Are there any Ruby ORMs which use cursors or smart fetch?

问题

I'm looking for a Ruby ORM to replace ActiveRecord. I've been looking at Sequel and DataMapper. They look pretty good however none of them seems to do the basic: not loading everything in memory when you don't need it.

I mean I've tried the following (or equivalent) on ActiveRecord and Sequel on table with lots of rows:

 posts.each { |p| puts p }

Both of them go crazy on memory. They seem to load everything in memory rather than fetching stuff when needed. I used the find_in_batches in ActiveRecord, but it's not an acceptable solution:

ActiveRecord is not an acceptable solution because we had too many problems with it.
Why should my code be aware of a paging mechanism? I'm happy to configure somewhere the size of the page but that's it. With find_in_batches you need to do something like:

post.find_in_batches { |batch| batch.each { |p| puts p } }

But that should be transparent.

So is there somewhere a reliable Ruby ORM which does the fetch properly?

Update:

As Sergio mentioned, in Rails 3 you can use find_each which exactly what I want. However as ActiveRecord is not an option, except if someone can really convince me to use it, the questions are:

Which ORMs support the equivalent of find_each?
How to do it?
Why do we need a find_each, while find should do it, shouldn't it?

回答1:

Sequel's Dataset#each does yield individual rows at a time, but most database drivers will load the entire result in memory first.

If you are using Sequel's Postgres adapter, you can choose to use real cursors:

posts.use_cursor.each{|p| puts p}

This fetches 1000 rows at a time by default, but you can use an option to specify the amount of rows to grab per cursor fetch:

posts.use_cursor(:rows_per_fetch=>100).each{|p| puts p}

If you aren't using Sequel's Postgres adapter, you can use Sequel's pagination extension:

Sequel.extension :pagination
posts.order(:id).each_page(1000){|ds| ds.each{|p| puts p}}

However, like ActiveRecord's find_in_batches/find_each, this does separate queries, so you need to be careful if there are concurrent modifications to the dataset you are retrieving.

The reason this isn't the default in Sequel is probably the same reason it isn't the default in ActiveRecord, which is that it isn't a good default in the general case. Only queries with large result sets really need to worry about it, and most queries don't return large result sets.

At least with the Postgres adapter cursor support, it's fairly easy to make it the default for your model:

Post.dataset = Post.dataset.use_cursor

For the pagination extension, you can't really do that, but you can wrap it in a method that makes it mostly transparent.

回答2:

Sequel.extension :pagination
posts.order(:id).each_page(1000) do |ds|
  ds.each { |p| puts p }
end

It is very very slow on large tables!

It becomes clear, looked at the method body: http://sequel.rubyforge.org/rdoc-plugins/classes/Sequel/Dataset.html#method-i-paginate

# File lib/sequel/extensions/pagination.rb, line 11

def paginate(page_no, page_size, record_count=nil)
  raise(Error, "You cannot paginate a dataset that already has a limit") if @opts[:limit]
  paginated = limit(page_size, (page_no - 1) * page_size)
  paginated.extend(Pagination)
  paginated.set_pagination_info(page_no, page_size, record_count || count)
end

回答3:

ActiveRecord actually has an almost transparent batch mode:

User.find_each do |user|
  NewsLetter.weekly_deliver(user)
end

回答4:

This code works faster than find_in_batches in ActiveRecord

id_max = table.get(:max[:id])
id_min = table.get(:min[:id])
n=1000
(0..(id_max-id_min)/n).map.each do |i|
    table.filter(:id >= id_min+n*i, :id < id_min+n*(i+1)).each {|row|}
end

回答5:

Maybe you can consider Ohm, that is based on Redis NoSQL store.

来源：https://stackoverflow.com/questions/8849340/are-there-any-ruby-orms-which-use-cursors-or-smart-fetch

标签

ruby

orm

datamapper

sequel