Pass env_name as a string in test databases

In 154abcab we switched from using `Rails.env` to fetch the `env_name` to
`ActiveRecord::ConnectionHandling::DEFAULT_ENV.call.to_sym` which
changed the type from a `String` to a `Symbol`.

This commit brings things back to the original state, so we can find the
configurations correctly!

It also modifies the configuration in the configurations array, so that
future connections can find the database with the updated keyword value.

activerecord/lib/active_record/test_databases.rb

@@ -5,7 +5,7 @@
 module ActiveRecord
   module TestDatabases # :nodoc:
     ActiveSupport::Testing::Parallelization.after_fork_hook do |i|
-      create_and_load_schema(i, env_name: ActiveRecord::ConnectionHandling::DEFAULT_ENV.call.to_sym)
+      create_and_load_schema(i, env_name: ActiveRecord::ConnectionHandling::DEFAULT_ENV.call)
     end
 
     def self.create_and_load_schema(i, env_name:)
@@ -20,7 +20,12 @@ def self.create_and_load_schema(i, env_name:)
           db_config.configuration_hash.merge(database: database)
         )
 
+        # Reconstruct with the new configuration
         ActiveRecord::Tasks::DatabaseTasks.reconstruct_from_schema(db_config_copy, ActiveRecord::Base.schema_format, nil)
+
+        # Replace the original configuration with our replacement
+        ActiveRecord::Base.configurations.configurations.delete(db_config)
+        ActiveRecord::Base.configurations.configurations.push(db_config_copy)
       end
     ensure
       ActiveRecord::Base.establish_connection(ActiveRecord::ConnectionHandling::DEFAULT_ENV.call.to_sym)

activerecord/test/cases/test_databases_test.rb

 # frozen_string_literal: true
 
 require "cases/helper"
+require "active_record/test_databases"
 
 class TestDatabasesTest < ActiveRecord::TestCase
   unless in_memory_db?
     def test_databases_are_created
       previous_env, ENV["RAILS_ENV"] = ENV["RAILS_ENV"], "arunit"
+      prev_configs, ActiveRecord::Base.configurations = ActiveRecord::Base.configurations, {
+        "arunit" => {
+          "primary" => { "adapter" => "sqlite3", "database" => "db/primary.sqlite3" }
+        }
+      }
 
       base_db_config = ActiveRecord::Base.configurations.configs_for(env_name: "arunit", spec_name: "primary")
       expected_database = "#{base_db_config.database}-2"
@@ -16,6 +22,34 @@ def test_databases_are_created
         ActiveRecord::TestDatabases.create_and_load_schema(2, env_name: "arunit")
       end
     ensure
+      ActiveRecord::Base.configurations = prev_configs
+      ActiveRecord::Base.establish_connection(:arunit)
+      ENV["RAILS_ENV"] = previous_env
+    end
+
+    def test_create_databases_after_fork
+      previous_env, ENV["RAILS_ENV"] = ENV["RAILS_ENV"], "arunit"
+      prev_configs, ActiveRecord::Base.configurations = ActiveRecord::Base.configurations, {
+        "arunit" => {
+          "primary" => { "adapter" => "sqlite3", "database" => "db/primary.sqlite3" }
+        }
+      }
+
+      idx = 42
+      base_db_config = ActiveRecord::Base.configurations.configs_for(env_name: "arunit", spec_name: "primary")
+      expected_database = "#{base_db_config.database}-#{idx}"
+
+      ActiveRecord::Tasks::DatabaseTasks.stub(:reconstruct_from_schema, ->(db_config, _, _) {
+        assert_equal expected_database, db_config.database
+      }) do
+        ActiveSupport::Testing::Parallelization.after_fork_hooks.each { |cb| cb.call(idx) }
+      end
+
+      # Updates the databse configuration
+      assert_equal expected_database, ActiveRecord::Base.configurations.configs_for(env_name: "arunit", spec_name: "primary").database
+    ensure
+      ActiveRecord::Base.configurations = prev_configs
+      ActiveRecord::Base.establish_connection(:arunit)
       ENV["RAILS_ENV"] = previous_env
     end
   end