title: Mongodb : Concat Month and Year
body: <p>I'm using a aggregation to determine the month when a user is connecting to my platform.</p>
  
  <p>For that, I've got this aggregation :</p>
  
  <pre><code> { 
          "$project" : {
            cuid: 1, username: 1, creationDate: 1, language: 1, favorites: 1, active: 1, company: { cuid: 1, name: 1, logoUrl: 1, type: 1 }, connectionLog: 1,
        profile: { $cond: ['$active', '$profile', null] },
              "connectionMonth" : {
                  "$cond" : [
                      {
                          "$ifNull" : [
                              "$connectionLog", 
                              0.0
                          ]
                      }, 
                      {
                           $month: "$connectionLog" 
                      }, 
                      -1.0
                  ]
              }, 
  
          }
      }, 
  </code></pre>
  
  <p>And it's working as expected, only problem, I want to have out connectionMonth output the Month + Year. Something like 07.2019 will be great. I try to concat but it didn't work.</p>