user.js

  var limitsByDate = [
          // event starts into current period
          {
              start: {
                  $gt: start
                , $lt: end
              }
          }

          // event ends into current period
        , {
              end: {
                  $gt: start
                , $lt: end
              }
          }

          // event not starts and not ends into current period
        , {
              start: {
                  $lt: start
              }

            , end: {
                  $gt: end
              }
          }

      ]

    , belongTo = [
          // events belongs to classroom calendar
          {
              _classroom: {
                  $in: this.allowedClassrooms
              }
          }

          // eventos belongs to current user
        , {
              _user: this.id
          }
      ]
      
    , ors = [];

    // mixing OR conditions
    for (var k = 0; k < belongTo.length; k++)
      for (var l = 0; l < limitsByDate.length; l++) {

        // cloning belongTo[k] object
        var belong = {};
        for (var m in belongTo[k])
          belong[m] = belongTo[k][m];

        ors.push (object.merge(belong, limitsByDate[l]));
      }

    console.log("-- ors ----------------------------------------------------");
    console.log(ors);
    console.log("---------------------------------------------------- ors --");

  return this.model('Calevent').find({ $or: ors }, fn);

};