FooModel.pluck(:hoge_id, :id)
# => [[1, 1], [1, 2], [2, 3], [3, 4], [3, 5]]

# ↑これを↓こんな風にhoge_id毎にまとめたい

# => {1 => [1, 2], 2 => [3], 3 => [4, 5]}