Develop Reference

How to write a query to get distinct values from mongodb collection?

I need to get distinct values from a collection. Those data is in a field of a collection. It means I need to get set of name from the user collection.I tried it using cmd and I get the result as I need. But I can't understand how to write the query in the spring file.Since I'm new to java I have not enough knowledge how to handle this.
Given below is a image of the database collection
Services.java
package limark.internal.css.services;
import limark.internal.css.core.model.User;
import limark.internal.css.exceptions.ResourceNotFoundException;
import limark.internal.css.persistence.UserRepository;
import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import javax.validation.constraints.NotNull;
import java.lang.reflect.Array;
import java.time.OffsetDateTime;
import java.util.List;
import static limark.internal.css.core.MessageConstants.USER_NOT_FOUND;
#Service
#Slf4j
public class UserService {
private final UserRepository userRepository;
#Autowired
public UserService(final UserRepository userRepository){
this.userRepository = userRepository;
}
/**
* Creates a new User
* #param user object
* #return user object
*/
public User create(User user){
user.setCreatedByLoginId("");
user.setCreatedTs(OffsetDateTime.now());
return userRepository.save(user);
}
/**
* Returns list of users.
*/
public List<User> findUsers(){
return userRepository.findAll();
}
/**
* Find user by id
* #param id userId
* #return user
*/
public User findById(#NotNull String id){
return userRepository.findById(id).orElseThrow(()->new ResourceNotFoundException(USER_NOT_FOUND,
id));
}
public User update(#NotNull User user){
user.setLastUpdatedByLoginId("");
user.setLastUpdatedTs(OffsetDateTime.now());
return userRepository.save(user);
}
/**
* sets a user to not active on delete with the given id
*
* #param userId valid user id
* #throws ResourceNotFoundException when user is not found
*/
public void deleteUser(String userId) {
User user =
userRepository
.findById(userId)
.orElseThrow(() -> new ResourceNotFoundException(USER_NOT_FOUND, userId));
user.setActive(false);
user.setLastUpdatedByLoginId("");
user.setLastUpdatedTs(OffsetDateTime.now());
userRepository.save(user);
log.info("The user {} is deleted successfully.", user.getId());
}
/**
* Returns list of users.
*/
public Array findUsersList(){
return userRepository.distinct( "firstName" );
}
}
I need to add this query inside
/**
* Returns list of users.
*/
public Array findUsersList(){
return userRepository.distinct( "firstName" );
}

You can introduce a method in the UserRepository to retrieve the distinct firstName field values and return a List<String>.
public interface UserRepository extends MongoRepository<User, String> {
#Aggregation(pipeline = { "{ '$group': { '_id' : '$firstName' } }" })
List<String> findDistinctFirstNames();
}
The call to get the list of distinct first names:
List<String> firstNamesDistinct = userRepository.findDistinctFirstNames();
This worked fine using Spring Data MongoDB v2.4 and MongoDB v4.2.

JdbcTemplate queryForMap get() returns null for valid key

A simple JdbcTemplate.queryForMap() call on MySQL table yields this strange explainable behavior. The map output returns null for get() and expected results for keySet() and values().
Edited:
-Posted full class code
- Appended query with RowMapper() implementation.
Here's the table schema:
create table Product(
id int not null primary key auto_increment,
`name` varchar(30),
price decimal(10,2)
);
mysql> select * from Product;
+----+--------+-------+
| id | name | price |
+----+--------+-------+
| 1 | iPhone | 10.00 |
+----+--------+-------+
1 row in set (0.00 sec)
Complete Java class:
package com.test1;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import org.apache.commons.dbcp2.BasicDataSource;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.jdbc.core.RowMapper;
/**
*
* #author user2176499
*
*/
public class Main {
private static final Logger logger = LoggerFactory.getLogger(Main.class);
public static void main(String[] args) throws Exception {
BasicDataSource dataSource = new BasicDataSource();
dataSource.setDriverClassName("com.mysql.cj.jdbc.Driver");
dataSource.setUrl("jdbc:mysql://127.0.0.1:3306/testdb");
dataSource.setUsername("user");
dataSource.setPassword("password");
JdbcTemplate jdbcTemplate = new JdbcTemplate(dataSource);
Map<String, Object> row = jdbcTemplate.queryForMap("select * from Product where id=?", 1);
System.out.println(row.getClass());
System.out.println(row.keySet());
System.out.println(row.values());
System.out.println(row.containsKey("name"));
System.out.println(row.get("name"));
Iterator<String> iterator = row.keySet().iterator();
while (iterator.hasNext()) {
String key = iterator.next();
System.out.println(key + " - " + row.get(key));
}
System.out.println("==================");
List<Map<String, Object>> rows = jdbcTemplate.query("select * from Product where id=?", new Object[] { 1 },
new RowMapper<Map<String, Object>>() {
#Override
public Map<String, Object> mapRow(ResultSet rs, int rowNum) throws SQLException {
return Map.ofEntries(Map.entry("id", rs.getInt("id")), Map.entry("name", rs.getString("name")),
Map.entry("price", rs.getDouble("price")));
}
});
System.out.println(rows);
}
}
Output:
class org.springframework.util.LinkedCaseInsensitiveMap
[id, name, price]
[1, iPhone, 10.00]
false
null
id - null
name - null
price - null
==================
[{name=iPhone, price=10.0, id=1}]
Spring Version: spring-jdbc-5.2.2-release

Why does a #Query annotated method require a body?

