Question

I seem to run into this very often. I need to build a Hash from an array using an attribute of each object in the array as the key.

Lets say I need a hash of example uses ActiveRecord objecs keyed by their ids Common way:

ary = [collection of ActiveRecord objects] hash = ary.inject({}) {|hash, obj| hash[obj.id] = obj } 

Another Way:

ary = [collection of ActiveRecord objects] hash = Hash[*(ary.map {|obj| [obj.id, obj]}).flatten] 

Dream Way: I could and might create this myself, but is there anything in Ruby or Rails that will this?

ary = [collection of ActiveRecord objects] hash = ary.to_hash &:id #or at least hash = ary.to_hash {|obj| obj.id} 

Answer 1

There is already a method in ActiveSupport that does this.

['an array', 'of active record', 'objects'].index_by(&:id) 

And just for the record, here’s the implementation:

def index_by   inject({}) do |accum, elem|     accum[yield(elem)] = elem     accum   end end 

Which could have been refactored into (if you’re desperate for one-liners):

def index_by   inject({}) {|hash, elem| hash.merge!(yield(elem) => elem) } end