The #tally method currently does this:

class Band
  include Mongoid::Document
  field :genres, type: Array
end

Band.create!(genres: [1, 2])
Band.create!(genres: [1, 2])
Band.create!(genres: [1, 3])

Band.tally(:genres)
  #=> { [1, 2] => 2,
        [1, 3] => 1 }

This is perhaps useful in some cases, but usually you'd want to see Array values split out. I'd like to add this:

Band.tally(:genres, splat_arrays: true)
  #=> { 1 => 3,
        2 => 2,
        3 => 1 }

The change can be done with low complexity.

The default value of :splat_arrays would be false, preserving the existing behavior.

I may call this option "unwind" instead because that's the MongoDB operator name that does this. TBD.