Feature request: Consider support for Postgres' CREATE DATABASE WITH TEMPLATE; or other 'clone a whole database' approaches

[doconnor-clintel]

Currently; the default assumption is to create, migrate, seed, etc N databases; and this is generally pretty sensible.

  desc "Create test databases via db:create --> parallel:create[num_cpus]"
  task :create, :count do |_, args|
    ParallelTests::Tasks.run_in_parallel(
      "#{ParallelTests::Tasks.rake_bin} db:create RAILS_ENV=#{ParallelTests::Tasks.rails_env}", args
    )
  end

For CI pipelines with a large number of migrations or complex seeding however would benefit from:

• Migrate, seed once - all of the "prepare" tasks
• Snapshot
• Create databases as a snapshot of only the end state.

Right now there are some strategies for dumping + loading schema; but less so with fully seeded data.
https://github.com/grosser/parallel_tests/blob/master/lib/parallel_tests/tasks.rb#L121

Postgres offers support for CREATE DATABASE x WITH TEMPLATE y; and it seems supported in activerecord - see https://dockyard.com/blog/2014/03/03/using-database-templates

SQLIte databases could in theory support this with a copy on the filesystem or inbuilt commands:
https://database.guide/clone-sqlite-database/

MySQL - doesn't seem supported without a dump + load.
SQL server - ditto.
Oracle - similar.

Docker based solutions - a similar effect could probably be achieved with a snapshot + starting many containers; but by this point you might not choose to use parallel_tests.

[grosser]

if you can somehow make this work as an additional task and not too much hackery. then PR welcome ... just don't want this to be the default behavior since I expect lots of bugs (at least initially) and it's not what simple users would expect or need

[adis-io]

https://gist.github.com/robworley/4346649