1 files changed, 99 insertions, 0 deletions
diff --git a/app/services/loose_foreign_keys/cleaner_service.rb b/app/services/loose_foreign_keys/cleaner_service.rb
new file mode 100644
index 00000000000..8fe053e2edf
--- /dev/null
+++ b/app/services/loose_foreign_keys/cleaner_service.rb
@@ -0,0 +1,99 @@
+# frozen_string_literal: true
+
+module LooseForeignKeys
+  # rubocop: disable CodeReuse/ActiveRecord
+  class CleanerService
+    DELETE_LIMIT = 1000
+    UPDATE_LIMIT = 500
+
+    delegate :connection, to: :model
+
+    def initialize(model:, foreign_key_definition:, deleted_parent_records:, with_skip_locked: false)
+      @model = model
+      @foreign_key_definition = foreign_key_definition
+      @deleted_parent_records = deleted_parent_records
+      @with_skip_locked = with_skip_locked
+    end
+
+    def execute
+      result = connection.execute(build_query)
+
+      { affected_rows: result.cmd_tuples, table: foreign_key_definition.to_table }
+    end
+
+    def async_delete?
+      foreign_key_definition.on_delete == :async_delete
+    end
+
+    def async_nullify?
+      foreign_key_definition.on_delete == :async_nullify
+    end
+
+    private
+
+    attr_reader :model, :foreign_key_definition, :deleted_parent_records, :with_skip_locked
+
+    def build_query
+      query = if async_delete?
+                delete_query
+              elsif async_nullify?
+                update_query
+              else
+                raise "Invalid on_delete argument: #{foreign_key_definition.on_delete}"
+              end
+
+      unless query.include?(%{"#{foreign_key_definition.column}" IN (})
+        raise("FATAL: foreign key condition is missing from the generated query: #{query}")
+      end
+
+      query
+    end
+
+    def arel_table
+      @arel_table ||= model.arel_table
+    end
+
+    def primary_keys
+      @primary_keys ||= connection.primary_keys(model.table_name).map { |key| arel_table[key] }
+    end
+
+    def quoted_table_name
+      @quoted_table_name ||= Arel.sql(connection.quote_table_name(model.table_name))
+    end
+
+    def delete_query
+      query = Arel::DeleteManager.new
+      query.from(quoted_table_name)
+
+      add_in_query_with_limit(query, DELETE_LIMIT)
+    end
+
+    def update_query
+      query = Arel::UpdateManager.new
+      query.table(quoted_table_name)
+      query.set([[arel_table[foreign_key_definition.column], nil]])
+
+      add_in_query_with_limit(query, UPDATE_LIMIT)
+    end
+
+    # IN query with one or composite primary key
+    # WHERE (primary_key1, primary_key2) IN (subselect)
+    def add_in_query_with_limit(query, limit)
+      columns = Arel::Nodes::Grouping.new(primary_keys)
+      query.where(columns.in(in_query_with_limit(limit))).to_sql
+    end
+
+    # Builds the following sub-query
+    # SELECT primary_keys FROM table WHERE foreign_key IN (1, 2, 3) LIMIT N
+    def in_query_with_limit(limit)
+      in_query = Arel::SelectManager.new
+      in_query.from(quoted_table_name)
+      in_query.where(arel_table[foreign_key_definition.column].in(deleted_parent_records.map(&:primary_key_value)))
+      in_query.projections = primary_keys
+      in_query.take(limit)
+      in_query.lock(Arel.sql('FOR UPDATE SKIP LOCKED')) if with_skip_locked
+      in_query
+    end
+  end
+  # rubocop: enable CodeReuse/ActiveRecord
+end