When processing large numbers of records, it's often a good idea to do so in batches to prevent memory ballooning.

Methods
F
Instance Public methods
find_each(options = {})

Yields each record that was found by the find options. The find is performed by #find_in_batches with a batch size of 1000 (or as specified by the :batch_size option).

Example:

Person.find_each(:conditions => "age > 21") do |person|
  person.party_all_night!
end

Note: This method is only intended to use for batch processing of large amounts of records that wouldn't fit in memory all at once. If you just need to loop over less than 1000 records, it's probably better just to use the regular find methods.

# File activerecord/lib/active_record/batches.rb, line 24
def find_each(options = {})
  find_in_batches(options) do |records|
    records.each { |record| yield record }
  end
  self
end
find_in_batches(options = {})

Yields each batch of records that was found by the find optionsas an array. The size of each batch is set by the :batch_sizeoption; the default is 1000.

You can control the starting point for the batch processing by supplying the :startoption. 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 :startoption 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 mean that this method only works with integer-based primary keys. You can't set the limit either, that's used to control the the batch sizes.

Example:

Person.find_in_batches(:conditions => "age > 21") do |group|
  sleep(50) # Make sure it doesn't get too crowded in there!
  group.each { |person| person.party_all_night! }
end
# File activerecord/lib/active_record/batches.rb, line 55
def find_in_batches(options = {})
  raise "You can't specify an order, it's forced to be #{batch_order}" if options[:order]
  raise "You can't specify a limit, it's forced to be the batch_size"  if options[:limit]
  start = options.delete(:start).to_i
  batch_size = options.delete(:batch_size) || 1000
  proxy = scoped(options.merge(:order => batch_order, :limit => batch_size))
  records = proxy.find(:all, :conditions => [ "#{table_name}.#{primary_key} >= ?", start ])
  while records.any?
    yield records
    break if records.size < batch_size
    last_value = records.last.id
    raise "You must include the primary key if you define a select" unless last_value.present?
    records = proxy.find(:all, :conditions => [ "#{table_name}.#{primary_key} > ?", last_value ])
  end
end