add index

Author: BenjaminChenTW

models/DB/messageDB.js

@@ -25,7 +25,7 @@ var schema = mongoose.Schema({
     logTime: { type: Date, default: Date.now }
 });
 
-schema.index({ "logTime": -1 });
+schema.index({ "logTime": -1, "img.id": 1 });
 
 // create the model for users and expose it to our app
 module.exports = mongoose.model('Message', schema);