1 files changed, 43 insertions, 0 deletions
diff --git a/app/models/preloaders/environments/deployment_preloader.rb b/app/models/preloaders/environments/deployment_preloader.rb
new file mode 100644
index 00000000000..fcf892698bb
--- /dev/null
+++ b/app/models/preloaders/environments/deployment_preloader.rb
@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+module Preloaders
+  module Environments
+    # This class is to batch-load deployments of multiple environments.
+    # The deployments to batch-load are fetched using UNION of N selects in a single query instead of default scoping with `IN (environment_id1, environment_id2 ...)`.
+    # See https://gitlab.com/gitlab-org/gitlab/-/issues/345672#note_761852224 for more details.
+    class DeploymentPreloader
+      attr_reader :environments
+
+      def initialize(environments)
+        @environments = environments
+      end
+
+      def execute_with_union(association_name, association_attributes)
+        load_deployment_association(association_name, association_attributes)
+      end
+
+      private
+
+      def load_deployment_association(association_name, association_attributes)
+        return unless environments.present?
+
+        union_arg = environments.inject([]) do |result, environment|
+          result << environment.association(association_name).scope
+        end
+
+        union_sql = Deployment.from_union(union_arg).to_sql
+
+        deployments = Deployment
+                        .from("(#{union_sql}) #{::Deployment.table_name}")
+                        .preload(association_attributes)
+
+        deployments_by_environment_id = deployments.index_by(&:environment_id)
+
+        environments.each do |environment|
+          environment.association(association_name).target = deployments_by_environment_id[environment.id]
+          environment.association(association_name).loaded!
+        end
+      end
+    end
+  end
+end