Make PrimaryKeyBatchingStrategy easy to extend

Add a hook to make it easy to apply additional filters to the relation
being iterated.

lib/gitlab/background_migration/batching_strategies/primary_key_batching_strategy.rb

@@ -23,6 +23,7 @@ module Gitlab
 
           quoted_column_name = model_class.connection.quote_column_name(column_name)
           relation = model_class.where("#{quoted_column_name} >= ?", batch_min_value)
+          relation = apply_additional_filters(relation)
           next_batch_bounds = nil
 
           relation.each_batch(of: batch_size, column: column_name) do |batch| # rubocop:disable Lint/UnreachableLoop
@@ -33,6 +34,20 @@ module Gitlab
 
           next_batch_bounds
         end
+
+        # Strategies based on PrimaryKeyBatchingStrategy can use
+        # this method to easily apply additional filters.
+        #
+        # Example:
+        #
+        #   class TypeIsNotNull < PrimaryKeyBatchingStrategy
+        #     def apply_additional_filters(relation)
+        #       relation.where.not(type: nil)
+        #     end
+        #   end
+        def apply_additional_filters(relation)
+          relation
+        end
       end
     end
   end

spec/lib/gitlab/background_migration/batching_strategies/primary_key_batching_strategy_spec.rb

@@ -44,4 +44,23 @@ RSpec.describe Gitlab::BackgroundMigration::BatchingStrategies::PrimaryKeyBatchi
       expect(batch_bounds).to be_nil
     end
   end
+
+  context 'additional filters' do
+    let(:strategy_with_filters) do
+      Class.new(described_class) do
+        def apply_additional_filters(relation)
+          relation.where.not(type: 'Project')
+        end
+      end
+    end
+
+    let(:batching_strategy) { strategy_with_filters.new(connection: ActiveRecord::Base.connection) }
+    let!(:namespace5) { namespaces.create!(name: 'batchtest5', path: 'batch-test5', type: 'Project') }
+
+    it 'applies additional filters' do
+      batch_bounds = batching_strategy.next_batch(:namespaces, :id, batch_min_value: namespace4.id, batch_size: 3, job_arguments: nil)
+
+      expect(batch_bounds).to eq([namespace4.id, namespace4.id])
+    end
+  end
 end