$ npm install --save mongoose

import * as mongoose from 'mongoose';

export const databaseProviders = [
 {
   provide: 'DATABASE_CONNECTION',
   useFactory: (): Promise<typeof mongoose> =>
     mongoose.connect('mongodb://localhost/nest'),
 },
];

import { Module } from '@nestjs/common';
import { databaseProviders } from './database.providers';

@Module({
 providers: [...databaseProviders],
 exports: [...databaseProviders],
})
export class DatabaseModule {}

import * as mongoose from 'mongoose';

export const CatSchema = new mongoose.Schema({
 name: String,
 age: Number,
 breed: String,
});

import { Connection } from 'mongoose';
import { CatSchema } from './schemas/cat.schema';

export const catsProviders = [
 {
   provide: 'CAT_MODEL',
   useFactory: (connection: Connection) => connection.model('Cat', CatSchema),
   inject: ['DATABASE_CONNECTION'],
 },
];

import { Model } from 'mongoose';
import { Injectable, Inject } from '@nestjs/common';
import { Cat } from './interfaces/cat.interface';
import { CreateCatDto } from './dto/create-cat.dto';

@Injectable()
export class CatsService {
 constructor(
   @Inject('CAT_MODEL')
   private catModel: Model<Cat>,
 ) {}

 async create(createCatDto: CreateCatDto): Promise<Cat> {
   const createdCat = new this.catModel(createCatDto);
   return createdCat.save();
 }

 async findAll(): Promise<Cat[]> {
   return this.catModel.find().exec();
 }
}

import { Document } from 'mongoose';

export interface Cat extends Document {
  readonly name: string;
  readonly age: number;
  readonly breed: string;
}

import { Module } from '@nestjs/common';
import { CatsController } from './cats.controller';
import { CatsService } from './cats.service';
import { catsProviders } from './cats.providers';
import { DatabaseModule } from '../database/database.module';

@Module({
 imports: [DatabaseModule],
 controllers: [CatsController],
 providers: [
   CatsService,
   ...catsProviders,
 ],
})
export class CatsModule {}