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I get this error:

Server running...
 MongoNetworkError: failed to connect to server [mongo:27017] on first connect [MongoNetworkError: getaddrinfo EAI_AGAIN mongo mongo:27017]
 at Pool.<anonymous> (/usr/src/app/node_modules/mongodb-core/lib/topologies/server.js:562:11)
 at Pool.emit (events.js:182:13)
 at Connection.<anonymous> (/usr/src/app/node_modules/mongodb-core/lib/connection/pool.js:316:12)
 at Object.onceWrapper (events.js:273:13)
 at Connection.emit (events.js:182:13)
 at Socket.<anonymous> (/usr/src/app/node_modules/mongodb-core/lib/connection/connection.js:245:50)
 at Object.onceWrapper (events.js:273:13)
 at Socket.emit (events.js:182:13)
 at emitErrorNT (internal/streams/destroy.js:82:8)
 at emitErrorAndCloseNT (internal/streams/destroy.js:50:3)
 at process._tickCallback (internal/process/next_tick.js:63:19)
 name: 'MongoNetworkError',
 message:
 'failed to connect to server [mongo:27017] on first connect [MongoNetworkError: getaddrinfo EAI_AGAIN mongo mongo:27017]',
 errorLabels: [ 'TransientTransactionError' ],
 [Symbol(mongoErrorContextSymbol)]: 

this is my docker-compose.yml

version: '3'
services:
 app:
 container_name: docker-node-mongo
 restart: always
 build: .
 ports:
 - '3001:3000'

this is how i tried to connet from node script:

mongoose
 .connect(
 'mongodb://127.0.0.1:27017/test2',
 useNewUrlParser: true 
 )
 .then(() => console.log('MongoDB Connected'))
 .catch(err => console.log("errorErrorError"));

this is the output of docker container ls -a

6f14a98e2d51 dockernodemongo_app "npm start" 11 minutes ago Up 11 minutes 0.0.0.0:3001->3000/tcp docker-node-mongo
f3fb55dad886 mongo "docker-entrypoint.s…" 24 minutes ago Up 24 minutes 127.0.0.1:27017->27017/tcp mongo

this is how i run mongo from a another docker-compose.yml file:

 mongo:
 restart: always
 image: mongo
 container_name: mongo
 network_mode: "bridge"
# expose:
# - 27017
 volumes:
 - ./mongo_data:/data/db
 ports:
 - "127.0.0.1:27017:27017"

You cannot connect your application to mongo that way. 127.0.0.1 refers to the localhost of the container, not the host. You have two options:

• Create a network and add both containers to it. Then use mongodb://mongo:27017/test2 to reach the mongo container. Docker will use the service name as a host so you can reach it by that name.




• Bind mongo to 0.0.0.0 (omit the 127.0.0.1 from the port declaration) and connect to the database using the bridge ip address.

Consider the first option, because the second one will expose your mongo database to the internet if you don't have a firewall in place.

For example:

version: '3.5'
services:
 app:
 container_name: docker-node-mongo
 restart: always
 build: .
 ports:
 - '3001:3000'
 networks:
 - mongo

networks:
 mongo:
 external: true

Then on your mongo compose file:

version: '3.5'
services:
 mongo:
 restart: always
 image: mongo
 container_name: mongo
 expose:
 - 27017
 volumes:
 - ./mongo_data:/data/db
 ports:
 - "127.0.0.1:27017:27017"
 networks:
 - mongo

networks:
 mongo:
 name: mongo

Then on your code:

mongoose
 .connect(
 'mongodb://mongo:27017/test2',
 useNewUrlParser: true 
 )
 .then(() => console.log('MongoDB Connected'))
 .catch(err => console.log("errorErrorError"));

edit: updated network names