Add support for Cloud Spanner Emulator

[aeroastro]

I would like to use Cloud Spanner Emulator to develop this gem and to develop Rails application using gem. Although, there are limitations and differences, the emulator does not cost money 💸 and the separated environments are suited for local development.

https://cloud.google.com/spanner/docs/emulator

Then, I've found out that the current implementation of this library does not support emulator.
This is because Google::Cloud.spanner is used instead of Google::Cloud::Spanner.new. The former method pass the limited portion of parameters to the latter.

ruby-spanner-activerecord/lib/activerecord_spanner_adapter/connection.rb

Lines 28 to 36 in 86909be

	@spanners[database_path(config)] ||= Google::Cloud.spanner(
	config[:project],
	config[:credentials],
	scope: config[:scope],
	timeout: config[:timeout],
	client_config: config[:client_config]&.symbolize_keys,
	lib_name: "spanner-activerecord-adapter",
	lib_version: ActiveRecordSpannerAdapter::VERSION
	)

If we directly call Google::Cloud::Spanner.new(config), we can pass the emulator_host parameter in config, which enable developers to use Cloud Spanner Emulator.

https://github.com/googleapis/google-cloud-ruby/blob/06c73b3b4b09e5f4dded27b61662fe110fea57bb/google-cloud-spanner/lib/google/cloud/spanner.rb#L91-L101

Although, this breaks config[:project], config[:credentials] compatibility, I think keeping consistency with Google::Cloud::Spanner.new reduces maintenance cost.
Of course, If required, I can write backward-compatible code.

If the above strategy is O.K., I am willing to work on this issue.

[skuruppu]

@jiren would you please be able to answer this question?

[jiren]

@aeroastro To run against emulator need to export env variable SPANNER_EMULATOR_HOST

i.e

export SPANNER_EMULATOR_HOST=localhost:9010

If env variable exported then goole-cloud-spanner lib will by default pickup spanner endpoint as emulator.

FYI, I have not tested activereocrd connector with emulator. I will try and let you know.

[aeroastro]

Thank you for your prompt response.
After I checked the following command in my local environment, I still have errors and I need to look deeper into the code.

$ SPANNER_EMULATOR_HOST=localhost:9010 bundle exec rake acceptance[test-project,test.json,test-instance]

Also, specifically speaking, what I would like to achieve here is to treat emulator environment just like the ordinary Cloud Spanner.

by specifying database.yml

development:
  adapter: "spanner"
  emulator_host: localhost:9010
  project: "emulator-project"
  instance: "emulator-instance"
  database: "app-dev"

production:
  adapter: "spanner"
  project: "<google project name>"
  instance: "<google instance name>"
  credentials: "<google credentails file path>"
  database: "app-prod"

or, by directly specifying settings as the arguments.

ActiveRecordSpannerAdapter::Connection.new(
  emulator_host: 'localhost:9010',
  keyfile: 'emulator.json',
  project: 'emulator-project'
)

This is already achieved in python library, and I think I can implement the similar design.

googleapis/python-spanner#87