title: Accessing controller's scope from Formly expressions
body: <p>What is the correct way to access controller's scope (i.e. <code>$scope</code>) in <a href="http://docs.angular-formly.com/v6.4.0/docs/formly-expressions" rel="nofollow">Formly's expressions</a>?</p>
  
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  <p>My use case:</p>
  
  <p>I have a property, e.g. <code>$scope.showEmailField = false</code> and I want to have my Formly field to be hidden when this property is set to <code>false</code>. I've tried to use <code>hideExpression: 'showEmailField'</code>, but it looks like Formly creates it's own child scope. The only way I found is to use <code>$parent.showEmailField</code> expression, but it's not a good practice and expression is not tracked correctly that way. I have had to move <code>showEmailField</code> to the model object and use the following expression: <code>model.showEmailField</code>. That works OK, but it is somewhat ugly workaround.</p>
  
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  <p>I've re-read the documentation on expressions, field configurations and other things several times, but I couldn't grasp the concept of how all this context and scopes work with Formly. Maybe we need a good explanatory article on this subject.</p>