diff --git a/db/post_migrate/20181130102132_backfill_hashed_project_repositories.rb b/db/post_migrate/20181130102132_backfill_hashed_project_repositories.rb
index b989d9fb43d..7814cdba58a 100644
--- a/db/post_migrate/20181130102132_backfill_hashed_project_repositories.rb
+++ b/db/post_migrate/20181130102132_backfill_hashed_project_repositories.rb
@@ -1,11 +1,11 @@
 # frozen_string_literal: true
 
-class BackfillHashedProjectRepositories < ActiveRecord::Migration[5.0]
+class BackfillHashedProjectRepositories < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
 
   DOWNTIME = false
   BATCH_SIZE     = 1_000
-  DELAY_INTERVAL = 1.minutes
+  DELAY_INTERVAL = 5.minutes
   MIGRATION      = 'BackfillHashedProjectRepositories'
 
   disable_ddl_transaction!
@@ -21,7 +21,6 @@ class BackfillHashedProjectRepositories < ActiveRecord::Migration[5.0]
   end
 
   def down
-    # Since there could have been existing rows before the migration
-    # do not remove anything
+    # no-op: since there could have been existing rows before the migration do not remove anything
   end
 end
diff --git a/lib/gitlab/background_migration/backfill_hashed_project_repositories.rb b/lib/gitlab/background_migration/backfill_hashed_project_repositories.rb
index 88696dd1aa6..2f76f2f7434 100644
--- a/lib/gitlab/background_migration/backfill_hashed_project_repositories.rb
+++ b/lib/gitlab/background_migration/backfill_hashed_project_repositories.rb
@@ -2,58 +2,60 @@
 
 module Gitlab
   module BackgroundMigration
-    # Class the will create rows in project_repositories for all
-    # projects that are on hashed storage
+    # Class that will create fill the project_repositories table
+    # for all projects that are on hashed storage and an entry is
+    # is missing in this table.
     class BackfillHashedProjectRepositories
-      # Model for a Shard
+      # Shard model
       class Shard < ActiveRecord::Base
         self.table_name = 'shards'
-
-        def self.by_name(name)
-          to_a.detect { |shard| shard.name == name } || create_by(name: name)
-        rescue ActiveRecord::RecordNotUnique
-          retry
-        end
       end
 
       # Class that will find or create the shard by name.
-      # There is only a small set of shards, which would not change quickly,
-      # so look them up from memory instead of hitting the DB each time.
+      # There is only a small set of shards, which would
+      # not change quickly, so look them up from memory
+      # instead of hitting the DB each time.
       class ShardFinder
-        def find(name)
-          shards.detect { |shard| shard.name == name } || create!(name)
+        def find_shard_id(name)
+          shard_id = shards.fetch(name, nil)
+          return shard_id if shard_id.present?
+
+          Shard.transaction(requires_new: true) do
+            create!(name)
+          end
         rescue ActiveRecord::RecordNotUnique
-          load!
+          reload!
           retry
         end
 
         private
 
         def create!(name)
-          Shard.create!(name: name).tap { |shard| @shards << shard }
+          Shard.create!(name: name).tap { |shard| @shards[name] = shard.id }
         end
 
         def shards
-          @shards || load!
+          @shards ||= reload!
         end
 
-        def load!
-          @shards = Shard.all.to_a
+        def reload!
+          @shards = Hash[*Shard.all.map { |shard| [shard.name, shard.id] }.flatten]
         end
       end
 
-      # Model for a ProjectRepository
+      # ProjectRegistry model
       class ProjectRepository < ActiveRecord::Base
         self.table_name = 'project_repositories'
 
         belongs_to :project, inverse_of: :project_repository
       end
 
-      # Model for a Project
+      # Project model
       class Project < ActiveRecord::Base
         self.table_name = 'projects'
 
