This is an attempt to efficiently upsert/overwrite a large chunk of records' (items) associated rows (terms).

upsert_all_or_else_delete.rb

Item.where("updated_at > ?", after).includes(:english_terms).find_in_batches(batch_size: 5000).with_index do |items, i|
  good_term_attrs = []
  bad_term_ids = []

  items.each do |item|
    # This just gathers all the potential valid english terms associated with a dictionary entry:
    term_texts = @expands_parentheticals.call(item.meaning_texts).map { |s|
      @massages_english.call(s)
    }.uniq

    good_term_attrs += term_texts.map { |text|
      {item_id: item.id, type: item.type, text: text}
    }
    bad_term_ids += item.english_terms.select { |existing_term|
      !term_texts.include?(existing_term.text)
    }.map(&:id)
  end

  if good_term_attrs.present?
    EnglishTerm.upsert_all(good_term_attrs, unique_by: [:item_id, :text])
  end

  if bad_term_ids.present?
    EnglishTerm.delete_by(id: bad_term_ids)
  end

  puts "Updated english_terms through #{items.size + i * 5000} items"
end