Switch extended configs

[faroit]

The documentation states how to extend configs. I have a hard time understanding how to design a pattern so that one can change/override the defaults via CLI, though:

Existing example

# config.yaml
defaults:
  - base_mysql

user: omry
password: secret
port: 3307
encoding: utf8

# db/mysql.yaml
defaults:
  - db: mysql 

# db/base_mysql.yaml
host: localhost
port: 3306
user: ???
password: ???

Problem

Now if I would now add two new files called

# db/sqlite.yaml
defaults:
  - db: sqlite

# db/base_sqlite.yaml
...

how can I change the default extension from the commandline to switch between sqlite and mysql without setting db=sqlite?

Motivation/Use case

extending experiments with multiple levels: i have four files:

• experiments/model1/commons.yaml

defaults:
  - override /xzy: abc
  - ...

# ...
# large number of baseline params

• experiments/model1/fp16.yaml

defaults:
  - model1/commons

data:
  batch_size: 16

trainer:
  precision: 16 

• experiments/model1/fp32.yaml

defaults:
  - model1/commons

data:
  batch_size: 8

trainer:
  precision: 32

• experiments/run1.yaml

defaults:
  - model1/fp16

base:
  name: run1

# ...specific params

Now I want to switch between fp16 and fp32. How do I override this from cli?

I am sure, I missed important parts of hydra, so I would be happy if someone would point me to the right spot. My guess is that this is related to #171 but please correct me!