slim dynamic conditional class

I use array of classes and nil element if there is no need to include class in list, then compact array to remove nil elements and finally join all together.

div class=(["cday", "col-md-1", day.day == 1 ? "col-md-offset-#{day.cwday-1}" : nil].compact.join(' '))

See the examples below:

div class=(is_active? ? 'active' : 'inactive')
div class=('active' if is_active?)

The same approach can be used to assign dynamic values to other attributes.