Angular JS cleanest way to conditionally append string to tag attribute

I have a very newbie question about angularJS:

I have two <a> tag very similar to show their href depends only if the variable foo is set:

if foo == null:

<a ng-href="#/bar/{{col.slug}}">{{col.title}}</a>

else

<a ng-href="#/{{foo}}/bar/{{col.slug}}">{{col.title}}</a>

As you can see the tags are pretty the same except for the foo variable appended at the beginning of the url.

I know there are commands like ng-show and ng-hide, ng-if or ternary operation.

How to make this operation in the cleanest way?

Thanks!