How to generate database.yml

I generate new rails application with --skip-activerecord, because I'm
going to use with MongoDB.
But right now I want my APP to support MySQL as well.
Any solutions?

Are you saying that for now, you want to use ActiveRecord as well as MySQL? You can change directory to your app’s directory and run: $ rails new my-existing-app-name --database=mysql .
It will try to generate all of the files and there will be conflicts. You can keep saying No to the overwrite prompts and it will eventually get to database.yml, which you can create/overwrite.

Just generate a completely new rails app, specifying mysql, and look
at that database.yml to see what yours should contain.