Connect docker compose containers without links - node.js

Question

https://docs.docker.com/compose/networking/
At above official docker document, I found the part
version: "3"
services:
web:
build: .
ports:
- "8000:8000"
db:
image: postgres
ports:
- "8001:5432"
Each container can now look up the hostname web or db and get back the appropriate container’s IP address. For example, web’s application code could connect to the URL postgres://db:5432 and start using the Postgres database.
So I understood this paragraph that I can connect docker containers each other without links: or networks: explicitly. because above docker-compose.yml snippet doesn't have links or networks: part. and the document say web’s application code could connect to the URL postgres://db:5432
So I tried to test simple docker-compose with nodejs express, mongodb together using above way. I thought I can connect mongodb in express app with just mongodb://mongo:27017/myapp But I cannot connect mongodb in express container. I think I followed docker's official manual but I don't know why it's not working. Of course I can connect mongodb using links: or networks: But I heard links is depreciated and I cannot find the proper way to use networks:
I think I might be misunderstood, Please fix me.
Below is my docker-compose.yml
version: '3'
services:
app:
container_name: node
restart: always
build: .
ports:
- '3000:3000'
mongo:
image: mongo
ports:
- '27017:27017'
In express app, I connect to mongodb with
mongoose.connect('mongodb://mongo:27017/myapp', {
useMongoClient: true
});
// also doesn't work with mongodb://mongo/myapp
Plus) Dockerfile
FROM node:10.17-alpine3.9
ENV NODE_ENV development
WORKDIR /usr/src/app
COPY ["package*.json", "npm-shrinkwrap.json*", "./"]
RUN rm -rf node_modules
RUN apk --no-cache --virtual build-dependencies add \
python \
make \
g++ \
&& npm install \
&& apk del build-dependencies
COPY . .
EXPOSE 3000
CMD npm start

Answer 1

If you want to connect mongo with local then you should have to select network mode.
docket-compose.yml file content.
version: '2.1'
services:
z2padmin_docker:
image: z2padmin_docker
build: .
environment:
NODE_ENV: production
volumes: [/home/ankit/Z2PDATAHUB/uploads:/mnt/Z2PDATAHUB/uploads]
ports:
- 5000:5000
network_mode: host