Question

This code should update the entire table by applying a filter to its “name” values:

entries = select('id, name').all
entries.each do |entry|
  puts entry.id
  update(entry.id, { :name => sanitize(entry.name) })
end

I am pretty new to Ruby on Rails and found it interesting, that my selection query is split into the single row selections:

SELECT `entries`.* FROM `entries` WHERE (`entries`.`id` = 1) LIMIT 1
SELECT `entries`.* FROM `entries` WHERE (`entries`.`id` = 2) LIMIT 1
SELECT `entries`.* FROM `entries` WHERE (`entries`.`id` = 3) LIMIT 1
...

As I understand, it’s a kind of optimization, provided by Rails – to select a row only when it’s needed (every cycle) and not the all entries at once.

However, is it really more efficient in this case? I mean, if I have 1000 records in my database table – is it better to make 1000 queries than a single one? If not, how can I force Rails to select more than one row per query?

Another question is: not all rows are updated by this query. Does Rails ignore the update query, if the provided values are the same which already exist (in other words, if entry.name == sanitize(entry.name))?

Answer 1

Entry.find_each do |entry|
  #...
end

That fetches all entries (100 per query) and exposes each entry for your pleasure.

If attributes are not changed, Rails will not perform an UPDATE query.