Question

This happens in Ruby on Rails’s View, where there is a hash for another partial. This hash has about 20 key/value pairs.

There is (in HAML)

- if (some_conditon)
  = render :partial => 'some_name', :locals => a_hash.merge({ :extra => true })
- else
  -# a lot more processing, including concatenating the partials and return as json
  - some_var.each do |item| 
    - result_html << (render :partial => 'some_name', :locals => a_hash )
    -# etc
  - response.content_type = "application/json"
  = result_html.to_json

So the question is, should the merge be written as merge! instead? Because it is no longer needed later, if a new hash is created, a lot of time will be spent for creating this new hash (20 items in hash). If an in-place modification is done, it can use the existing hash structure and add one item to it, which will be a lot faster?

Answer 1

If you are certain that modifying the existing hash is not a problem, then you could certainly use merge!. However, I’m not sure if it will be “a lot” faster than simply using merge. copying a hash of 20 or so objects probably isn’t an extremely time consuming operation. But, if it is a concern, you could benchmark the different implementations and see how much you will gain by doing one over the other.