I'm new to Spring.
I wrote a class:
import org.springframework.data.jpa.repository.Query;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;
#Controller
public class Kontroller
{
#Query( value="select * from teszt"
, nativeQuery = true
)
List<Teszt> osszesSor(); // <-- error
//.....
#RequestMapping("/szam")
public #ResponseBody String szamossag()
{
List<Teszt> sokasag = osszesSor();
return("számosság="+sokasag.size());
}
}
And it says (it = the IDE (STS), and at runtime when I call osszesSor()):
This method requires a body instead of a semicolon.
The Teszt class is:
import java.io.Serializable;
import javax.persistence.*;
#Entity
#IdClass(Teszt.class)
public class Teszt implements Serializable {
#Id
private String kulcs;
#Id
private String ertek;
//getters, setters
}
It's a very simple table (that's why it becomes very complex in spring, because it has not a one-column key):
create table TESZT
(
KULCS VARCHAR2(2000) not null,
ERTEK VARCHAR2(2000) not null,
constraint TESZT_UN
unique (KULCS, ERTEK)
)

Now I know.
I had to create a repo interface:
public interface TesztRepo extends Repository<Teszt, Teszt>
{
#Query( value="select * from teszt"
, nativeQuery = true
)
List<Teszt> sokasag();
//...
}
and autowire it in the controller.
#Autowired
TesztRepo dao;
//...
List<Teszt> sokasag = dao.sokasag();
Not that complex.

Connecting Spring Boot JDBCTemplate to SQL Server (MSSQL)

I'm very new to Spring Boot and I'm having trouble trying to set my project up so that it can communicate to SQL Server - more specifically, my JDBCTemplate instance variable is null and for some reason isn't being 'autowired' with the datasource I've specified in my application.properties file. These are the steps I've taken so far:
Using STS I created a new Spring Boot project using the Spring Start Project template.
I selected Gradle to by the 'Type' and ticked JDBC.
I then followed the following tutorial to create an abstract interface (DAO) to SQL Server (http://www.tutorialspoint.com/spring/spring_jdbc_example.htm).
If you scroll down the tutorial page to the MainApp.java bit, the first 4 lines of the main method I did not use - because I don't have a beans.xml file. This is where I presume Spring Boot's #Autowired annotation comes in and creates my beans for me?
I downloaded the SQL Server jar file from Microsoft and added it to my project as an external JAR by right clicking on my project -> Build Path -> Configure Build Path -> Add External JARs..'. Doing this removed an error I had which indicated that the driverClassName I specified in my application.properties file couldn't be found.
I'll start by displaying the contents of my 'application.properties' file:
spring.datasource.url=jdbc:sqlserver://localhost:1433;databaseName=testdb
spring.datasource.username=sa
spring.datasource.password=myPassword
spring.datasource.driverClassName=com.microsoft.sqlserver.jdbc.SQLServerConnection
spring.datasource.initialize=true
Below is my 'JDBCTemplate.java' class which contains my CRUD methods:
package demo;
import java.util.List;
import org.apache.tomcat.jdbc.pool.DataSource;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.jdbc.core.JdbcTemplate;
public class BranchJDBCTemplate implements BranchDAO {
private DataSource dataSource;
#Autowired
protected JdbcTemplate jdbcTemplateObject;
#Autowired
#Override
public void setDataSource(DataSource ds) {
this.dataSource = ds;
this.jdbcTemplateObject = new JdbcTemplate(dataSource);
}
#Override
public void create(String name) {
String SQL = "insert into branches (name) values (?)";
jdbcTemplateObject.update(SQL, name);
System.out.println("Created Record Name = " + name);
return;
}
#Override
public Branch getBranch(Integer id) {
String SQL = "select * from branches where id = ?";
Branch student = jdbcTemplateObject.queryForObject(SQL,
new Object[]{id}, new BranchMapper());
return student;
}
#Override
public List<Branch> listBranches() {
String SQL = "select * from branches";
List <Branch> branches = jdbcTemplateObject.query(SQL, new BranchMapper());
return branches;
}
#Override
public void delete(Integer id) {
String SQL = "delete from branches where id = ?";
jdbcTemplateObject.update(SQL, id);
System.out.println("Deleted Record with ID = " + id );
return;
}
#Override
public void update(Integer id, String name) {
String SQL = "update Student set name = ? where id = ?";
jdbcTemplateObject.update(SQL, id);
System.out.println("Updated Record with ID = " + id );
return;
}
}
And finally, here is my 'CustController.java' class which contains the request mapping where I use the JDBCTemplate class to perform a database operation:
package demo;
import java.util.List;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;
#RestController
public class CustController {
#RequestMapping("/customer")
public Cust customer(#RequestParam(value="name", required=false, defaultValue="World") String name) {
BranchJDBCTemplate branchTemplate = new BranchJDBCTemplate();
List<Branch> branchesList = branchTemplate.listBranches();
for (Branch branch : branchesList) {
System.out.print("ID : " + branch.getId());
}
return new Cust(12, "Test", "Test");
}
}
The issue I'm encountering as mentioned previously is that my jdbcTemplateObject instance ...
protected JdbcTemplate jdbcTemplateObject;
is null and therefore throwing an exception on the following line:
List <Branch> branches = jdbcTemplateObject.query(SQL, new BranchMapper());
It isn't being initialised automatically, can anyone point out what I'm doing wrong?
Many thanks!
Tony

