Ruby函数编程的困惑—（入门）

Question

I know I'm looping twice in the following code for no reason. How could I go the following loop of records in one step instead of first selecting them and then performing actions on it.

merchants = Merchant.all.select do |merchant|
  merchant.url.present? && merchant.url != merchant.new_data.url
end

merchants.each do |merchant|
  merchant.url = merchant.new_data.url
  merchant.save!
end

I bet it's something very simple -- I'm new to Ruby so all this is new to me.

Answer 1

It's conceptually ok to iterate over data on separate steps (this allows modularization and composability). Granted, this may be a bit inefficient when using strict structures (as opposed to lazy ones) like Ruby arrays, but no worries for small/medium arrays. Anyway, of course you can join the logic in one step, like this:

Merchant.all.each do |merchant|
  if merchant.url.present? && merchant.url != merchant.new_data.url
    merchant.url = merchant.new_data.url
    merchant.save!
  end
end

Answer 2

It will be quite inefficient doing Merchant.all when you have millions of rows, especially if they have many large fields. I would move the selection logic into the ActiveRecord query, something like this (untested):

Merchant.includes(:new_data).select(:url).where("url IS NOT NULL").where("url != new_data.url")

Answer 3

This will check for nil merchant url

merchants.each do |merchant|
  merchant.url = merchant.url || merchant.new_data.url
  merchant.save!
end