how to sort array in ruby if i have nil object inside array?

Here's a nice short way to do it, but it will only work if you can come up with a string that's guaranteed to be "greater than" all the strings in the array (which was easy here):

my_array.sort_by { |x| x || "Z" }
=> ["12", "12 months", "13 months", nil, nil]

You can compare a 2 dimensions array: whether string is nil & the string itself.

my_array = ["12 months", "13 months", nil, nil, "12"]

my_array.sort_by { |x| [x ? 0 : 1, x] }
=> ["12", "12 months", "13 months", nil, nil]

Create a Array#sort_with_nils method.

If you need this more than once or twice, it could be useful to add an initializer and add a #sort_with_nils method to Array. Something like this:

config/initializers/array.rb

def sort_with_nils( nils_first: true )
  nils = [ nil ] * ( self.tally[ nil ] || 0 ) # Counts the number of `nil` values and create an Array of that number of `nil` values. We will add this to the beginning or the end of the Array after sorting.

  if nils_first
    nils + ( self - [ nil ] ).sort
  else
    ( self - [ nil ] ).sort + nils
  end
end

An example:

myarray = [ 1, 2, nil, 4, nil ]
#=> [1, 2, nil, 4, nil]
myarray.sort_with_nils
#=> [nil, nil, 1, 2, 4]
myarray
#=> [1, 2, nil, 4, nil]
myarray.sort_with_nils( nils_first: false )
#=> [1, 2, 4, nil, nil]
myarray
#=> [1, 2, nil, 4, nil]

What I like about this is that:

1. It doesn't matter if your Array contains Strings or Integers or anything else so you don't need to use arbitrary values to assign to nil to sort them last or first.

2. It uses the standard .sort method so nothing is overwritten and leverages the power already there.

3. You can sort nil values first (default) or last (by passing nils_first: false param).

4. It retains the number of nil values, if that's important, and then you can use .uniq to reduce it to a single nil value if you like.

5. The operation is not in-place and so it doesn't affect the original Array object.