Only create parallel test databases on schema changes

activerecord/lib/active_record/migration.rb

@@ -587,8 +587,10 @@ def check_pending!(connection = Base.connection)
       end
 
       def load_schema_if_pending!
-        if Base.connection.migration_context.needs_migration? || !Base.connection.migration_context.any_migrations?
-          # Roundtrip to Rake to allow plugins to hook into database initialization.
+        all_configs = ActiveRecord::Base.configurations.configs_for(env_name: Rails.env)
+
+        unless all_configs.all? { |config| Tasks::DatabaseTasks.schema_up_to_date?(config.config) }
+          # Roundrip to Rake to allow plugins to hook into database initialization.
           root = defined?(ENGINE_ROOT) ? ENGINE_ROOT : Rails.root
           FileUtils.cd(root) do
             current_config = Base.connection_config
@@ -597,8 +599,9 @@ def load_schema_if_pending!
             # Establish a new connection, the old database may be gone (db:test:prepare uses purge)
             Base.establish_connection(current_config)
           end
-          check_pending!
         end
+
+        check_pending!
       end
 
       def maintain_test_schema! #:nodoc:

activerecord/lib/active_record/tasks/database_tasks.rb

@@ -332,10 +332,36 @@ def load_schema(configuration, format = ActiveRecord::Base.schema_format, file =
         end
         ActiveRecord::InternalMetadata.create_table
         ActiveRecord::InternalMetadata[:environment] = environment
+        ActiveRecord::InternalMetadata[:schema_sha1] = schema_sha1(file)
       ensure
         Migration.verbose = verbose_was
       end
 
+      def schema_up_to_date?(configuration, format = ActiveRecord::Base.schema_format, file = nil, environment = env, spec_name = "primary")
+        file ||= dump_filename(spec_name, format)
+
+        return true unless File.exist?(file)
+
+        ActiveRecord::Base.establish_connection(configuration)
+        return false unless ActiveRecord::InternalMetadata.table_exists?
+        ActiveRecord::InternalMetadata[:schema_sha1] == schema_sha1(file)
+      end
+
+      def reset_to_schema(configuration, format = ActiveRecord::Base.schema_format, file = nil, environment = env, spec_name = "primary") # :nodoc:
+        file ||= dump_filename(spec_name, format)
+
+        check_schema_file(file)
+
+        ActiveRecord::Base.establish_connection(configuration)
+
+        if schema_up_to_date?(configuration, format, file, environment, spec_name)
+          truncate_tables(configuration)
+        else
+          purge(configuration)
+          load_schema(configuration, format, file, environment, spec_name)
+        end
+      end
+
       def dump_schema(configuration, format = ActiveRecord::Base.schema_format, spec_name = "primary") # :nodoc:
         require "active_record/schema_dumper"
         filename = dump_filename(spec_name, format)
@@ -467,6 +493,10 @@ def each_local_configuration
         def local_database?(configuration)
           configuration["host"].blank? || LOCAL_HOSTS.include?(configuration["host"])
         end
+
+        def schema_sha1(file)
+          Digest::SHA1.hexdigest(File.read(file))
+        end
     end
   end
 end

activerecord/lib/active_record/test_databases.rb

@@ -8,31 +8,16 @@ module TestDatabases # :nodoc:
       create_and_load_schema(i, env_name: Rails.env)
     end
 
-    ActiveSupport::Testing::Parallelization.run_cleanup_hook do
-      drop(env_name: Rails.env)
-    end
-
     def self.create_and_load_schema(i, env_name:)
       old, ENV["VERBOSE"] = ENV["VERBOSE"], "false"
 
       ActiveRecord::Base.configurations.configs_for(env_name: env_name).each do |db_config|
         db_config.config["database"] += "-#{i}"
-        ActiveRecord::Tasks::DatabaseTasks.create(db_config.config)
-        ActiveRecord::Tasks::DatabaseTasks.load_schema(db_config.config, ActiveRecord::Base.schema_format, nil, env_name, db_config.spec_name)
+        ActiveRecord::Tasks::DatabaseTasks.reset_to_schema(db_config.config, ActiveRecord::Base.schema_format, nil, env_name, db_config.spec_name)
       end
     ensure
       ActiveRecord::Base.establish_connection(Rails.env.to_sym)
       ENV["VERBOSE"] = old
     end
-
-    def self.drop(env_name:)
-      old, ENV["VERBOSE"] = ENV["VERBOSE"], "false"
-
-      ActiveRecord::Base.configurations.configs_for(env_name: env_name).each do |db_config|
-        ActiveRecord::Tasks::DatabaseTasks.drop(db_config.config)
-      end
-    ensure
-      ENV["VERBOSE"] = old
-    end
   end
 end

railties/test/application/test_test.rb

@@ -232,10 +232,7 @@ class UserTest < ActiveSupport::TestCase
       assert_successful_test_run("models/user_test.rb")
     end
 
-    # TODO: would be nice if we could detect the schema change automatically.
-    # For now, the user has to synchronize the schema manually.
-    # This test case serves as a reminder for this use case.
-    test "manually synchronize test schema after rollback" do
+    test "automatically synchronizes test schema after rollback" do
       output  = rails("generate", "model", "user", "name:string")
       version = output.match(/(\d+)_create_users\.rb/)[1]
 
@@ -268,10 +265,6 @@ class UserTest < ActiveSupport::TestCase
         end
       RUBY
 
-      assert_successful_test_run "models/user_test.rb"
-
-      rails "db:test:prepare"
-
       assert_unsuccessful_run "models/user_test.rb", <<-ASSERTION
 Expected: ["id", "name"]
   Actual: ["id", "name", "age"]