feat(mapped-types): add skip null properties option to partial type

With this change, you can create a partial class which disallows null values, but allows
undefined values. Previously, every class created with PartialType allowed null values for
every property, which may be undesired if you are defining the DTO for a PATCH endpoint.
If the option is not defined, the behaviour is unchanged from the previous behaviour.

lib/index.ts

@@ -5,6 +5,7 @@ export * from './partial-type.helper';
 export * from './pick-type.helper';
 export {
   applyIsOptionalDecorator,
+  applyValidateIfDefinedDecorator,
   inheritPropertyInitializers,
   inheritTransformationMetadata,
   inheritValidationMetadata,

lib/partial-type.helper.ts

@@ -2,13 +2,27 @@ import { Type } from '@nestjs/common';
 import { MappedType } from './mapped-type.interface';
 import {
   applyIsOptionalDecorator,
+  applyValidateIfDefinedDecorator,
   inheritPropertyInitializers,
   inheritTransformationMetadata,
   inheritValidationMetadata,
 } from './type-helpers.utils';
 import { RemoveFieldsWithType } from './types/remove-fields-with-type.type';
 
-export function PartialType<T>(classRef: Type<T>) {
+export function PartialType<T>(
+  classRef: Type<T>,
+  /**
+   *  Configuration options.
+   */
+  options: {
+    /**
+     * If true, validations will be ignored on a property if it is either null or undefined. If
+     * false, validations will be ignored only if the property is undefined.
+     * @default true
+     */
+    skipNullProperties?: boolean;
+  } = {},
+) {
   abstract class PartialClassType {
     constructor() {
       inheritPropertyInitializers(this, classRef);
@@ -20,7 +34,9 @@ export function PartialType<T>(classRef: Type<T>) {
 
   if (propertyKeys) {
     propertyKeys.forEach((key) => {
-      applyIsOptionalDecorator(PartialClassType, key);
+      options.skipNullProperties === false
+        ? applyValidateIfDefinedDecorator(PartialClassType, key)
+        : applyIsOptionalDecorator(PartialClassType, key);
     });
   }
 

lib/type-helpers.utils.ts

@@ -15,6 +15,20 @@ export function applyIsOptionalDecorator(
   decoratorFactory(targetClass.prototype, propertyKey);
 }
 
+export function applyValidateIfDefinedDecorator(
+  targetClass: Function,
+  propertyKey: string,
+) {
+  if (!isClassValidatorAvailable()) {
+    return;
+  }
+  const classValidator: typeof import('class-validator') = require('class-validator');
+  const decoratorFactory = classValidator.ValidateIf(
+    (_, value) => value !== undefined,
+  );
+  decoratorFactory(targetClass.prototype, propertyKey);
+}
+
 export function inheritValidationMetadata(
   parentClass: Type<any>,
   targetClass: Function,

tests/partial-type.helper.spec.ts

@@ -81,4 +81,44 @@ describe('PartialType', () => {
       expect(updateUserDto.login).toEqual('defaultLogin');
     });
   });
+
+  describe('Configuration options', () => {
+    it('should not ignore validations for null properties when `skipNullProperties` is false', async () => {
+      class UpdateUserDtoDisallowNull extends PartialType(CreateUserDto, {
+        skipNullProperties: false,
+      }) {}
+
+      const updateDto = new UpdateUserDtoDisallowNull();
+      updateDto.password = null as any;
+
+      const validationErrors = await validate(updateDto);
+
+      expect(validationErrors.length).toBe(1);
+      expect(validationErrors[0].constraints).toEqual({
+        isString: 'password must be a string',
+      });
+    });
+
+    it('should ignore validations on null properties when `skipNullProperties` is undefined', async () => {
+      const updateDto = new UpdateUserDto();
+      updateDto.password = null as any;
+
+      const validationErrors = await validate(updateDto);
+
+      expect(validationErrors.length).toBe(0);
+    });
+
+    it('should ignore validations on null properties when `skipNullProperties` is true', async () => {
+      class UpdateUserDtoAllowNull extends PartialType(CreateUserDto, {
+        skipNullProperties: true,
+      }) {}
+
+      const updateDto = new UpdateUserDtoAllowNull();
+      updateDto.password = null as any;
+
+      const validationErrors = await validate(updateDto);
+
+      expect(validationErrors.length).toBe(0);
+    });
+  });
 });