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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::Base.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 SchemaStatements are very useful.

Namespace
Methods
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Included Modules
Constants
ADAPTER_NAME = "Abstract"
 
COMMENT_REGEX = %r{(?:\-\-.*\n)*|/\*(?:[^\*]|\*[^/])*\*/}m
 
SIMPLE_INT = /\A\d+\z/
 
Attributes
[R] in_use?
[R] lock
[R] logger
[R] owner
[RW] pool
[R] visitor
Class Public methods
database_exists?(config)

Does the database for this adapter exist?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 267
def self.database_exists?(config)
  raise NotImplementedError
end
type_cast_config_to_boolean(config)
# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 58
def self.type_cast_config_to_boolean(config)
  if config == "false"
    false
  else
    config
  end
end
type_cast_config_to_integer(config)
# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 48
def self.type_cast_config_to_integer(config)
  if config.is_a?(Integer)
    config
  elsif SIMPLE_INT.match?(config)
    config.to_i
  else
    config
  end
end
Instance Public methods
active?()

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.

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 470
def active?
end
adapter_name()

Returns the human-readable name of the adapter. Use mixed case - one can always use downcase if needed.

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 262
def adapter_name
  self.class::ADAPTER_NAME
end
clear_cache!()

Clear any caching the database adapter may be doing.

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 521
def clear_cache!
  @lock.synchronize { @statements.clear } if @statements
end
close()

Check the connection back in to the connection pool

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 572
def close
  pool.checkin self
end
disable_extension(name)

This is meant to be implemented by the adapters that support extensions

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 423
def disable_extension(name)
end
disable_referential_integrity()

Override to turn off referential integrity while executing &block.

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 461
def disable_referential_integrity
  yield
end
discard!()

Immediately forget this connection ever existed. Unlike disconnect!, this will not communicate with the server.

After calling this method, the behavior of all other methods becomes undefined. This is called internally just before a forked process gets rid of a connection that belonged to its parent.

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 494
def discard!
  # This should be overridden by concrete adapters.
  #
  # Prevent @connection's finalizer from touching the socket, or
  # otherwise communicating with its server, when it is collected.
  if schema_cache.connection == self
    schema_cache.connection = nil
  end
end
disconnect!()

Disconnects from the database if already connected. Otherwise, this method does nothing.

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 483
def disconnect!
  clear_cache!
  reset_transaction
end
enable_extension(name)

This is meant to be implemented by the adapters that support extensions

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 427
def enable_extension(name)
end
expire()

this method must only be called while holding connection pool's mutex

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 219
def expire
  if in_use?
    if @owner != Thread.current
      raise ActiveRecordError, "Cannot expire connection, " \
        "it is owned by a different thread: #{@owner}. " \
        "Current thread: #{Thread.current}."
    end

    @idle_since = Concurrent.monotonic_time
    @owner = nil
  else
    raise ActiveRecordError, "Cannot expire connection, it is not currently leased."
  end
end
extensions()

A list of extensions, to be filled in by adapters that support them.

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 449
def extensions
  []
end
index_algorithms()

A list of index algorithms, to be filled by adapters that support them.

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 454
def index_algorithms
  {}
end
lease()

this method must only be called while holding connection pool's mutex

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 190
def lease
  if in_use?
    msg = +"Cannot lease connection, "
    if @owner == Thread.current
      msg << "it is already leased by the current thread."
    else
      msg << "it is already in use by a different thread: #{@owner}. " \
             "Current thread: #{Thread.current}."
    end
    raise ActiveRecordError, msg
  end

  @owner = Thread.current
end
prefetch_primary_key?(table_name = nil)

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.

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 294
def prefetch_primary_key?(table_name = nil)
  false
end
prepared_statements()
# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 158
def prepared_statements
  @prepared_statements && !prepared_statements_disabled_cache.include?(object_id)
end
preventing_writes?()

Determines whether writes are currently being prevents.

Returns true if the connection is a replica.

If the application is using legacy handling, returns true if `connection_handler.prevent_writes` is set.

If the application is using the new connection handling will return true based on `current_preventing_writes`.

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 123
def preventing_writes?
  return true if replica?
  return ActiveRecord::Base.connection_handler.prevent_writes if ActiveRecord::Base.legacy_connection_handling
  return false if owner_name.nil?

  klass = self.owner_name.safe_constantize
  klass&.current_preventing_writes
end
raw_connection()

Provides access to the underlying database driver for this adapter. For example, this method returns a Mysql2::Client object in case of Mysql2Adapter, and a PG::Connection object in case of PostgreSQLAdapter.

This is useful for when you need to call a proprietary method such as PostgreSQL's lo_* methods.

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 543
def raw_connection
  disable_lazy_transactions!
  @connection
end
reconnect!()

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.

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 476
def reconnect!
  clear_cache!
  reset_transaction
end
replica?()
# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 106
def replica?
  @config[:replica] || false
end
requires_reloading?()

Returns true if its required to reload the connection between requests for development mode.