         HASHED_PATH_PREFIX = '@hashed'
+
         HASHED_STORAGE_FEATURES = {
           repository: 1,
           attachments: 2
@@ -63,33 +65,26 @@ module Gitlab
 
         class << self
           def on_hashed_storage
-            where(arel_table[:storage_version].gteq(HASHED_STORAGE_FEATURES[:repository]))
+            where(Project.arel_table[:storage_version]
+              .gteq(HASHED_STORAGE_FEATURES[:repository]))
           end
 
           def without_project_repository
-            cond = ProjectRepository.arel_table[:project_id].eq(nil)
-            left_outer_joins(:project_repository).where(cond)
+            joins(left_outer_join_project_repository)
+              .where(ProjectRepository.arel_table[:project_id].eq(nil))
           end
 
-          def left_outer_joins(relation)
-            return super if Gitlab.rails5?
+          def left_outer_join_project_repository
+            projects_table = Project.arel_table
+            repository_table = ProjectRepository.arel_table
 
-            # TODO Rails 4?
+            projects_table
+              .join(repository_table, Arel::Nodes::OuterJoin)
+              .on(projects_table[:id].eq(repository_table[:project_id]))
+              .join_sources
           end
         end
 
-        def project_repository_attributes(shard_finder)
-          return unless hashed_storage?
-
-          {
-            project_id: id,
-            shard_id: shard_finder.find(repository_storage).id,
-            disk_path: hashed_disk_path
-          }
-        end
-
-        private
-
         def hashed_storage?
           self.storage_version && self.storage_version >= 1
         end
@@ -99,7 +94,7 @@ module Gitlab
         end
 
         def disk_hash
-          @disk_hash ||= Digest::SHA2.hexdigest(id.to_s) if id
+          @disk_hash ||= Digest::SHA2.hexdigest(id.to_s)
         end
       end
 
@@ -110,12 +105,27 @@ module Gitlab
       private
 
       def project_repositories(start_id, stop_id)
-        Project.on_hashed_storage.without_project_repository
+        Project.on_hashed_storage
+          .without_project_repository
           .where(id: start_id..stop_id)
-          .map { |project| project.project_repository_attributes(shard_finder) }
+          .map { |project| build_attributes_for_project(project) }
           .compact
       end
 
+      def build_attributes_for_project(project)
+        return unless project.hashed_storage?
+
+        {
+          project_id: project.id,
+          shard_id:   find_shard_id(project.repository_storage),
+          disk_path:  project.hashed_disk_path
+        }
+      end
+
+      def find_shard_id(repository_storage)
+        shard_finder.find_shard_id(repository_storage)
+      end
+
       def shard_finder
         @shard_finder ||= ShardFinder.new
       end
diff --git a/spec/lib/gitlab/background_migration/backfill_hashed_project_repositories_spec.rb b/spec/lib/gitlab/background_migration/backfill_hashed_project_repositories_spec.rb
index d2f499ffa64..b6c1edbbf8b 100644
--- a/spec/lib/gitlab/background_migration/backfill_hashed_project_repositories_spec.rb
+++ b/spec/lib/gitlab/background_migration/backfill_hashed_project_repositories_spec.rb
@@ -3,59 +3,31 @@
 require 'spec_helper'
 
 describe Gitlab::BackgroundMigration::BackfillHashedProjectRepositories, :migration, schema: 20181130102132 do
-  let(:shards) { table(:shards) }
   let(:namespaces) { table(:namespaces) }
-  let(:projects) { table(:projects) }
   let(:project_repositories) { table(:project_repositories) }
+  let(:projects) { table(:projects) }
+  let(:shards) { table(:shards) }
   let(:group) { namespaces.create!(name: 'foo', path: 'foo') }
-  let(:default_shard) { shards.create!(name: 'default') }
+  let(:shard) { shards.create!(name: 'default') }
 
   describe described_class::ShardFinder do
-    describe '#find' do
-      subject(:finder) { described_class.new }
-
-      it 'creates the shard by name' do
-        expect(finder).to receive(:create!).and_call_original
-
-        expect(finder.find('default')).to be_present
-      end
-
-      it 'does not try to create existing shards' do
-        shards.create(name: 'default')
-
-        expect(finder).not_to receive(:create!)
-
-        finder.find('default')
-      end
-
-      it 'only queries the database once for shards' do
-        finder.find('default')
-
-        expect do
-          finder.find('default')
-        end.not_to exceed_query_limit(0)
-      end
-
+    describe '#find_shard_id' do
       it 'creates a new shard when it does not exist yet' do
-        expect do
-          finder.find('other')
-        end.to change(shards, :count).by(1)
+        expect { subject.find_shard_id('other') }.to change(shards, :count).by(1)
       end
 
