Column_names being deleted when deleting from array

Question

If I do:

name_array = Model.column_names
name_array.delete_if {|name| ["id","created_at","updated_at"].include?(name)}
pp Model.column_names

The Model.column_names command will return an array without the id, created_at & updated_at column names. It was my presumption that the name_array would have the names deleted and not the Model.column_names array.

Rails 3.0.13 Ruby 1.9.3

Here is a similar question with an answer: http://stackoverflow.com/questions/2635156/object-assignment-and-pointers — iouri, Oct 11 '12 at 04:15
iouri Your comment was what I needed. If you want to make it an answer I will accept. — ScottJShea, Oct 11 '12 at 15:15
If you accept the answer of that link that would be ok with me. — iouri, Oct 11 '12 at 22:06

score 5 · Accepted Answer · answered Oct 11 '12 at 04:01

See the Array documentation for delete_if:

Deletes every element of self for which block evaluates to true. The array is changed instantly every time the block is called and not after the iteration is over.

So when you delete elements from name_array, you are actually deleting them from Model.column_names since they're pointing to the same thing.

If you don't want to do that, duplicate the array before deleting:

name_array = Model.column_names.dup
name_array.delete_if {|name| ["id","created_at","updated_at"].include?(name)}

Thank you. Between you and iouri's link it cleared up my issues. — ScottJShea, Oct 11 '12 at 22:10

score 2 · Answer 2 · answered Oct 11 '12 at 04:09

2

u can also use reject

a = [1,2,3]
a.reject{|x| x>=2}
=> [1]

answered Oct 11 '12 at 04:09

Prasad Surase

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score 2 · Answer 3 · answered Oct 11 '12 at 05:23

Dup can help, but there is an restruction. The fact that dup creates 'shallow copy', it has the following non-obvious consequences:

> arr = Model.column_names.dup
=> ["id", "name", "klasse", "cars_count", "klasse_id"] 
> arr.delete('id')
=> "id" 
> arr
=> ["name", "klasse", "cars_count", "klasse_id"] 
> Model.column_names
=> ["id", "name", "klasse", "cars_count", "klasse_id"] 
> arr.each &:upcase!
=> ["NAME", "KLASSE", "CARS_COUNT", "KLASSE_ID"] 
> Model.column_names
=> ["id", "NAME", "KLASSE", "CARS_COUNT", "KLASSE_ID"]

Column_names being deleted when deleting from array

3 Answers3