Ruby Quicktips

Merge or reverse_merge a Hash

Ruby’s Hash#merge lets you merge two Hashes. Duplicate entries in the merged Hash taking precedence over then ones in the calling Hash:

h1 = { "a" => 100, "b" => 200 }
h2 = { "b" => 254, "c" => 300 }
h1.merge(h2)   #=> {"a"=>100, "b"=>254, "c"=>300}
h1             #=> {"a"=>100, "b"=>200}

Rails’ Hash#reverse_merge takes the opposite approach when it comes to handle duplicate entries. The API docs perfectly describe what it’s good for, giving an example of a very common practise regarding a method’s options:

Allows for reverse merging two hashes where the keys in the calling hash take precedence over those in the other_hash. This is particularly useful for initializing an option hash with default values:

def setup(options = {})
  options.reverse_merge! :size => 25, :velocity => 10
end

Using merge, the above example would look as follows:

def setup(options = {})
  { :size => 25, :velocity => 10 }.merge(options)
end

The default :size and :velocity are only set if the options hash passed in doesn’t already have the respective key.

Both methods are also available in their Bang versions merge! and reverse_merge!.

Here are the docs: Hash#merge (Ruby) and Hash#reverse_merge (Rails).

All code examples taken from the respective API docs.