target_project)
+  def create_mr_path(from:, source_project:, to: nil, target_project: nil, mr_params: {})
+    merge_request_params = {
+      source_branch: from
+    }
+
+    merge_request_params[:target_project_id] = target_project.id if target_project
+    merge_request_params[:target_branch] = to if to
+
     project_new_merge_request_path(
-      target_project,
-      merge_request: {
-        source_project_id: source_project.id,
-        source_branch: to,
-        target_project_id: target_project.id,
-        target_branch: from
-      }
+      source_project,
+      merge_request: merge_request_params.merge(mr_params)
     )
   end
 
@@ -32,14 +37,32 @@ module CompareHelper
   def project_compare_selector_data(project, merge_request, params)
     {
       project_compare_index_path: project_compare_index_path(project),
-      refs_project_path: refs_project_path(project),
+      source_project: { id: project.id, name: project.full_path }.to_json,
+      target_project: { id: @target_project.id, name: @target_project.full_path }.to_json,
+      source_project_refs_path: refs_project_path(project),
+      target_project_refs_path: refs_project_path(@target_project),
       params_from: params[:from],
-      params_to: params[:to],
-      project_merge_request_path: merge_request.present? ? project_merge_request_path(project, merge_request) : '',
-      create_mr_path: create_mr_button? ? create_mr_path : ''
+      params_to: params[:to]
     }.tap do |data|
-      data[:project_to] = { id: project.id, name: project.full_path }.to_json
-      data[:projects_from] = target_projects(project).map { |project| { id: project.id, name: project.full_path } }.to_json
+      data[:projects_from] = target_projects(project).map do |target_project|
+        { id: target_project.id, name: target_project.full_path }
+      end.to_json
+
+      data[:project_merge_request_path] =
+        if merge_request.present?
+          project_merge_request_path(project, merge_request)
+        else
+          ''
+        end
+
+      # The `from` and `to` params are inverted in the compare page. The route is `/compare/:from...:to`, but the UI
+      # correctly shows `:to` as the "Source" (i.e. the `from` for MR), and `:from` as "Target" (i.e. the `to` for MR).
+      data[:create_mr_path] =
+        if create_mr_button?(from: params[:to], to: params[:from], source_project: project, target_project: @target_project)
+          create_mr_path(from: params[:to], to: params[:from], source_project: project, target_project: @target_project)
+        else
+          ''
+        end
     end
   end
 end