Fix Rails environment when running tests with Ruby

railties/lib/rails/generators/rails/app/templates/test/test_helper.rb.tt

@@ -1,3 +1,4 @@
+ENV['RAILS_ENV'] ||= 'test'
 require_relative '../config/environment'
 require 'rails/test_help'
 

railties/lib/rails/generators/rails/plugin/templates/test/test_helper.rb.tt

@@ -1,3 +1,6 @@
+# Configure Rails Environment
+ENV["RAILS_ENV"] = "test"
+
 require_relative "<%= File.join('..', options[:dummy_path], 'config/environment') -%>"
 <% unless options[:skip_active_record] -%>
 ActiveRecord::Migrator.migrations_paths = [File.expand_path("../<%= options[:dummy_path] -%>/db/migrate", __dir__)]

railties/lib/rails/test_unit/runner.rb

@@ -13,7 +13,7 @@ class Runner
       class << self
         def attach_before_load_options(opts)
           opts.on("--warnings", "-w", "Run with Ruby warnings enabled") {}
-          opts.on("--environment", "-e", "Run tests in the ENV environment") {}
+          opts.on("-e", "--environment ENV", "Run tests in the ENV environment") {}
         end
 
         def parse_options(argv)

railties/test/application/test_runner_test.rb

@@ -569,6 +569,40 @@ def test_rake_passes_TESTOPTS_to_minitest
       assert_match "AccountTest#test_truth", output, "passing TEST= should run selected test"
     end
 
+    def test_running_with_ruby_gets_test_env_by_default
+      # Subshells inherit `ENV`, so we need to ensure `RAILS_ENV` is set to
+      # nil before we run the tests in the test app.
+      re, ENV["RAILS_ENV"] = ENV["RAILS_ENV"], nil
+
+      file = create_test_for_env("test")
+      results = Dir.chdir(app_path) {
+        `ruby -Ilib:test #{file}`.each_line.map { |line| JSON.parse line }
+      }
+      assert_equal 1, results.length
+      failures = results.first["failures"]
+      flunk(failures.first) unless failures.empty?
+
+    ensure
+      ENV["RAILS_ENV"] = re
+    end
+
+    def test_running_with_ruby_can_set_env_via_cmdline
+      # Subshells inherit `ENV`, so we need to ensure `RAILS_ENV` is set to
+      # nil before we run the tests in the test app.
+      re, ENV["RAILS_ENV"] = ENV["RAILS_ENV"], nil
+
+      file = create_test_for_env("development")
+      results = Dir.chdir(app_path) {
+        `RAILS_ENV=development ruby -Ilib:test #{file}`.each_line.map { |line| JSON.parse line }
+      }
+      assert_equal 1, results.length
+      failures = results.first["failures"]
+      flunk(failures.first) unless failures.empty?
+
+    ensure
+      ENV["RAILS_ENV"] = re
+    end
+
     def test_rake_passes_multiple_TESTOPTS_to_minitest
       create_test_file :models, "account"
       output = Dir.chdir(app_path) { `bin/rake test TESTOPTS='-v --seed=1234'` }
@@ -727,6 +761,45 @@ def create_schema
         app_file "db/schema.rb", ""
       end
 
+      def create_test_for_env(env)
+        app_file "test/models/environment_test.rb", <<-RUBY
+          require 'test_helper'
+          class JSONReporter < Minitest::AbstractReporter
+            def record(result)
+              puts JSON.dump(klass: result.class.name,
+                             name: result.name,
+                             failures: result.failures,
+                             assertions: result.assertions,
+                             time: result.time)
+            end
+          end
+
+          def Minitest.plugin_json_reporter_init(opts)
+            Minitest.reporter.reporters.clear
+            Minitest.reporter.reporters << JSONReporter.new
+          end
+
+          Minitest.extensions << "rails"
+          Minitest.extensions << "json_reporter"
+
+          # Minitest uses RubyGems to find plugins, and since RubyGems
+          # doesn't know about the Rails installation we're pointing at,
+          # Minitest won't require the Rails minitest plugin when we run
+          # these integration tests.  So we have to manually require the
+          # Minitest plugin here.
+          require 'minitest/rails_plugin'
+
+          class EnvironmentTest < ActiveSupport::TestCase
+            def test_environment
+              test_db = ActiveRecord::Base.configurations[#{env.dump}]["database"]
+              db_file = ActiveRecord::Base.connection_config[:database]
+              assert_match(test_db, db_file)
+              assert_equal #{env.dump}, ENV["RAILS_ENV"]
+            end
+          end
+        RUBY
+      end
+
       def create_test_file(path = :unit, name = "test", pass: true)
         app_file "test/#{path}/#{name}_test.rb", <<-RUBY
           require 'test_helper'

railties/test/isolation/abstract_unit.rb

@@ -358,10 +358,12 @@ def remove_from_file(file, str)
     end
 
     def app_file(path, contents, mode = "w")
-      FileUtils.mkdir_p File.dirname("#{app_path}/#{path}")
-      File.open("#{app_path}/#{path}", mode) do |f|
+      file_name = "#{app_path}/#{path}"
+      FileUtils.mkdir_p File.dirname(file_name)
+      File.open(file_name, mode) do |f|
         f.puts contents
       end
+      file_name
     end
 
     def remove_file(path)