Question

I’m getting strange behavior when trying to select from a hash created with group_by:

When I run

all_records.group_by(&:opportunity).map{|foo| foo[1].length != 1 }.select{|x| x}

I get some elements back: => [true, true]

Yet when I try and select, with the exact block I mapped:

all_records.group_by(&:opportunity).select{|foo| foo[1].length != 1 }

I get no results: => {}

Just as a sanity check, it works as expected when i first convert the hash to an array with sort:

all_records.group_by(&:opportunity).sort.select{|foo| foo[1].length != 1 }.length

Result: => 2

It’s strange to me, because the first result indicates that the hash recognized the foo[1] command perfectly. What’s causing this?

Answer 1

In the first snippet you are doing a Enumerable#map on a hash, with a block that gets a single argument (why don’t you unpack it?), and here you get a pair (as you expected). In the second snippet you are doing a Hash#select again with a single argument (and again you should unpack the key/value), but here you get only the key, not the pair (because of how the method is implemented, check the source code for details).

>> {a: 1, b: 2}.map { |x| p x }
[:a, 1]
[:b, 2]
>> {a: 1, b: 2}.select { |x| p x }
:a
:b

If you go to the docs for Hash#select, you’ll see it explicitly requires unpacked arguments. Conclusion: always unpack the key/value when iterating hashes with any method:

records.group_by(&:opportunity).select { |key, values| values.length > 1 }