i_just_want_to_pluck_from_ast.rb

# this works in Ruby 2.3 / Rails 4.2. Untested in other versions.

# ActiveRecord::Calculations.pluck implementation is essentially this with additional
# relation / scoping helpers (for chaining & type saftey) and caching logic. 
def pluck(relation, *column_names)
  relation.select_values = column_names
  ActiveRecord::Base.connection.select_all(relation.arel).cast_values
end

# this totally sucks though, because if I want to pass Arel::Attributes or use Arel::SelectManager,
# I'm SOL.

# better, maybe?
def exec_to_array(relation)
  ActiveRecord::Base.connection.select_all(relation).cast_values
end