module ActiveRecord::Batches

# File activerecord/lib/active_record/relation/batches.rb, line 67
def find_each(start: nil, finish: nil, batch_size: 1000, error_on_ignore: nil)
  if block_given?
    find_in_batches(start: start, finish: finish, batch_size: batch_size, error_on_ignore: error_on_ignore) do |records|
      records.each { |record| yield record }
    end
  else
    enum_for(:find_each, start: start, finish: finish, batch_size: batch_size, error_on_ignore: error_on_ignore) do
      relation = self
      apply_limits(relation, start, finish).size
    end
  end
end

Looping through a collection of records from the database (using the ActiveRecord::Scoping::Named::ClassMethods#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.find_each do |person|
  person.do_awesome_stuff
end

Person.where("age > 21").find_each do |person|
  person.party_all_night!
end

If you do not provide a block to find_each, it will return an Enumerator for chaining with other methods:

Person.find_each.with_index do |person, index|
  person.award_trophy(index + 1)
end

Options

• :batch_size - Specifies the size of the batch. Defaults to 1000.

• :start - Specifies the primary key value to start from, inclusive of the value.

• :finish - Specifies the primary key value to end at, inclusive of the value.

• :error_on_ignore - Overrides the application config to specify if an error should be raised when an order is present in the relation.

Limits are honored, and if present there is no requirement for the batch size: it can be less than, equal to, or greater than the limit.

The options start and finish are especially useful if you want multiple workers dealing with the same processing queue. You can make worker 1 handle all the records between id 1 and 9999 and worker 2 handle from 10000 and beyond by setting the :start and :finish option on each worker.

# In worker 1, let's process until 9999 records.
Person.find_each(finish: 9_999) do |person|
  person.party_all_night!
end

# In worker 2, let's process from record 10_000 and onwards.
Person.find_each(start: 10_000) do |person|
  person.party_all_night!
end

NOTE: 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 when the primary key is orderable (e.g. an integer or string).

NOTE: By its nature, batch processing is subject to race conditions if other processes are modifying the database.

# File activerecord/lib/active_record/relation/batches.rb, line 126
def find_in_batches(start: nil, finish: nil, batch_size: 1000, error_on_ignore: nil)
  relation = self
  unless block_given?
    return to_enum(:find_in_batches, start: start, finish: finish, batch_size: batch_size, error_on_ignore: error_on_ignore) do
      total = apply_limits(relation, start, finish).size
      (total - 1).div(batch_size) + 1
    end
  end

  in_batches(of: batch_size, start: start, finish: finish, load: true, error_on_ignore: error_on_ignore) do |batch|
    yield batch.to_a
  end
end

Yields each batch of records that was found by the find options as an array.

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

If you do not provide a block to find_in_batches, it will return an Enumerator for chaining with other methods:

Person.find_in_batches.with_index do |group, batch|
  puts "Processing group ##{batch}"
  group.each(&:recover_from_last_night!)
end

To be yielded each record one by one, use find_each instead.

Options

• :batch_size - Specifies the size of the batch. Defaults to 1000.

• :start - Specifies the primary key value to start from, inclusive of the value.

• :finish - Specifies the primary key value to end at, inclusive of the value.

• :error_on_ignore - Overrides the application config to specify if an error should be raised when an order is present in the relation.

Limits are honored, and if present there is no requirement for the batch size: it can be less than, equal to, or greater than the limit.

The options start and finish are especially useful if you want multiple workers dealing with the same processing queue. You can make worker 1 handle all the records between id 1 and 9999 and worker 2 handle from 10000 and beyond by setting the :start and :finish option on each worker.

# Let's process from record 10_000 on.
Person.find_in_batches(start: 10_000) do |group|
  group.each { |person| person.party_all_night! }
end

NOTE: 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 when the primary key is orderable (e.g. an integer or string).

NOTE: By its nature, batch processing is subject to race conditions if other processes are modifying the database.

# File activerecord/lib/active_record/relation/batches.rb, line 201
def in_batches(of: 1000, start: nil, finish: nil, load: false, error_on_ignore: nil)
  relation = self
  unless block_given?
    return BatchEnumerator.new(of: of, start: start, finish: finish, relation: self)
  end

  if arel.orders.present?
    act_on_ignored_order(error_on_ignore)
  end

  batch_limit = of
  if limit_value
    remaining   = limit_value
    batch_limit = remaining if remaining < batch_limit
  end

  relation = relation.reorder(batch_order).limit(batch_limit)
  relation = apply_limits(relation, start, finish)
  relation.skip_query_cache! # Retaining the results in the query cache would undermine the point of batching
  batch_relation = relation

  loop do
    if load
      records = batch_relation.records
      ids = records.map(&:id)
      yielded_relation = where(primary_key => ids)
      yielded_relation.load_records(records)
    else
      ids = batch_relation.pluck(primary_key)
      yielded_relation = where(primary_key => ids)
    end

    break if ids.empty?

    primary_key_offset = ids.last
    raise ArgumentError.new("Primary key not included in the custom select clause") unless primary_key_offset

    yield yielded_relation

    break if ids.length < batch_limit

    if limit_value
      remaining -= ids.length

      if remaining == 0
        # Saves a useless iteration when the limit is a multiple of the
        # batch size.
        break
      elsif remaining < batch_limit
        relation = relation.limit(remaining)
      end
    end

    batch_relation = relation.where(
      bind_attribute(primary_key, primary_key_offset) { |attr, bind| attr.gt(bind) }
    )
  end
end

Yields ActiveRecord::Relation objects to work with a batch of records.

Person.where("age > 21").in_batches do |relation|
  relation.delete_all
  sleep(10) # Throttle the delete queries
end

If you do not provide a block to in_batches, it will return a BatchEnumerator which is enumerable.

Person.in_batches.each_with_index do |relation, batch_index|
  puts "Processing relation ##{batch_index}"
  relation.delete_all
end

Examples of calling methods on the returned BatchEnumerator object:

Person.in_batches.delete_all
Person.in_batches.update_all(awesome: true)
Person.in_batches.each_record(&:party_all_night!)

Options

• :of - Specifies the size of the batch. Defaults to 1000.

• :load - Specifies if the relation should be loaded. Defaults to false.

• :start - Specifies the primary key value to start from, inclusive of the value.

• :finish - Specifies the primary key value to end at, inclusive of the value.

• :error_on_ignore - Overrides the application config to specify if an error should be raised when an order is present in the relation.

Limits are honored, and if present there is no requirement for the batch size, it can be less than, equal, or greater than the limit.

The options start and finish are especially useful if you want multiple workers dealing with the same processing queue. You can make worker 1 handle all the records between id 1 and 9999 and worker 2 handle from 10000 and beyond by setting the :start and :finish option on each worker.

# Let's process from record 10_000 on.
Person.in_batches(start: 10_000).update_all(awesome: true)

An example of calling where query method on the relation:

Person.in_batches.each do |relation|
  relation.update_all('age = age + 1')
  relation.where('age > 21').update_all(should_party: true)
  relation.where('age <= 21').delete_all
end

NOTE: If you are going to iterate through each record, you should call each_record on the yielded BatchEnumerator:

Person.in_batches.each_record(&:party_all_night!)

NOTE: 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 consistent. Therefore the primary key must be orderable, e.g. an integer or a string.

NOTE: By its nature, batch processing is subject to race conditions if other processes are modifying the database.