Question: Just to help other developers, because there is no similar question on SO. 

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

```

Answer: 
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(' '))

```
If you have multiple conditions I am doing right now something like

```
div class=(('foo ' if is_foo?) + ('bar' if is_bar?))

```

Though I feel it to be a blemish if is\_bar? return false and the generated HTML results in

```
<div class="foo "></div>

```

(the blemish is the blank character after the `foo`). If someone had a solution for that would be awesome.