Spring MongoDB _id field mapping conventions

Java Spring MongoDB  : '_id' field mapping :

MongoDB requires that you have an '_id' field for all documents. If you don’t provide one the driver will assign a ObjectId with a generated value. The "_id" field can be of any type the, other than arrays.

The following outlines what field will be mapped to the '_id' document field:
  • A field annotated with @Id (org.springframework.data.annotation.Id) will be mapped to the '_id' field.
  • A field without an annotation but named 'id' will be mapped to the '_id' field.
  • The default field name for identifiers is '_id' and can be customized via the @Field annotation.

Examples for the translation of '_id'-field definitions

Field definition Resulting Id-Fieldname in MongoDB
String id
@Field String id
@Field('x') String id
@Id String x
@Field('x') @Id String x

Source :  How the '_id' field is handled in the mapping layer

MongoDB Multiple insert at once

We can insert multiple documents (bulk insert) to a collection using a single insert command :

Syntax :
   db.[COLLECTION].insert( [ { DOC1 }, {DOC2} ] ) ;

Example :
   db.people.insert( [ {name: "Ganesh", age: 24, country : "Nepal"}, {name: "John", age : 20} ]);

In Java  ( Using Spring Data JPA Repository )

   personRepository.insert( personList);
   --> It follows org.springframework.data.mongodb.repository.MongoRepository#insert(Iterable<S> entities) method's syntax

MongoDB rename a database

As of now, there is not a straight forward way to rename a MongoDB database. But you can use the following steps to achieve so :

Here, we are trying to rename OLD_DB to NEW_DB

Steps :
1) copy old database to new one using db.CopyDatabase( OLD_DB, NEW_DB )
2) switching to OLD_DB and dropping the database

Mongo Command :

  #Step 1 : Copy database to new name

  #Step 2 and 3, drop OLD_DB
     use OLD_DB

MongoDB Drop Database

We can use the following query to drop ( DELETE EVERYTHING) from a MongoDB database

( please be careful on running the query, it deletes everything )

   use [database]

MongoDB Like Query

We can do the following to achieve SQL's LIKE equivalent on a Query :

MongoDB :  db.users.find({"name": /jo/})

MYSQL Equivalent:   select * from users where name like '%jo%' 

MongoDB - show all database in server

We can use following commands to view all databases in the server
   show dbs
   show databases

MongoDB - List all collection from selected DB

There are two ways to do so, so far :

1 ) using  reference variable 'db'

2) using show command
    show collections

MongoDB : use database / switch to different database

MongoDB uses same syntax as SQL/MySQL to switch to different database :

MongoDB Syntax :
SQL Syntax :
     use DATABASE_NAME ;

Speech Recognition using HMM and VQ - Code Updates

Hey guys,

I've done some minor changes to the Speech Recognition code that I pushed to github/googlecode couple of years ago. As I received several attention/queries from readers/students all over the world regarding couple of minor bugs within the project, I thought its time ( better late than never :P ) to address them into the code.

Significant changes are on:

- Exception handling
- File handling
- File (recorded WAVE file, VQ codebook, HMM models) save path corrected
- Delta calculation fixed when regression window is less than number of frames ( when a short audio sample is recorded)

I've also uploaded training audio samples for few  words, trained Vector Quantization coebook and trained HMM model file for the words.

If you wish to add/train more words, please use the GUI - "HMM_VQ_Speech_Recognition" class. Just a note, you need to re-train both VQ and HMM for each word that you add.

Code :


Report File (contains both Speech and Speaker recognition)


I hope everyone will benefit from this. Keep coding, keep learning. Please let me know if you see any issues.

Thank you!

Ynaxdh - answer acronym in Yeoman generator

Meaning of Ynaxdh in Yeoman generator

Here the complete list of options/meanings:

  • Y: yes (Default)
  • n: no
  • a: always yes (yes to this question and all others)
  • x: exit
  • d: show the differences between the old and the new file
  • h: help, list all options

Notes :   

  • If you don't remember this definitions you can always enter h and see the list.
  • Also in the new version, when you enter one of the letters you will see the definition showing behind the question so that you can verify your answer before pressing Enter key.
  • The one in uppercase is the default one. If that is your choice, you can just hit enter.