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module ActiveRecord::ConnectionAdapters::DatabaseStatements

Public Class Methods

# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 4
def initialize
  super
  reset_transaction
end
Calls superclass method

Public Instance Methods

add_transaction_record(record) Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 233
def add_transaction_record(record)
  current_transaction.add_record(record)
end

Register a record with the current transaction so that its after_commit and after_rollback callbacks can be called.

begin_db_transaction() Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 242
def begin_db_transaction()    end

Begins the transaction (and turns off auto-committing).

begin_isolated_db_transaction(isolation) Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 256
def begin_isolated_db_transaction(isolation)
  raise ActiveRecord::TransactionIsolationError, "adapter does not support setting transaction isolation"
end

Begins the transaction with the isolation level set. Raises an error by default; adapters that support setting the isolation level should implement this method.

commit_db_transaction() Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 261
def commit_db_transaction()   end

Commits the transaction (and turns on auto-committing).

delete(arel, name = nil, binds = []) Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 118
def delete(arel, name = nil, binds = [])
  exec_delete(to_sql(arel, binds), name, binds)
end

Executes the delete statement and returns the number of rows affected.

exec_delete(sql, name, binds) Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 82
def exec_delete(sql, name, binds)
  exec_query(sql, name, binds)
end

Executes delete sql statement in the context of this connection using binds as the bind substitutes. name is logged along with the executed sql statement.

exec_insert(sql, name, binds, pk = nil, sequence_name = nil) Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 75
def exec_insert(sql, name, binds, pk = nil, sequence_name = nil)
  exec_query(sql, name, binds)
end

Executes insert sql statement in the context of this connection using binds as the bind substitutes. name is logged along with the executed sql statement.

exec_query(sql, name = 'SQL', binds = []) Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 69
def exec_query(sql, name = 'SQL', binds = [])
end

Executes sql statement in the context of this connection using binds as the bind substitutes. name is logged along with the executed sql statement.

exec_update(sql, name, binds) Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 94
def exec_update(sql, name, binds)
  exec_query(sql, name, binds)
end

Executes update sql statement in the context of this connection using binds as the bind substitutes. name is logged along with the executed sql statement.

execute(sql, name = nil) Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 62
def execute(sql, name = nil)
end

Executes the SQL statement in the context of this connection.

insert(arel, name = nil, pk = nil, id_value = nil, sequence_name = nil, binds = []) Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 106
def insert(arel, name = nil, pk = nil, id_value = nil, sequence_name = nil, binds = [])
  sql, binds = sql_for_insert(to_sql(arel, binds), pk, id_value, sequence_name, binds)
  value      = exec_insert(sql, name, binds, pk, sequence_name)
  id_value || last_inserted_id(value)
end

Returns the last auto-generated ID from the affected table.

id_value will be returned unless the value is nil, in which case the database will attempt to calculate the last inserted id and return that value.

If the next id was calculated in advance (as in Oracle), it should be passed in as id_value.

rollback_db_transaction() Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 265
def rollback_db_transaction
  exec_rollback_db_transaction
end

Rolls back the transaction (and turns on auto-committing). Must be done if the transaction block raises an exception or returns false.

select_all(arel, name = nil, binds = []) Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 30
def select_all(arel, name = nil, binds = [])
  arel, binds = binds_from_relation arel, binds
  select(to_sql(arel, binds), name, binds)
end

Returns an ActiveRecord::Result instance.

select_one(arel, name = nil, binds = []) Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 37
def select_one(arel, name = nil, binds = [])
  select_all(arel, name, binds).first
end

Returns a record hash with the column names as keys and column values as values.

select_rows(sql, name = nil, binds = []) Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 57
def select_rows(sql, name = nil, binds = [])
end

Returns an array of arrays containing the field values. Order is the same as that returned by columns.

select_value(arel, name = nil, binds = []) Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 42
def select_value(arel, name = nil, binds = [])
  if result = select_one(arel, name, binds)
    result.values.first
  end
end

Returns a single value from a record

select_values(arel, name = nil) Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 50
def select_values(arel, name = nil)
  arel, binds = binds_from_relation arel, []
  select_rows(to_sql(arel, binds), name, binds).map(&:first)
end

