AngularJS 1.5.x

var app = angular.module('plunker', []);

app.controller('MainCtrl', function($scope) {
  $scope.name = 'World';
  
  $scope.clients = [
    {'client_id' : '1', 'client_name' : 'Anne'}, 
    {'client_id' : '2', 'client_name' : 'Tim'},
    {'client_id' : '3', 'client_name' : 'John'}
    ];
  
  $scope.courses = [
    {'ordered_by' : '1', 'course_name' : 'Marketing'}, 
    {'ordered_by' : '1', 'course_name' : 'Project Management'}, 
    {'ordered_by' : '1', 'course_name' : 'Analytics'}, 
    {'ordered_by' : '2', 'course_name' : 'Big Data'},
    {'ordered_by' : '2', 'course_name' : 'Data Visualization'},
    {'ordered_by' : '3', 'course_name' : 'PHP'}
    ];
  
});

<!DOCTYPE html>
<html ng-app="plunker">

<head>
  <meta charset="utf-8" />
  <title>AngularJS Plunker</title>
  <script>
    document.write('<base href="' + document.location + '" />');
  </script>
  <link rel="stylesheet" href="style.css" />
  <script data-require="angular.js@1.5.x" src="https://code.angularjs.org/1.5.8/angular.js" data-semver="1.5.8"></script>
  <script src="app.js"></script>
</head>

<body ng-controller="MainCtrl">
  <p>Course list:</p>

  <div ng-repeat="person in clients">
    <strong><h2>{{person.client_name}} (ID: {{person.client_id}})</h2></strong>
    <ul>
      
      <!-- I wish it was this simple:
          <li ng-repeat="course in courses | filter : {'ordered_by' : '{{person.client_id}}' }"> -->

      <li ng-repeat="course in courses | filter : {'ordered_by' : '' }">
        {{course.course_name}}<br><span>(Ordered by person with the ID: {{course.ordered_by}})</span>
      </li>
    </ul>

  </div>


</body>

</html>

/* Put your css in here */
span {
  font-size: 0.8em;
  color: silver;
}