In my angular, I define a scope variable $scope.letter_content. When the view is loaded, I load string from my database and set it to $scope.letter_conten
can you ever heard of angular's dot notation?
put letter_content in an object. froalaDetails.letter_content for example. then update that value and everything should work exactly like you want.
watch this very short video to get the full details: https://egghead.io/lessons/angularjs-the-dot