使用nil键和数组值对哈希排序

Question

I have a hash like this (it is the result of the group_by method):

{nil => [#<ActiveRecord id ..., ...], #<ActiveRecord ...> => [#<ActiveRecord id ..., ...], ...}

I need to sort it so that the nil elements would be the first, and then all the other ActiveRecord objects. How can I do this? Thanx

PS

1. Yes, I need an ActiveRecord objects as the keys (not symbols or some else)
2. I can't do the order in my DB due to complex SQL request.

I need only to sort the hash by the keys.

Answer 1

You cannot sort a hash, but :

Hashes enumerate their values in the order that the corresponding keys were inserted.

To get a specific key at the beginning, just make sure to insert it first. Here's an example:

array = [Integer, Range, BasicObject]

hash = array.group_by(&:superclass)
#=> {Numeric=>[Integer], Object=>[Range], nil=>[BasicObject]}

To get nil first, create a hash with a nil key and merge! the new values:

hash = {nil => nil}
hash.merge!(array.group_by(&:superclass))
#=> {nil=>[BasicObject], Numeric=>[Integer], Object=>[Range]}

Answer 2

Assuming you have your hash in h :

Hash[h.sort { |a,b| 
  NilClass === a.first ? 
      (NilClass === b.first ? 0 : -1) : 
      (NilClass === b.first ? 1 : a.first <=> b.first) 
}]

Here we explicitly define sort function, which will place nil s in front, following by native ordering of other elements by keys ( ActiveRecord instances in your case.)

Sidenote: you always can do sorting in database.