Looping through a collection of records from the database (using the
all
method, for example) is very inefficient since it will try
to instantiate all the objects at once.
In that case, batch processing methods allow you to work with the records in batches, thereby greatly reducing memory consumption.
The find_each method uses find_in_batches with a
batch size of 1000 (or as specified by the :batch_size
option).
Person.all.find_each do |person| person.do_awesome_stuff end Person.where("age > 21").find_each do |person| person.party_all_night! end You can also pass the +:start+ option to specify an offset to control the starting point.
Yields each batch of records that was found by the find
options
as an array. The size of each batch is set by the
:batch_size
option; the default is 1000.
You can control the starting point for the batch processing by supplying
the :start
option. This is especially useful if you want
multiple workers dealing with the same processing queue. You can make
worker 1 handle all the records between id 0 and 10,000 and worker 2 handle
from 10,000 and beyond (by setting the :start
option on that
worker).
It’s not possible to set the order. That is automatically set to ascending on the primary key (“id ASC”) to make the batch ordering work. This also means that this method only works with integer-based primary keys. You can’t set the limit either, that’s used to control the batch sizes.
Person.where("age > 21").find_in_batches do |group| sleep(50) # Make sure it doesn't get too crowded in there! group.each { |person| person.party_all_night! } end # Let's process the next 2000 records Person.all.find_in_batches(start: 2000, batch_size: 2000) do |group| group.each { |person| person.party_all_night! } end
# File activerecord/lib/active_record/relation/batches.rb, line 56 def find_in_batches(options = {}) options.assert_valid_keys(:start, :batch_size) relation = self if logger && (arel.orders.present? || arel.taken.present?) logger.warn("Scoped order and limit are ignored, it's forced to be batch order and batch size") end start = options.delete(:start) batch_size = options.delete(:batch_size) || 1000 relation = relation.reorder(batch_order).limit(batch_size) records = start ? relation.where(table[primary_key].gteq(start)).to_a : relation.to_a while records.any? records_size = records.size primary_key_offset = records.last.id yield records break if records_size < batch_size if primary_key_offset records = relation.where(table[primary_key].gt(primary_key_offset)).to_a else raise "Primary key not included in the custom select clause" end end end