title: MongoDB Filter for both matched and un-matched conditions
body: <p>I have a scenario where I <em>think</em> I might need to use $cond to set an attribute in $project (aggregation) based on whether an existing attribute exists in an array of strings.</p>
  
  <p>Take the following data as a simplified example...</p>
  
  <pre><code>var teamMemberFilter = ['Dave', 'Kate'];
  
  [
      {
          ticketId: '1',
          ticketDesc: 'Dummy ticket 1',
          assignments: [
              {
                  asignee: 'Team',
                  email: 'supportinbox@mysite.com'
              },
              {
                  asignee: 'Dave',
                  email: 'dave@mysite.com'
              },
              {
                  asignee: 'Kate',
                  email: 'kate@mysite.com'
              }
          ]
      },
      {
          ticketId: '2',
          ticketDesc: 'Dummy ticket 2',
          assignments: [
              {
                  asignee: 'Team',
                  email: 'supportinbox@mysite.com'
              },
              {
                  asignee: 'Rob',
                  email: 'rob@mysite.com'
              }
          ]
      },
      {
          ticketId: '3',
          ticketDesc: 'Dummy ticket 3',
          assignments: [
              {
                  asignee: 'Team',
                  email: 'supportinbox@mysite.com'
              },
              {
                  asignee: 'Dave',
                  email: 'dave@mysite.com'
              },
              {
                  asignee: 'Mark',
                  email: 'mark@mysite.com'
              }
          ]
      }
  ]
  </code></pre>
  
  <p>I'd like a query that returns all tickets with a team assignment and does not have an assignment for a team member.</p>
  
  <p>In the above example, only ticket 2 should be returned as it has a team assignment, and the only other assignment in the same ticket belongs to Rob, which is not part of that team (as specified in teamMemberFilter).</p>