Returns an array of the values of the first column in a select:

select_values("SELECT id FROM companies LIMIT 3") => [1,2,3]
supports_statement_cache?() Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 124
def supports_statement_cache?
  false
end

Returns true when the connection adapter supports prepared statement caching, otherwise returns false

to_sql(arel, binds = []) Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 10
def to_sql(arel, binds = [])
  if arel.respond_to?(:ast)
    collected = visitor.accept(arel.ast, collector)
    collected.compile(binds.dup, self)
  else
    arel
  end
end

Converts an arel AST to SQL

transaction(options = {}) { || ... } Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 204
def transaction(options = {})
  options.assert_valid_keys :requires_new, :joinable, :isolation

  if !options[:requires_new] && current_transaction.joinable?
    if options[:isolation]
      raise ActiveRecord::TransactionIsolationError, "cannot set isolation when joining a transaction"
    end
    yield
  else
    transaction_manager.within_new_transaction(options) { yield }
  end
rescue ActiveRecord::Rollback
  # rollbacks are silently swallowed
end

Runs the given block in a database transaction, and returns the result of the block.

Nested transactions support

Most databases don't support true nested transactions. At the time of writing, the only database that supports true nested transactions that we're aware of, is MS-SQL.

In order to get around this problem, transaction will emulate the effect of nested transactions, by using savepoints: dev.mysql.com/doc/refman/5.0/en/savepoint.html Savepoints are supported by MySQL and PostgreSQL. SQLite3 version >= '3.6.8' supports savepoints.

It is safe to call this method if a database transaction is already open, i.e. if transaction is called within another transaction block. In case of a nested call, transaction will behave as follows:

  • The block will be run without doing anything. All database statements that happen within the block are effectively appended to the already open database transaction.

  • However, if :requires_new is set, the block will be wrapped in a database savepoint acting as a sub-transaction.

Caveats

MySQL doesn't support DDL transactions. If you perform a DDL operation, then any created savepoints will be automatically released. For example, if you've created a savepoint, then you execute a CREATE TABLE statement, then the savepoint that was created will be automatically released.

This means that, on MySQL, you shouldn't execute DDL operations inside a transaction call that you know might create a savepoint. Otherwise, transaction will raise exceptions when it tries to release the already-automatically-released savepoints:

Model.connection.transaction do  # BEGIN
  Model.connection.transaction(requires_new: true) do  # CREATE SAVEPOINT active_record_1
    Model.connection.create_table(...)
    # active_record_1 now automatically released
  end  # RELEASE SAVEPOINT active_record_1  <--- BOOM! database error!
end

Transaction isolation

If your database supports setting the isolation level for a transaction, you can set it like so:

Post.transaction(isolation: :serializable) do
  # ...
end

Valid isolation levels are:

  • :read_uncommitted

  • :read_committed

  • :repeatable_read

  • :serializable

You should consult the documentation for your database to understand the semantics of these different levels:

An ActiveRecord::TransactionIsolationError will be raised if:

  • The adapter does not support setting the isolation level

  • You are joining an existing open transaction

  • You are creating a nested (savepoint) transaction

The mysql, mysql2 and postgresql adapters support setting the transaction isolation level. However, support is disabled for MySQL versions below 5, because they are affected by a bug which means the isolation level gets persisted outside the transaction.

transaction_isolation_levels() Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 244
def transaction_isolation_levels
  {
    read_uncommitted: "READ UNCOMMITTED",
    read_committed:   "READ COMMITTED",
    repeatable_read:  "REPEATABLE READ",
    serializable:     "SERIALIZABLE"
  }
end
transaction_open?() Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 223
def transaction_open?
  current_transaction.open?
end
transaction_state() Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 237
def transaction_state
  current_transaction.state
end
truncate(table_name, name = nil) Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 87
def truncate(table_name, name = nil)
  raise NotImplementedError
end

Executes the truncate statement.

update(arel, name = nil, binds = []) Show source 
# File activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb, line 113
def update(arel, name = nil, binds = [])
  exec_update(to_sql(arel, binds), name, binds)
end

Executes the update statement and returns the number of rows affected.

© 2004–2018 David Heinemeier Hansson
Licensed under the MIT License.