Dynamic Validations using CRUD Listeners

---

Introduction

In this document, learn how we can use DB CRUD listeners to perform dynamic validations. The WaveMaker runtime framework publishes pre and post-events for each CRUD operation (Create, Read, Update and Delete) performed on the table. Here, in the following example, we use the pre-create event to check if the username already exists in the table. For this, we use the HRDB USER table to demonstrate.

note

Configure an HRDB database in the project from WaveMaker sample databases.

Create a Java Service

Create a Java Service in the project and add the preCreate event listener for the user entity, as shown below.

• The beforeUserCreate method gets invoked before a new user is created.
• We can get the user object inside the listener by using the expression entityPreCreateEvent.getEntity().
• In the next lines of code, we are then using UserService to check if the username is present in the table or not. If the user is already present, we throw an error message saying "Username Not Available".

package com.sample.myjavaservice;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.event.EventListener;

import com.sample.hrdb.User;
import com.sample.hrdb.service.UserService;
import com.wavemaker.commons.WMRuntimeException;
import com.wavemaker.runtime.data.event.EntityPreCreateEvent;
import com.wavemaker.commons.MessageResource;

@ExposeToClient
public class MyJavaService {

    private static final Logger logger = LoggerFactory.getLogger(MyJavaService.class);
    
    @Autowired
    private UserService userService;
    
    @EventListener
    public void beforeUserCreate(EntityPreCreateEvent<User> entityPreCreateEvent){
    String inputUsername = entityPreCreateEvent.getEntity().getUsername();
    logger.info("Inside [EntityPreCreateEvent], username {}",inputUsername);
    String query="username='"+inputUsername+"'";
    if(userService.count(query) > 0) {
        throw new WMRuntimeException(MessageResource.create("Username Not Available"));
    } } 
    
    }

Create a Data Table

1. Drag and drop a Data Table widget.
2. Bind the Data Table to the User CRUD variable.
3. Insert a new record with an existing username For example, admin, and submit.
   • Once submitting the record, see that an error is shown "Username not available", see the image below.

See Also

Database CRUD Operations Event Listeners
Record Transactional History of an Entity
Custom Business Logic using DB CRUD Event Listeners