1 files changed, 95 insertions, 0 deletions
diff --git a/app/models/concerns/loose_foreign_key.rb b/app/models/concerns/loose_foreign_key.rb
new file mode 100644
index 00000000000..4e822a04869
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+++ b/app/models/concerns/loose_foreign_key.rb
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+# frozen_string_literal: true
+
+module LooseForeignKey
+  extend ActiveSupport::Concern
+
+  # This concern adds loose foreign key support to ActiveRecord models.
+  # Loose foreign keys allow delayed processing of associated database records
+  # with similar guarantees than a database foreign key.
+  #
+  # TODO: finalize this later once the async job is in place
+  #
+  # Prerequisites:
+  #
+  # To start using the concern, you'll need to install a database trigger to the parent
+  # table in a standard DB migration (not post-migration).
+  #
+  # > add_loose_foreign_key_support(:projects, :gitlab_main)
+  #
+  # Usage:
+  #
+  # > class Ci::Build < ApplicationRecord
+  # >
+  # >   loose_foreign_key :security_scans, :build_id, on_delete: :async_delete, gitlab_schema: :gitlab_main
+  # >
+  # >   # associations can be still defined, the dependent options is no longer necessary:
+  # >   has_many :security_scans, class_name: 'Security::Scan'
+  # >
+  # > end
+  #
+  # Options for on_delete:
+  #
+  # - :async_delete - deletes the children rows via an asynchronous process.
+  # - :async_nullify - sets the foreign key column to null via an asynchronous process.
+  #
+  # Options for gitlab_schema:
+  #
+  # - :gitlab_ci
+  # - :gitlab_main
+  #
+  # The value can be determined by calling `Model.gitlab_schema` where the Model represents
+  # the model for the child table.
+  #
+  # How it works:
+  #
+  # When adding loose foreign key support to the table, a DELETE trigger is installed
+  # which tracks the record deletions (stores primary key value of the deleted row) in
+  # a database table.
+  #
+  # These deletion records are processed asynchronously and records are cleaned up
+  # according to the loose foreign key definitions described in the model.
+  #
+  # The cleanup happens in batches, which reduces the likelyhood of statement timeouts.
+  #
+  # When all associations related to the deleted record are cleaned up, the record itself
+  # is deleted.
+  included do
+    class_attribute :loose_foreign_key_definitions, default: []
+  end
+
+  class_methods do
+    def loose_foreign_key(to_table, column, options)
+      symbolized_options = options.symbolize_keys
+
+      unless base_class?
+        raise <<~MSG
+        loose_foreign_key can be only used on base classes, inherited classes are not supported.
+        Please define the loose_foreign_key on the #{base_class.name} class.
+        MSG
+      end
+
+      on_delete_options = %i[async_delete async_nullify]
+      gitlab_schema_options = [ApplicationRecord.gitlab_schema, Ci::ApplicationRecord.gitlab_schema]
+
+      unless on_delete_options.include?(symbolized_options[:on_delete]&.to_sym)
+        raise "Invalid on_delete option given: #{symbolized_options[:on_delete]}. Valid options: #{on_delete_options.join(', ')}"
+      end
+
+      unless gitlab_schema_options.include?(symbolized_options[:gitlab_schema]&.to_sym)
+        raise "Invalid gitlab_schema option given: #{symbolized_options[:gitlab_schema]}. Valid options: #{gitlab_schema_options.join(', ')}"
+      end
+
+      definition = ActiveRecord::ConnectionAdapters::ForeignKeyDefinition.new(
+        table_name.to_s,
+        to_table.to_s,
+        {
+          column: column.to_s,
+          on_delete: symbolized_options[:on_delete].to_sym,
+          gitlab_schema: symbolized_options[:gitlab_schema].to_sym
+        }
+      )
+
+      self.loose_foreign_key_definitions += [definition]
+    end
+  end
+end