Added ExportedProtectedBranch model

Model to manage protected branch import export.
Added specs accordingly.

app/models/exported_protected_branch.rb

+# frozen_string_literal: true
+
+class ExportedProtectedBranch < ProtectedBranch
+  has_many :push_access_levels, -> { where(deploy_key_id: nil) }, class_name: "ProtectedBranch::PushAccessLevel", foreign_key: :protected_branch_id
+end

app/models/project.rb

@@ -222,6 +222,7 @@ class Project < ApplicationRecord
   has_many :snippets, class_name: 'ProjectSnippet'
   has_many :hooks, class_name: 'ProjectHook'
   has_many :protected_branches
+  has_many :exported_protected_branches
   has_many :protected_tags
   has_many :repository_languages, -> { order "share DESC" }
   has_many :designs, inverse_of: :project, class_name: 'DesignManagement::Design'

app/presenters/projects/import_export/project_export_presenter.rb

@@ -15,6 +15,10 @@ module Projects
         self.respond_to?(:override_description) ? override_description : super
       end
 
+      def protected_branches
+        project.exported_protected_branches
+      end
+
       private
 
       def converted_group_members

doc/user/project/settings/import_export.md

@@ -51,6 +51,9 @@ Note the following:
   then new branches associated with such merge requests will be created
   within a project during the import/export. Thus, the number of branches
   in the exported project could be bigger than in the original project.
+- Deploy keys allowed to push to protected branches are not exported. Therefore,
+  you will need to recreate this association by first enabling these deploy keys in your
+  imported project and then updating your protected branches accordingly.
 
 ## Version history
 

lib/gitlab/import_export/project/import_export.yml

@@ -169,6 +169,7 @@ excluded_attributes:
     - :compliance_framework_setting
     - :show_default_award_emojis
     - :services
+    - :exported_protected_branches
   namespaces:
     - :runners_token
     - :runners_token_encrypted

spec/factories/exported_protected_branches.rb

+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :exported_protected_branch, class: 'ExportedProtectedBranch', parent: :protected_branch
+end

spec/lib/gitlab/import_export/all_models.yml

@@ -552,6 +552,7 @@ project:
 - pipeline_artifacts
 - terraform_states
 - alert_management_http_integrations
+- exported_protected_branches
 award_emoji:
 - awardable
 - user

spec/models/exported_protected_branch_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe ExportedProtectedBranch do
+  describe 'Associations' do
+    it { is_expected.to have_many(:push_access_levels) }
+  end
+
+  describe '.push_access_levels' do
+    it 'returns the correct push access levels' do
+      exported_branch = create(:exported_protected_branch, :developers_can_push)
+      deploy_key = create(:deploy_key)
+      create(:deploy_keys_project, :write_access, project: exported_branch.project, deploy_key: deploy_key )
+      create(:protected_branch_push_access_level, protected_branch: exported_branch, deploy_key: deploy_key)
+      dev_push_access_level = exported_branch.push_access_levels.first
+
+      expect(exported_branch.push_access_levels).to contain_exactly(dev_push_access_level)
+    end
+  end
+end