feat(backups): add support for multi-database configurations

Rails has multiple database support since 6.0, but as Parity doesn't
parse three-tier database configuration files correctly, it still
doesn't work on Rails applications set up in this way.

This change detects primary configurations. If it finds one, then it
will use it while restoring a backup. Else, it will default to the
first available configuration.

README.md

@@ -103,7 +103,7 @@ heroku git:remote -r staging -a your-staging-app
 heroku git:remote -r production -a your-production-app
 ```
 * There is a `config/database.yml` file that can be parsed as YAML for
-  `['development']['database']`.
+  `['development']['database']` or, alternatively, `['development']['primary']['database']` for multi-database configurations.
 
 Pipelines
 ---------

lib/parity/backup.rb

@@ -5,6 +5,7 @@ class Backup
     BLANK_ARGUMENTS = "".freeze
     DATABASE_YML_RELATIVE_PATH = "config/database.yml".freeze
     DEVELOPMENT_ENVIRONMENT_KEY_NAME = "development".freeze
+    PRIMARY_DATABASE_KEY_NAME = "primary".freeze
     DATABASE_KEY_NAME = "database".freeze
 
     def initialize(args)
@@ -108,9 +109,15 @@ def remote_db_backup_url
     end
 
     def development_db
-      YAML.load(database_yaml_file).
-        fetch(DEVELOPMENT_ENVIRONMENT_KEY_NAME).
-        fetch(DATABASE_KEY_NAME)
+      db_configuration = YAML.safe_load(database_yaml_file, aliases: true).
+        fetch(DEVELOPMENT_ENVIRONMENT_KEY_NAME)
+
+      if db_configuration.key?(PRIMARY_DATABASE_KEY_NAME)
+        db_configuration[PRIMARY_DATABASE_KEY_NAME].
+          fetch(DATABASE_KEY_NAME)
+      else
+        db_configuration.fetch(DATABASE_KEY_NAME)
+      end
     end
 
     def database_yaml_file

spec/fixtures/database_three_tier.yml

@@ -0,0 +1,8 @@
+development:
+  primary:
+    database: parity_development
+    username: user
+  primary_replica:
+    database: parity_development
+    username: user_readonly
+    replica: true

spec/parity/backup_spec.rb

@@ -169,6 +169,32 @@
         to have_received(:system).
         with(delete_local_temp_backup_command)
     end
+
+    it "is able to load a database.yml file with 3-tier configuration" do
+      allow(IO).to receive(:read).and_return(database_fixture_three_tier)
+      allow(Kernel).to receive(:system)
+
+      Parity::Backup.new(
+        from: "production",
+        to: "development",
+      ).restore
+
+      expect(Kernel).
+        to have_received(:system).
+        with(make_temp_directory_command)
+      expect(Kernel).
+        to have_received(:system).
+        with(download_remote_database_command)
+      expect(Kernel).
+        to have_received(:system).
+        with(drop_development_database_drop_command)
+      expect(Kernel).
+        to have_received(:system).
+        with(restore_from_local_temp_backup_command(cores: 1))
+      expect(Kernel).
+        to have_received(:system).
+        with(delete_local_temp_backup_command)
+    end
   end
 
   it "restores backups to staging from production" do
@@ -231,6 +257,10 @@ def database_fixture_with_erb
     IO.read(fixture_path("database_with_erb.yml"))
   end
 
+  def database_fixture_three_tier
+    IO.read(fixture_path("database_three_tier.yml"))
+  end
+
   def fixture_path(filename)
     File.join(File.dirname(__FILE__), "..", "fixtures", filename)
   end