Building a hash in a conditional way

I am using Ruby on Rails 3.0.10 and I would like to build an hash key\value pairs in a conditional way. That is, I would like to add a key and its related value if a condition is matched:

hash = {  :key1 => value1,  :key2 => value2, # This key2\value2 pair should be added only 'if condition' is 'true'  :key3 => value3,  ...}

How can I do that and keep a "good" readability for the code? Am I "forced" to use the merge method?