How to set up networking in docker-compose?

Question

Docker

Two environments configured via docker-compose run on one physical machine. Let there be E1 and E2.
In E1 there is a database server to which the service from E2 should go.
By hand, I can connect the corresponding container from E2 to the E1 network:

docker network connect odfpmcaddy_default ordergrabber_grabber_1

I read the docker-compose networks manual, but I’m dumb, whether it is or not, and if it is, how to configure it correctly.
I really don’t want to combine E1 and E2, E1 is already spreading, and their tasks are completely different.
Thank you.