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I am new to nestjs and was trying to convert my backend from nodejs to nestjs. Hope that makes sense? I am using `typeorm. But I am not sure as what could be the best of way of catching errors.

entity.ts

import { Entity, Column, PrimaryGeneratedColumn, PrimaryColumn } from 'typeorm';

@Entity()
export class Course {

  @PrimaryColumn()
  course: string;

  @Column("varchar", { array: true })
  subject: string[];
}

controller.ts

import { Controller, Get, Post, Body } from '@nestjs/common';
import { CourseService } from './course.service';
import { Course } from './course.entity';

@Controller('course')
export class CourseController {
    constructor(private courseService: CourseService) {}

    @Get()
    getCourses(): Promise<Course[]> {
        return this.courseService.findAll();
    }

    @Post()
    addCourse(@Body() courseDto: Course[]) {
        return this.courseService.create(courseDto);
    }
}

service.ts

import { Injectable, Catch, ExceptionFilter, ArgumentsHost, ConflictException } from '@nestjs/common';
import { InjectRepository } from '@nestjs/typeorm';
import { Repository, QueryFailedError } from 'typeorm';
import { Course } from './course.entity';

@Injectable()
export class CourseService {
    constructor(
        @InjectRepository(Course)
        private courseRepository: Repository<Course>,
    ) { }

    catch(exception: any, host: ArgumentsHost) {
        throw new Error("Error in course service." + exception.code);
    }

    findAll(): Promise<Course[]> {
        return this.courseRepository.find();
    }

    create(courseDto) {
        return this.courseRepository.insert(courseDto)
            .catch((err: any) => {
                // throw new ConflictException();

                switch (err.name) {
                    case 'QueryFailedError':
                        console.log("**++**" + JSON.stringify(err));
                        // throw new Error("Error" + err.message + "" + err.detail);
                        // throw new ConflictException();
                        throw JSON.stringify(err); //"Error creating a course" + err.message + "::: " + err.detail;
                    default:
                        throw err; 

                }
            });
    }
}

Now, all I am able to throw is throw new ConflictException();. I wanted to throw different errors based on the result, like - 1. For duplicate record 2. Missing mandatory fields 3. etc

But not sure how can we handle and customize the same and also make full use of nestjs.

Like I see the below trace in console but 500, Internal server in postman -

{"message":"duplicate key value violates unique constraint \"PK_d7fc152bc721b3f55a56ed3ad33\"","name":"QueryFailedError","length":293,"severity":"ERROR","code":"23505","detail":"Key (course)=(II) already exists.","schema":"public","table":"course","constraint":"PK_d7fc152bc721b3f55a56ed3ad33","file":"d:\\pginstaller.auto\\postgres.windows-x64\\src\\backend\\access\\nbtree\\nbtinsert.c","line":"535","routine":"_bt_check_unique","query":"INSERT INTO \"course\"(\"course\", \"subject\") VALUES ($1, $2)","parameters":["II",["A","B","C"]]}
[Nest] 12152   - 04/10/2020, 1:18:40 PM   [ExceptionsHandler] {"message":"duplicate key value violates unique constraint \"PK_d7fc152bc721b3f55a56ed3ad33\"","name":"QueryFailedError","length":293,"severity":"ERROR","code":"23505","detail":"Key (course)=(II) already exists.","schema":"public","table":"course","constraint":"PK_d7fc152bc721b3f55a56ed3ad33","file":"d:\\pginstaller.auto\\postgres.windows-x64\\src\\backend\\access\\nbtree\\nbtinsert.c","line":"535","routine":"_bt_check_unique","query":"INSERT INTO \"course\"(\"course\", \"subject\") VALUES ($1, $2)","parameters":["II",["A","B","C"]]} +190924ms

Answers

How about passing the errors to your controller and let the controller throw those errors.

service.ts

create(courseDto) {
  return this.courseRepository.insert(courseDto)
}

and in your controller.ts

import {
  Controller,
  Get,
  Post,
  HttpException,
  HttpStatus,
} from '@nestjs/common';

...

@Post()
async addCourse(@Body() courseDto: Course[]) {
    return await this.courseService.create(courseDto).catch(err => {
      throw new HttpException({
        message: err.message
      }, HttpStatus.BAD_REQUEST);
    })
}

https://docs.nestjs.com/exception-filters