Sync test DB from schema using its SHA1

Previously, we used the migration status to determine whether the test
database(s) needed to be reloaded from the schema. This worked in most
cases, but if a schema.rb was modified outside of migrations or if a
migration was rolled back, it would require a manual db:test:prepare.

This commit updates load_schema to record the SHA1 of the loaded schema
file inside of the ar_internal_metadata table. We can then use this SHA
to determine whether we should reload the schema.

This ensures that the test DB stays exactly in sync with the schema
file, including rollbacks which fixes a test marked TODO.

activerecord/lib/active_record/migration.rb

@@ -587,18 +587,22 @@ module ActiveRecord
       end
 
       def load_schema_if_pending!
-        if Base.connection.migration_context.needs_migration? || !Base.connection.migration_context.any_migrations?
+        current_config = Base.connection_config
+        all_configs = ActiveRecord::Base.configurations.configs_for(env_name: Rails.env)
+
+        unless all_configs.all? { |config| Tasks::DatabaseTasks.schema_up_to_date?(config.config) }
           # Roundtrip 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
             Base.clear_all_connections!
             system("bin/rails db:test:prepare")
-            # Establish a new connection, the old database may be gone (db:test:prepare uses purge)
-            Base.establish_connection(current_config)
           end
-          check_pending!
         end
+
+        # Establish a new connection, the old database may be gone (db:test:prepare uses purge)
+        Base.establish_connection(current_config)
+
+        check_pending!
       end
 
       def maintain_test_schema! #:nodoc:

activerecord/lib/active_record/tasks/database_tasks.rb

@@ -332,10 +332,21 @@ module ActiveRecord
         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 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 +478,10 @@ module ActiveRecord
         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

railties/test/application/test_test.rb

@@ -232,10 +232,7 @@ module ApplicationTests
       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 @@ module ApplicationTests
         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"]