Conditional key/value in a ruby hash

Is there a nice (one line) way of writing a hash in ruby with some entry only there if a condition is fulfilled? I thought of

{:a => 'a', :b => ('b' if condition)}

But that leaves :b == nil if the condition is not fulfilled. I realize this could be done easily in two lines or so, but it would be much nicer in one line (e.g. when passing the hash to a function).

Am I missing (yet) another one of ruby's amazing features here? ;)

UPDATE Ruby 2.4+

Since ruby 2.4.0, you can use the compact method:

{ a: 'a', b: ('b' if cond) }.compact

Original answer (Ruby 1.9.2)

You could first create the hash with key => nil for when the condition is not met, and then delete those pairs where the value is nil. For example:

{ :a => 'a', :b => ('b' if cond) }.delete_if{ |k,v| v.nil? }

yields, for cond == true:

{:b=>"b", :a=>"a"}

and for cond == false

{:a=>"a"} 

UPDATE for ruby 1.9.3

This is equivalent - a bit more concise and in ruby 1.9.3 notation:

{ a: 'a', b: ('b' if cond) }.reject{ |k,v| v.nil? }

From Ruby 1.9+, if you want to build a hash based on conditionals you can use tap, which is my new favourite thing. This breaks it onto multiple lines but is more readable IMHO:

{}.tap do |my_hash| 
  my_hash[:a] = 'a'
  my_hash[:b] = 'b' if condition
end

>= Ruby 2.4:

{a: 'asd', b: nil}.compact
=> {:a=>"asd"}