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

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.

Source: show | on GitHub 

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

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

Source: show | on GitHub 

# 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
    raise "Primary key not included in the custom select clause" unless primary_key_offset

    yield records

    break if records_size < batch_size

    records = relation.where(table[primary_key].gt(primary_key_offset)).to_a
  end
end