NestJS Mongoose

Connecting Mongoose in Nest

@nestjs/mongoose provides MongooseModule.forRoot() for the connection and forFeature() for registering schemas that become injectable models.

MongooseModule.forRoot(process.env.MONGODB_URI);

@Schema()
export class Cat {
  @Prop({ required: true })
  name: string;

  @Prop()
  age: number;
}

export const CatSchema = SchemaFactory.createForClass(Cat);

@Schema() / @Prop() keep Nest-style decorator syntax while generating a Mongoose schema via SchemaFactory.

Injecting a Model

@Injectable()
export class CatsService {
  constructor(@InjectModel(Cat.name) private catModel: Model<Cat>) {}

  create(dto: CreateCatDto) {
    return this.catModel.create(dto);
  }
}

• Register the schema with MongooseModule.forFeature([{ name: Cat.name, schema: CatSchema }]).
• @InjectModel(Cat.name) injects the compiled Mongoose model.
• Documents return lean objects when you call .lean() for better performance on reads.
• Use indexes and unique constraints in schema props for fields queried often.

Mongoose Nest Cheatsheet

Key decorators and APIs for Nest + MongoDB.

Embedded Documents and References

MongoDB favors embedding related data when it is owned by a parent document. Use ObjectId references and populate() when data is shared across collections.

Validation Layers

Still validate HTTP input with Nest DTOs and ValidationPipe. Schema-level Mongoose validation is a second line of defense, not a replacement for API validation.

Common Mistakes

• Treating Mongoose documents as plain objects without converting when needed (toJSON / lean).
• Forgetting unique indexes and then handling duplicates only as generic 500 errors.
• Over-using populate() on large lists without pagination.
• Hard-coding MongoDB URIs instead of ConfigModule environment variables.

Key Takeaways

• @nestjs/mongoose registers connections and feature schemas as Nest providers.
• Class-based schemas with @Schema/@Prop keep models consistent with Nest style.
• Inject models with @InjectModel and keep queries in services.
• Combine Nest DTO validation with schema constraints for safer writes.