title: Merge value from one array of JSON into corresponding JSON of another array based on key
body: <p>I have two arrays of JSON objects. One with contractor details and another with projects. Both have a common field user_id.</p>
  
  <p>I want a resultant array of objects such that each contractor also has an array of his active projects. Example:</p>
  
  <pre><code>arr1 = [{
      name: 'Contractor A',
      user_id: 3,
      mobile_number: '9999999999',
      active_projects: []
  },
  {
      name: 'Contractor B',
      user_id: 6,
      mobile_number: '9999999999',
      active_projects: []
  }]
  
  arr2 = [{
      user_id: 3, project_name: 'Project A'
  },
  {
      user_id: 3, project_name: 'Project B'
  },
  {
      user_id: 6, project_name: 'Project C'
  }]
  
  </code></pre>
  
  <p>The final array should be:</p>
  
  <pre><code>arr1 = [{
      name: 'Contractor A',
      user_id: 3,
      mobile_number: '9999999999',
      active_projects: ['Project A', 'Project B']
  },
  {
      name: 'Contractor B',
      user_id: 6,
      mobile_number: '9999999999',
      active_projects: ['Project C']
  }]
  </code></pre>
  
  <p>What is the best/cleanest way to achieve this?</p>