I'm executing an active record command Product.pluck(:category_id, :price), which returns an array of 2 element arrays:
[
[1, 500],
[1, 100],
[2, 300]
]
I want to group on the basis of the first element, creating a hash that looks like:
{1 => [500, 100], 2 => [300]}
group_by seems logical, but replicates the entire array. I.e. a.group_by(&:first) produces:
{1=>[[1, 500], [1, 100]], 2=>[[2, 300]]}
You can do a secondary transform to it:
Hash[
array.group_by(&:first).collect do |key, values|
[ key, values.collect { |v| v[1] } ]
end
]
Alternatively just map out the logic directly:
array.each_with_object({ }) do |item, result|
(result[item[0]] ||= [ ]) << item[1]
end
This one-liner seemed to work for me.
array.group_by(&:first).map { |k, v| [k, v.each(&:shift)] }.to_h
Since you're grouping by the first element, just remove it with shift and turn the result into a hash:
array.group_by(&:first).map do |key, value|
value = value.flat_map { |x| x.shift; x }
[key, value]
end #=> {1=>[500, 100], 2=>[300]}
I do not like the destructive operation.
array.group_by(&:first).map { |id, a| [id, a.map(&:last)] }.to_h
Used this functionality several times in my app, added extension to an array:
# config/initializers/array_ext.rb
class Array
# given an array of two-element arrays groups second element by first element, eg:
# [[1, 2], [1, 3], [2, 4]].group_second_by_first #=> {1 => [2, 3], 2 => [4]}
def group_second_by_first
each_with_object({}) { |(first, second), h| (h[first] ||= []) << second }
end
end
