title: How to differentiate when to use angular expression inside any of it's directive?
body: <p>I am confused about when to use expressions and when not to use inside default AngularJS directives such as <strong>ng-src</strong>, <strong>ng-href</strong> and other.</p>
  
  <p>According to my understanding when we use angular directive we have just use scope variable names to bind it's value. Following expression work properly.</p>
  
  <p><code>&lt;link ng-href="{{BASIC_PATH + '/relative-path-url/image.png'}}"/&gt;</code></p>
  
  <p>But consider an case of <strong>ng-model</strong> directive, following example is not valid way to bind variables.</p>
  
  <p><code>&lt;span ng-model="{{BASIC_PATH}}"&gt;&lt;/span&gt;</code></p>
  
  <p>Every time when I have to use angular expressions with directives, I used to write code in both format and then test. 
  So what is the basic fundamental way to use expressions with angular directives.</p>