#each_slice and #flat_map: A Perfect Pair

Sat, 23 Jan 2016 00:00:00 +0000

The Enumerable#each_slice method is a great way to break up a collection so you can work on chunks of it at a time.

irb> (1..10).each_slice(2) { |slice| pp slice }
[1, 2]
[3, 4]
[5, 6]
[7, 8]
[9, 10]
 => nil

I find it useful when I need to query a bunch of records from a database with a SQL IN clause. Let’s say you have a table of items, and you want to query order lines for the active items. Like this example:

item_numbers = Item.where(active: true).map(&:item_number)

lines = OrderLine.where(item_number: item_numbers)

# SELECT * FROM order_lines WHERE item_numbers IN ( ... )

Some databases put a limit on how many values you can use on an IN clause. So if your active items exceed that limit, the query will fail. That’s where #each_slice can help.

lines = []

item_numbers.each_slice(100) do |slice|
  lines += OrderLine.where(item_number: slice)
end

Now we’re performing multiple queries with only 100 item numbers at a time. Cool! That fixes our query problem, but this code is kinda ugly. I don’t like creating empty arrays just to build them with a loop unless there’s no other way to avoid it. That’s where Enumerable#flat_map comes to the rescue.

lines = item_numbers.each_slice(100).flat_map do |slice|
  OrderLine.where(item_number: slice)
end

#flat_map is the perfect companion to #each_slice. When #each_slice tears the collection apart, #flat_map is there to put all of the pieces back together.

I particularly enjoy how #each_slice returns an Enumerable if a block isn’t given, which means you can immediately call #flat_map on the return value. The slices are passed to the #flat_map method and then flattened back into a single collection when it’s done. Brilliant!

David Biehl

#each_slice and #flat_map: A Perfect Pair