How to confirm if update succeeds using mongoose and bluebird promise

I'm using bluebird and mongoose for a node page. I want to check if the update is successful before sending data back to clients via socket.js.Here's the part of the code that I can't figure out:

.then(function(a){     
          var g = collection3.update({_id:a.one[0]._id},{$set:{avg:a.one[0].avg}}).function(err,d){
            if(!err){
              return 1; // Here's the problem
            }
          })
          return {updated:g,info:a};
       }).then(function(c){
          console.log(c.updated); // I can't get the `1` value
          if(c == 1){
            io.sockets.in('index|1').emit("estimate",c.three);
          }
       })

Does mongoose return a success message after update? I can't return 1 from the update query and pass it to the next then function, instead, I'm getting this object:

{ _mongooseOptions: {},
  mongooseCollection:
   { collection:
      { db: [Object],
        collectionName: 'table',
        internalHint: null,
        opts: {},
        slaveOk: false,
        serializeFunctions: false,
        raw: false,
        pkFactory: [Object],
        serverCapabilities: undefined },
     opts: { bufferCommands: true, capped: false },
     name: 'table',
     conn:....

Here's the full code:

  socket.on("input",function(d){ 
    Promise.props({
       one: collection2.aggregate([
        {
         $match:{post_id:mongoose.Types.ObjectId(d.id)}
        },
        {
         $group:{
                 _id:"$post_id",
                 avg:{$avg:"$rating"}
                }
        }
       ]).exec();
   }).then(function(a){     
      var g = collection3.update({_id:a.one[0]._id},{$set:{avg:a.one[0].avg}}).function(err,d){
        if(!err){
          return 1; // Here's the problem
        }
      })
      return {updated:g,info:a};
   }).then(function(c){
      console.log(c.updated); // I can't get the `1` value
      if(c.updated == 1){
        io.sockets.in('index|1').emit("estimate",c.three);
      }
   }).catch(function (error) {
     console.log(error);
   })