add option in database config for metadata table

    - adds the option `metadata_table` to a database connection

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -111,6 +111,10 @@ def replica?
         @config[:replica] || false
       end
 
+      def use_metadata_table?
+        @config.fetch(:use_metadata_table, true)
+      end
+
       # Determines whether writes are currently being prevents.
       #
       # Returns true if the connection is a replica, or if +prevent_writes+

activerecord/lib/active_record/internal_metadata.rb

@@ -6,8 +6,15 @@
 module ActiveRecord
   # This class is used to create a table that keeps track of values and keys such
   # as which environment migrations were run in.
+  #
+  # It is possible to enable or disable this functionality by setting the
+  # adapter configuration option `use_metadata_table` to false
   class InternalMetadata < ActiveRecord::Base # :nodoc:
     class << self
+      def enabled?
+        ActiveRecord::Base.connection.use_metadata_table?
+      end
+
       def _internal?
         true
       end
@@ -21,15 +28,21 @@ def table_name
       end
 
       def []=(key, value)
+        return unless enabled?
+
         find_or_initialize_by(key: key).update!(value: value)
       end
 
       def [](key)
+        return unless enabled?
+
         where(key: key).pluck(:value).first
       end
 
       # Creates an internal metadata table with columns +key+ and +value+
       def create_table
+        return unless enabled?
+
         unless table_exists?
           key_options = connection.internal_string_options_for_primary_key
 
@@ -42,6 +55,8 @@ def create_table
       end
 
       def drop_table
+        return unless enabled?
+
         connection.drop_table table_name, if_exists: true
       end
     end

activerecord/lib/active_record/migration.rb

@@ -190,6 +190,13 @@ def initialize(current: nil, stored: nil)
     end
   end
 
+  class EnvironmentStorageError < ActiveRecordError #:nodoc:
+    def initialize
+      msg = +"You are attempting to store the environment in a database where metadata is disabled.\n"
+      msg << "Check your database configuration to see if this is inteded."
+    end
+  end
+
   # = Active Record Migrations
   #
   # Migrations can manage the evolution of a schema used by several physical
@@ -1143,6 +1150,7 @@ def protected_environment?
     end
 
     def last_stored_environment
+      return nil unless ActiveRecord::InternalMetadata.enabled?
       return nil if current_version == 0
       raise NoEnvironmentInSchemaError unless ActiveRecord::InternalMetadata.table_exists?
 

activerecord/lib/active_record/railties/databases.rake

@@ -8,6 +8,7 @@ databases = ActiveRecord::Tasks::DatabaseTasks.setup_initial_database_yaml
 db_namespace = namespace :db do
   desc "Set the environment value for the database"
   task "environment:set" => :load_config do
+    raise ActiveRecord::EnvironmentStorageError unless ActiveRecord::InternalMetadata.enabled?
     ActiveRecord::InternalMetadata.create_table
     ActiveRecord::InternalMetadata[:environment] = ActiveRecord::Base.connection.migration_context.current_environment
   end

activerecord/lib/active_record/tasks/database_tasks.rb

@@ -348,7 +348,10 @@ def schema_up_to_date?(configuration, format = ActiveRecord::Base.schema_format,
         return true unless File.exist?(file)
 
         ActiveRecord::Base.establish_connection(db_config)
+
+        return false unless ActiveRecord::InternalMetadata.enabled?
         return false unless ActiveRecord::InternalMetadata.table_exists?
+
         ActiveRecord::InternalMetadata[:schema_sha1] == schema_sha1(file)
       end
 

activerecord/test/cases/migration_test.rb

@@ -640,6 +640,16 @@ def test_internal_metadata_stores_environment_when_other_data_exists
     assert_equal "bar", ActiveRecord::InternalMetadata[:foo]
   end
 
+  def test_internal_metadata_not_used_when_not_enabled
+    ActiveRecord::InternalMetadata.stub(:enabled?, false) do
+      migrations_path = MIGRATIONS_ROOT + "/valid"
+
+      migrator = ActiveRecord::MigrationContext.new(migrations_path, @schema_migration)
+      migrator.up
+      assert_not ActiveRecord::InternalMetadata[:environment]
+    end
+  end
+
   def test_proper_table_name_on_migration
     reminder_class = new_isolated_reminder_class
     migration = ActiveRecord::Migration.new

guides/source/configuring.md

@@ -1258,6 +1258,16 @@ development:
 
 Change the username and password in the `development` section as appropriate.
 
+#### Configuring Metadata Storage
+
+By default Rails will store information about your environment and schema in an internal table. If you do not want this table to be created, and are willing to give up the protections it provides (useful when working with a shared database and a database user that can not create tables).
+
+```yaml
+development:
+  adapter: postgresql
+  use_metadata_table: false
+```
+
 ### Creating Rails Environments
 
 By default Rails ships with three environments: "development", "test", and "production". While these are sufficient for most use cases, there are circumstances when you want more environments.