-      it 'only creates a new shard once' do
-        finder.find('other')
+      it 'returns the shard when it exists' do
+        shards.create(id: 5, name: 'other')
 
-        expect do
-          finder.find('other')
-        end.not_to change(shards, :count)
+        shard_id = subject.find_shard_id('other')
+
+        expect(shard_id).to eq(5)
       end
 
-      it 'is not vulnerable to race conditions' do
-        finder.find('default')
+      it 'only queries the database once to retrieve shards' do
+        subject.find_shard_id('default')
 
-        other_shard = shards.create(name: 'other')
-
-        expect(finder.find('other').id).to eq(other_shard.id)
+        expect { subject.find_shard_id('default') }.not_to exceed_query_limit(0)
       end
     end
   end
@@ -63,93 +35,56 @@ describe Gitlab::BackgroundMigration::BackfillHashedProjectRepositories, :migrat
   describe described_class::Project do
     describe '.on_hashed_storage' do
       it 'finds projects with repository on hashed storage' do
-        hashed_projects = [
-          projects.create!(name: 'foo', path: 'foo', namespace_id: group.id, storage_version: 1),
-          projects.create!(name: 'bar', path: 'bar', namespace_id: group.id, storage_version: 2)
-        ]
+        projects.create!(id: 1, name: 'foo', path: 'foo', namespace_id: group.id, storage_version: 1)
+        projects.create!(id: 2, name: 'bar', path: 'bar', namespace_id: group.id, storage_version: 2)
+        projects.create!(id: 3, name: 'baz', path: 'baz', namespace_id: group.id, storage_version: 0)
+        projects.create!(id: 4, name: 'zoo', path: 'zoo', namespace_id: group.id, storage_version: nil)
 
-        projects.create!(name: 'baz', path: 'baz', namespace_id: group.id, storage_version: 0)
-        projects.create!(name: 'quz', path: 'quz', namespace_id: group.id, storage_version: nil)
-
-        expect(described_class.on_hashed_storage.pluck(:id)).to match_array(hashed_projects.map(&:id))
+        expect(described_class.on_hashed_storage.pluck(:id)).to match_array([1, 2])
       end
     end
 
     describe '.without_project_repository' do
-      it 'finds projects which do not have a projects_repositories row' do
-        without_project = projects.create!(name: 'foo', path: 'foo', namespace_id: group.id)
-        with_project = projects.create!(name: 'bar', path: 'bar', namespace_id: group.id)
-        project_repositories.create!(project_id: with_project.id, disk_path: '@phony/foo/bar', shard_id: default_shard.id)
+      it 'finds projects which do not have a projects_repositories entry' do
+        projects.create!(id: 1, name: 'foo', path: 'foo', namespace_id: group.id)
+        projects.create!(id: 2, name: 'bar', path: 'bar', namespace_id: group.id)
+        project_repositories.create!(project_id: 2, disk_path: '@phony/foo/bar', shard_id: shard.id)
 
