After= dependency is only effective when service including After= and service included by After= are both scheduled to start as part of your boot up.
Ex:
a.service
[Unit]
After=b.service
This way, if both a.service and b.service are enabled, then systemd will order b.service after a.service.
If I am not misunderstanding, what you are asking is how to start b.service when a.service starts even though b.service is not enabled.
The directive for this is Wants= or Requires= under [Unit].
website.service
[Unit]
Wants=mongodb.service
After=mongodb.service
The difference between Wants= and Requires= is that with Requires=, a failure to start b.service will cause the startup of a.service to fail, whereas with Wants=, a.service will start even if b.service fails. This is explained in detail on the man page of .unit.