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 526
def requires_reloading?
  false
end
reset!()

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.

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 510
def reset!
  # this should be overridden by concrete adapters
end
schema_cache()
# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 209
def schema_cache
  @pool.get_schema_cache(self)
end
schema_cache=(cache)
# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 213
def schema_cache=(cache)
  cache.connection = self
  @pool.set_schema_cache(cache)
end
supports_advisory_locks?()

Does this adapter support application-enforced advisory locking?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 287
def supports_advisory_locks?
  false
end
supports_bulk_alter?()
# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 277
def supports_bulk_alter?
  false
end
supports_check_constraints?()

Does this adapter support creating check constraints?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 349
def supports_check_constraints?
  false
end
supports_comments?()

Does this adapter support metadata comments on database objects (tables, columns, indexes)?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 374
def supports_comments?
  false
end
supports_comments_in_create?()

Can comments for tables, columns, and indexes be specified in create/alter table statements?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 379
def supports_comments_in_create?
  false
end
supports_common_table_expressions?()
# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 398
def supports_common_table_expressions?
  false
end
supports_datetime_with_precision?()

Does this adapter support datetime with precision?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 364
def supports_datetime_with_precision?
  false
end
supports_ddl_transactions?()

Does this adapter support DDL rollbacks in transactions? That is, would CREATE TABLE or ALTER TABLE get rolled back by a transaction?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 273
def supports_ddl_transactions?
  false
end
supports_explain?()

Does this adapter support explain?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 318
def supports_explain?
  false
end
supports_expression_index?()

Does this adapter support expression indices?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 313
def supports_expression_index?
  false
end
supports_extensions?()

Does this adapter support database extensions?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 328
def supports_extensions?
  false
end
supports_foreign_keys?()

Does this adapter support creating foreign key constraints?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 339
def supports_foreign_keys?
  false
end
supports_foreign_tables?()

Does this adapter support foreign/external tables?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 389
def supports_foreign_tables?
  false
end
supports_index_sort_order?()

Does this adapter support index sort order?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 303
def supports_index_sort_order?
  false
end
supports_indexes_in_create?()

Does this adapter support creating indexes in the same statement as creating the table?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 334
def supports_indexes_in_create?
  false
end
supports_insert_conflict_target?()
# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 418
def supports_insert_conflict_target?
  false
end
supports_insert_on_duplicate_skip?()
# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 410
def supports_insert_on_duplicate_skip?
  false
end
supports_insert_on_duplicate_update?()
# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 414
def supports_insert_on_duplicate_update?
  false
end
supports_insert_returning?()
# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 406
def supports_insert_returning?
  false
end
supports_json?()

Does this adapter support json data type?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 369
def supports_json?
  false
end
supports_lazy_transactions?()
# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 402
def supports_lazy_transactions?
  false
end
supports_materialized_views?()

Does this adapter support materialized views?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 359
def supports_materialized_views?
  false
end
supports_optimizer_hints?()

Does this adapter support optimizer hints?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 394
def supports_optimizer_hints?
  false
end
supports_partial_index?()

Does this adapter support partial indices?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 308
def supports_partial_index?
  false
end
supports_partitioned_indexes?()
# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 298
def supports_partitioned_indexes?
  false
end
supports_savepoints?()

Does this adapter support savepoints?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 282
def supports_savepoints?
  false
end
supports_transaction_isolation?()

Does this adapter support setting the isolation level for a transaction?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 323
def supports_transaction_isolation?
  false
end
supports_validate_constraints?()

Does this adapter support creating invalid constraints?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 344
def supports_validate_constraints?
  false
end
supports_views?()

Does this adapter support views?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 354
def supports_views?
  false
end
supports_virtual_columns?()

Does this adapter support virtual columns?

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 384
def supports_virtual_columns?
  false
end
throw_away!()

Removes the connection from the pool and disconnect it.

# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 515
def throw_away!
  pool.remove self
  disconnect!
end
unprepared_statement()
# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 253
def unprepared_statement
  cache = prepared_statements_disabled_cache.add(object_id) if @prepared_statements
  yield
ensure
  cache&.delete(object_id)
end
use_metadata_table?()
# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 110
def use_metadata_table?
  @config.fetch(:use_metadata_table, true)
end
verify!()

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 533
def verify!
  reconnect! unless active?
end
Instance Private methods
log(sql, name = "SQL", binds = [], type_casted_binds = [], statement_name = nil)
# File activerecord/lib/active_record/connection_adapters/abstract_adapter.rb, line 687
def log(sql, name = "SQL", binds = [], type_casted_binds = [], statement_name = nil) # :doc:
  @instrumenter.instrument(
    "sql.active_record",
    sql:               sql,
    name:              name,
    binds:             binds,
    type_casted_binds: type_casted_binds,
    statement_name:    statement_name,
    connection:        self) do
    @lock.synchronize do
      yield
    end
  rescue => e
    raise translate_exception_class(e, sql, binds)
  end
end