Fix Error: Cannot determine a type for the "CreateOrderDto.items" property. Make sure you've added the appropriate decorators. in NestJS

This error means NestJS Swagger cannot infer the type metadata for a DTO property because it lacks the required decorators. TypeScript metadata reflection does not preserve generic types like arrays. Fix it by adding @ApiProperty() with the type option and @Type() decorator from class-transformer.

Reading the Stack Trace

Here's what each line means:

• at SchemaObjectFactory.getTypeForArrayProperty (node_modules/@nestjs/swagger/dist/services/schema-object-factory.js:182:19): The Swagger schema factory could not determine the array item type because generic type information is lost at runtime.
• at SwaggerExplorer.exploreRouteModel (node_modules/@nestjs/swagger/dist/swagger-explorer.js:218:33): While exploring route models to build the OpenAPI spec, the explorer encountered a property without type metadata.
• at bootstrap (src/main.ts:18:28): The error occurs during app bootstrap when SwaggerModule.createDocument() scans all DTOs.

Common Causes

export class CreateOrderDto {
  @IsArray()
  items: OrderItemDto[]; // Swagger can't infer OrderItemDto from reflection
}

// tsconfig.json
{
  "compilerOptions": {
    "experimentalDecorators": true
    // Missing: "emitDecoratorMetadata": true
  }
}

export class ConfigDto {
  @ApiProperty()
  settings: Record<string, any>; // Type erased to Object at runtime
}

The Fix

Add @ApiProperty({ type: [OrderItemDto] }) to explicitly tell Swagger the array item type. Add @Type(() => OrderItemDto) from class-transformer so validation pipes can properly instantiate nested objects.

import { IsArray, IsNumber, IsString, ValidateNested } from 'class-validator';

export class OrderItemDto {
  @IsString()
  productId: string;

  @IsNumber()
  quantity: number;
}

export class CreateOrderDto {
  @IsArray()
  @ValidateNested({ each: true })
  items: OrderItemDto[];
}

import { IsArray, IsNumber, IsString, ValidateNested } from 'class-validator';
import { Type } from 'class-transformer';
import { ApiProperty } from '@nestjs/swagger';

export class OrderItemDto {
  @ApiProperty({ example: 'prod-123' })
  @IsString()
  productId: string;

  @ApiProperty({ example: 2 })
  @IsNumber()
  quantity: number;
}

export class CreateOrderDto {
  @ApiProperty({ type: [OrderItemDto] })
  @IsArray()
  @ValidateNested({ each: true })
  @Type(() => OrderItemDto)
  items: OrderItemDto[];
}

Testing the Fix

import { validate } from 'class-validator';
import { plainToInstance } from 'class-transformer';
import { CreateOrderDto, OrderItemDto } from './create-order.dto';

describe('CreateOrderDto', () => {
  it('validates a correct order', async () => {
    const dto = plainToInstance(CreateOrderDto, {
      items: [{ productId: 'prod-1', quantity: 2 }],
    });
    const errors = await validate(dto);
    expect(errors).toHaveLength(0);
  });

  it('rejects missing items', async () => {
    const dto = plainToInstance(CreateOrderDto, {});
    const errors = await validate(dto);
    expect(errors.length).toBeGreaterThan(0);
  });

  it('rejects invalid nested items', async () => {
    const dto = plainToInstance(CreateOrderDto, {
      items: [{ productId: 123, quantity: 'abc' }],
    });
    const errors = await validate(dto);
    expect(errors.length).toBeGreaterThan(0);
  });
});

Run your tests:

npm test

Pushing Through CI/CD

git checkout -b fix/nestjs-swagger-schema-error,git add src/order/dto/create-order.dto.ts src/order/dto/__tests__/create-order.dto.spec.ts,git commit -m "fix: add ApiProperty and Type decorators for Swagger schema generation",git push origin fix/nestjs-swagger-schema-error

Your CI config should look something like this:

name: CI
on:
  pull_request:
    branches: [main]
jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: '20'
          cache: 'npm'
      - run: npm ci
      - run: npm test -- --coverage
      - run: npm run build

The Full Manual Process: 18 Steps

Here's every step you just went through to fix this one bug:

1. Notice the error alert or see it in your monitoring tool
2. Open the error dashboard and read the stack trace
3. Identify the file and line number from the stack trace
4. Open your IDE and navigate to the file
5. Read the surrounding code to understand context
6. Reproduce the error locally
7. Identify the root cause
8. Write the fix
9. Run the test suite locally
10. Fix any failing tests
11. Write new tests covering the edge case
12. Run the full test suite again
13. Create a new git branch
14. Commit and push your changes
15. Open a pull request
16. Wait for code review
17. Merge and deploy to production
18. Monitor production to confirm the error is resolved

Total time: 30-60 minutes. For one bug.

npm install bugstack-sdk

import { initBugStack } from 'bugstack-sdk'

initBugStack({ apiKey: process.env.BUGSTACK_API_KEY })

Deploying the Fix (Manual Path)

1. Add @ApiProperty with explicit type for array and nested properties.
2. Add @Type() decorator for nested object validation.
3. Verify emitDecoratorMetadata is true in tsconfig.json.
4. Run tests and verify swagger.json generates correctly.
5. Open a PR, merge after CI, and check Swagger UI in staging.

Frequently Asked Questions