You are right, you need to have beans.xml with datasource configured in it.
In CustController class customer() method, you are using new operator as:
BranchJDBCTemplate branchTemplate = new BranchJDBCTemplate();
and so this branchTemplate instance is not spring manged and so datasource is not autowired resulting in null value of jdbctemplate.
Instead use the annotatioan as:
#Repository("branchDao")
public class BranchJDBCTemplate implements BranchDAO {
...
}
and access branchTemplate in CustController as:
#RestController
public class CustController {
#Autowired
#Qualifier("branchDao")
BranchJDBCTemplate branchTemplate;
...
}

try using the following in your application.properties file
spring.datasource.url=jdbc:sqlserver://localhost:1433;databaseName=testdb;integratedSecurity=false;

Hibernate - Auto or Manual ID - On cluster environment

I have an situation where I need to save a hibernate object, but I am not sure whether an ID would be assigned by calling app (using special logic with in a range - unique) or not.
If the ID is not assigned, I need hibernate to generate an ID higher than the possible ID range which the app would input (i know the range). Else should go with what using app inputs.
I am working on MySQL - checking to see if I can custom generator like below
public class MyDOIdGenerator extends IdentityGenerator{
#Override
public Serializable generate(SessionImplementor session, Object obj) throws HibernateException {
if ((((MyDO) obj).getId()) == null) {
Serializable id = super.generate(session, obj) ;
return id;
} else {
return ((MyDO) obj).getId();
}
}
}
But my problem is, I dont know how the super.generate would behave in a clustered environment. Would it maintain the ID synchronization across servers? How do I specify the number to start from? (Because I need to exclude the app using id range when having hibernate generate it)
Please help
Thanks

In the DB, set your sequence to start with a specific number (beginning of the range you mentioned) and then use something like this for your ID annotation, it will ensure hibernate will use the next ID in the sequence and you won't have to worry about clustered/non-clustered environment issue:
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
#Column(name = "id")
private Long id;

I finally implemented this using a table in database as below
package com.yahoo.mb.do;
import java.io.Serializable;
import java.lang.invoke.MethodHandles;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.Calendar;
import org.hibernate.HibernateException;
import org.hibernate.engine.SessionImplementor;
import org.hibernate.id.IdentityGenerator;
import com.yahoo.sm.ads.common.log.Logger;
public class MyDOGenerator extends IdentityGenerator{
private static final Class clazz = MethodHandles.lookup().lookupClass();
private static final Logger logger = Logger.getLogger(clazz);
private static String selectQuery = "SELECT nextval FROM identity_value WHERE name='MY_DO' FOR UPDATE";
private static String updateQuery = "UPDATE identity_value SET nextval=?, last_updated=? WHERE name='MY_DO'";
#Override
public Serializable generate(SessionImplementor session, Object obj) throws HibernateException {
if ((((MyDO) obj).getId()) == null) {
Connection connection = session.connection();
try {
PreparedStatement selectStatement = connection.prepareStatement(selectQuery);
PreparedStatement updateStatement = connection.prepareStatement(updateQuery);
logger.info(clazz, "generate", "Generating nextval");
ResultSet rs = selectStatement.executeQuery();
if (rs.next()) {
Long id = rs.getLong("nextval");
logger.info(clazz, "generate", "Generated nextval: " + id);
updateStatement.setLong(1, id+1);
updateStatement.setDate(2, new java.sql.Date(new java.util.Date().getTime()));
logger.info(clazz, "generate", "Updating nextval: " + id);
updateStatement.executeUpdate();
return id;
}
} catch (SQLException e) {
logger.error(clazz, "generate", "Error generating ID" + e);
throw new HibernateException("Unable to generate MyDO id value");
}
return null;
} else {
return ((MyDO) obj).getId();
}
}
}