无法将主机名“postgres”转换为地址：名称或服务未知（PG :: ConnectionBad）

I'm working on a Rails 4.2.3 project with ruby 2.2.2 (Running ubuntu 16.04). It was working ok but after revisiting it after a while I can't serve. I've tried searching but all situations don't seem to be of help. Here's the error log:I'm working on a Rails 4.2.3 project with ruby