-        expect(described_class.without_project_repository.pluck(:id)).to contain_exactly(without_project.id)
-      end
-    end
-
-    describe '#project_repository_attributes' do
-      let(:shard_finder) { Gitlab::BackgroundMigration::BackfillHashedProjectRepositories::ShardFinder.new }
-
-      it 'composes the correct attributes for project_repository' do
-        shiny_shard = shards.create!(name: 'shiny')
-        project = projects.create!(id: 5, name: 'foo', path: 'foo', namespace_id: group.id, repository_storage: shiny_shard.name, storage_version: 1)
-
-        expected_attributes = {
-          project_id: project.id,
-          shard_id: shiny_shard.id,
-          disk_path: '@hashed/ef/2d/ef2d127de37b942baad06145e54b0c619a1f22327b2ebbcfbec78f5564afe39d'
-        }
-
-        expect(described_class.find(project.id).project_repository_attributes(shard_finder)).to eq(expected_attributes)
-      end
-
-      it 'returns nil for a project not on hashed storage' do
-        project = projects.create!(name: 'foo', path: 'foo', namespace_id: group.id, storage_version: 0)
-
-        expect(described_class.find(project.id).project_repository_attributes(shard_finder)).to be_nil
+        expect(described_class.without_project_repository.pluck(:id)).to contain_exactly(1)
       end
     end
   end
 
   describe '#perform' do
-    def perform!
-      described_class.new.perform(1, projects.last.id)
+    it 'creates a project_repository row for projects on hashed storage that need one' do
+      projects.create!(id: 1, name: 'foo', path: 'foo', namespace_id: group.id, storage_version: 1)
+      projects.create!(id: 2, name: 'bar', path: 'bar', namespace_id: group.id, storage_version: 2)
+
+      expect { described_class.new.perform(1, projects.last.id) }.to change(project_repositories, :count).by(2)
     end
 
-    it 'create project_repository row for hashed storage project' do
-      projects.create!(name: 'foo', path: 'foo', namespace_id: group.id, storage_version: 1)
+    it 'does nothing for projects on hashed storage that have already a project_repository row' do
+      projects.create!(id: 1, name: 'foo', path: 'foo', namespace_id: group.id, storage_version: 1)
+      project_repositories.create!(project_id: 1, disk_path: '@phony/foo/bar', shard_id: shard.id)
 
-      expect do
-        perform!
-      end.to change(project_repositories, :count).by(1)
-    end
-
-    it 'does nothing for projects that have already a project_repository' do
-      project = projects.create!(name: 'foo', path: 'foo', namespace_id: group.id, storage_version: 1)
-      project_repositories.create!(project_id: project.id, disk_path: '@phony/foo/bar', shard_id: default_shard.id)
-
-      expect do
-        perform!
-      end.not_to change(project_repositories, :count)
+      expect { described_class.new.perform(1, projects.last.id) }.not_to change(project_repositories, :count)
     end
 
     it 'does nothing for projects on legacy storage' do
       projects.create!(name: 'foo', path: 'foo', namespace_id: group.id, storage_version: 0)
 
-      expect do
-        perform!
-      end.not_to change(project_repositories, :count)
+      expect { described_class.new.perform(1, projects.last.id) }.not_to change(project_repositories, :count)
     end
 
     it 'inserts rows in a single query' do
-      projects.create!(name: 'foo', path: 'foo', namespace_id: group.id, storage_version: 1, repository_storage: default_shard.name)
+      projects.create!(name: 'foo', path: 'foo', namespace_id: group.id, storage_version: 1, repository_storage: shard.name)
 
-      control_count = ActiveRecord::QueryRecorder.new do
-        perform!
-      end
+      control_count = ActiveRecord::QueryRecorder.new { described_class.new.perform(1, projects.last.id) }
 
-      projects.create!(name: 'bar', path: 'bar', namespace_id: group.id, storage_version: 1, repository_storage: default_shard.name)
-      projects.create!(name: 'quz', path: 'quz', namespace_id: group.id, storage_version: 1, repository_storage: default_shard.name)
+      projects.create!(name: 'bar', path: 'bar', namespace_id: group.id, storage_version: 1, repository_storage: shard.name)
+      projects.create!(name: 'zoo', path: 'zoo', namespace_id: group.id, storage_version: 1, repository_storage: shard.name)
 
-      expect { perform! }.not_to exceed_query_limit(control_count)
+      expect { described_class.new.perform(1, projects.last.id) }.not_to exceed_query_limit(control_count)
     end
   end
 end