Class: OnlineMigrations::BatchIterator

Inherits:
Object
  • Object
show all
Defined in:
lib/online_migrations/batch_iterator.rb

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(relation) ⇒ BatchIterator

Returns a new instance of BatchIterator.



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# File 'lib/online_migrations/batch_iterator.rb', line 8

def initialize(relation)
  if !relation.is_a?(ActiveRecord::Relation)
    raise ArgumentError, "relation is not an ActiveRecord::Relation"
  end

  @relation = relation
end

Instance Attribute Details

#relation ⇒ Object (readonly)

Returns the value of attribute relation.



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# File 'lib/online_migrations/batch_iterator.rb', line 6

def relation
  @relation
end

Instance Method Details

#each_batch(of: 1000, column: relation.primary_key, start: nil, finish: nil, order: :asc) ⇒ Object 



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# File 'lib/online_migrations/batch_iterator.rb', line 16

def each_batch(of: 1000, column: relation.primary_key, start: nil, finish: nil, order: :asc)
  if ![:asc, :desc].include?(order)
    raise ArgumentError, ":order must be :asc or :desc, got #{order.inspect}"
  end

  relation = apply_limits(self.relation, column, start, finish, order)
  unscopes = Utils.ar_version < 7.1 ? [:includes] : [:includes, :preload, :eager_load]
  base_relation = relation.unscope(*unscopes).reselect(column).reorder(column => order)

  start_id = start || begin
    start_row = base_relation.uncached { base_relation.first }
    start_row[column] if start_row
  end

  arel_table = relation.arel_table

  while start_id
    if order == :asc
      start_cond = arel_table[column].gteq(start_id)
    else
      start_cond = arel_table[column].lteq(start_id)
    end

    last_row, stop_row = base_relation.uncached do
      base_relation
        .where(start_cond)
        .offset(of - 1)
        .first(2)
    end

    if last_row.nil?
      # We are at the end of the table.
      last_row, stop_row = base_relation.uncached do
        base_relation
          .where(start_cond)
          .last(2)
      end
    end

    batch_relation = relation.where(start_cond)

    if stop_row
      stop_id = stop_row[column]

      if order == :asc
        stop_cond = arel_table[column].lt(stop_id)
      else
        stop_cond = arel_table[column].gt(stop_id)
      end

      batch_relation = batch_relation.where(stop_cond)
    end

    # Any ORDER BYs are useless for this relation and can lead to less
    # efficient UPDATE queries, hence we get rid of it.
    batch_relation = batch_relation.except(:order)

    last_id = (last_row && last_row[column]) || finish

    # Retaining the results in the query cache would undermine the point of batching.
    batch_relation.uncached { yield batch_relation, start_id, last_id }

    break if last_id == finish

    start_id = stop_id
    stop_id = nil
  end
end