Active Record supports multiple database systems. AbstractAdapter and related classes form the abstraction layer which makes this possible. An AbstractAdapter represents a connection to a database, and provides an abstract interface for database-specific functionality such as establishing a connection, escaping values, building the right SQL fragments for ':offset' and ':limit' options, etc.
All the concrete database adapters follow the interface laid down in this class. ActiveRecord::Core#connection returns an AbstractAdapter object, which you can use.
Most of the methods in the adapter are useful during migrations. Most notably, the instance methods provided by SchemaStatement are very useful.
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- schema_cache=,
- schema_creation,
- substitute_at,
- supports_bulk_alter?,
- supports_count_distinct?,
- supports_ddl_transactions?,
- supports_explain?,
- supports_extensions?,
- supports_index_sort_order?,
- supports_migrations?,
- supports_partial_index?,
- supports_primary_key?,
- supports_savepoints?,
- supports_transaction_isolation?
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- ActiveRecord::ConnectionAdapters::Quoting
- ActiveRecord::ConnectionAdapters::DatabaseStatements
- ActiveRecord::ConnectionAdapters::SchemaStatements
- ActiveRecord::ConnectionAdapters::DatabaseLimits
- ActiveRecord::ConnectionAdapters::QueryCache
- ActiveSupport::Callbacks
- MonitorMixin
- ActiveRecord::ConnectionAdapters::ColumnDumper
| SIMPLE_INT | = | /\A\d+\z/ |
| [R] | in_use | |
| [R] | in_use? | |
| [R] | last_use | |
| [R] | logger | |
| [RW] | pool | |
| [R] | schema_cache | |
| [RW] | visitor |
Checks whether the connection to the database is still active. This includes checking whether the database is actually capable of responding, i.e. whether the connection isn't stale.
Adapter should redefine this if it needs a threadsafe way to approximate if the connection is active
Returns the human-readable name of the adapter. Use mixed case - one can always use downcase if needed.
Clear any caching the database adapter may be doing, for example clearing the prepared statement cache. This is database specific.
Check the connection back in to the connection pool
This is meant to be implemented by the adapters that support extensions
Override to turn off referential integrity while executing
&block.
Disconnects from the database if already connected. Otherwise, this method does nothing.
This is meant to be implemented by the adapters that support extensions
A list of extensions, to be filled in by adapters that support them. At the moment only postgresql does.
A list of index algorithms, to be filled by adapters that support them. MySQL and PostgreSQL have support for them right now.
Should primary key values be selected from their corresponding sequence before the insert statement? If true, next_sequence_value is called before each insert to set the record's primary key. This is false for all adapters but Firebird.
Provides access to the underlying database driver for this adapter. For example, this method returns a Mysql object in case of MysqlAdapter, and a PGconn object in case of PostgreSQLAdapter.
This is useful for when you need to call a proprietary method such as PostgreSQL's lo_* methods.
Disconnects from the database if already connected, and establishes a new connection with the database. Implementors should call super if they override the default implementation.
Returns true if its required to reload the connection between requests for development mode. This is not the case for Ruby/MySQL and it's not necessary for any adapters except SQLite.
Reset the state of this connection, directing the DBMS to clear transactions and other connection-related server-side state. Usually a database-dependent operation.
The default implementation does nothing; the implementation should be overridden by concrete adapters.
Returns a bind substitution value given a column and list of
current binds.
Does this adapter support using DISTINCT within COUNT? This is
true for all adapters except sqlite.
Does this adapter support DDL rollbacks in transactions? That is, would CREATE TABLE or ALTER TABLE get rolled back by a transaction? PostgreSQL, SQL Server, and others support this. MySQL and others do not.
Does this adapter support explain? As of this writing sqlite3, mysql2, and postgresql are the only ones that do.
Does this adapter support database extensions? As of this writing only postgresql does.
Does this adapter support index sort order?
Does this adapter support migrations? Backend specific, as the abstract
adapter always returns false.
Does this adapter support partial indices?
Can this adapter determine the primary key for tables not attached to an
Active Record class, such as join tables? Backend specific, as the abstract
adapter always returns false.
Does this adapter support savepoints? PostgreSQL and MySQL do, SQLite < 3.6.8 does not.
Does this adapter support setting the isolation level for a transaction?
# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 402 def transaction_joinable=(joinable) message = "#transaction_joinable= is deprecated. Please pass the :joinable option to #begin_transaction instead." ActiveSupport::Deprecation.warn message @transaction.joinable = joinable end
# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 201 def unprepared_statement old_prepared_statements, @prepared_statements = @prepared_statements, false old_visitor, @visitor = @visitor, unprepared_visitor yield ensure @visitor, @prepared_statements = old_visitor, old_prepared_statements end
Checks whether the connection to the database is still active (i.e. not
stale). This is done under the hood by calling active?. If the
connection is no longer active, then this method will reconnect to the
database.
# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 436 def log(sql, name = "SQL", binds = []) @instrumenter.instrument( "sql.active_record", :sql => sql, :name => name, :connection_id => object_id, :binds => binds) { yield } rescue => e message = "#{e.class.name}: #{e.message}: #{sql}" @logger.error message if @logger exception = translate_exception(e, message) exception.set_backtrace e.backtrace raise exception end