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, # will emulate the
effect of nested transactions, by using savepoints: dev.mysql.com/doc/refman/5.0/en/savepoints.html
Savepoints are supported by MySQL and PostgreSQL, but not SQLite3.
It is safe to call this method if a database transaction is already open,
i.e. if # is
called within another # block. In case of
a nested call, #
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 # call
that you know might create a savepoint. Otherwise, # 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
113: def transaction(options = {})
114: options.assert_valid_keys :requires_new, :joinable
115:
116: last_transaction_joinable = defined?(@transaction_joinable) ? @transaction_joinable : nil
117: if options.has_key?(:joinable)
118: @transaction_joinable = options[:joinable]
119: else
120: @transaction_joinable = true
121: end
122: requires_new = options[:requires_new] || !last_transaction_joinable
123:
124: transaction_open = false
125: @_current_transaction_records ||= []
126:
127: begin
128: if block_given?
129: if requires_new || open_transactions == 0
130: if open_transactions == 0
131: begin_db_transaction
132: elsif requires_new
133: create_savepoint
134: end
135: increment_open_transactions
136: transaction_open = true
137: @_current_transaction_records.push([])
138: end
139: yield
140: end
141: rescue Exception => database_transaction_rollback
142: if transaction_open && !outside_transaction?
143: transaction_open = false
144: decrement_open_transactions
145: if open_transactions == 0
146: rollback_db_transaction
147: rollback_transaction_records(true)
148: else
149: rollback_to_savepoint
150: rollback_transaction_records(false)
151: end
152: end
153: raise unless database_transaction_rollback.is_a?(ActiveRecord::Rollback)
154: end
155: ensure
156: @transaction_joinable = last_transaction_joinable
157:
158: if outside_transaction?
159: @open_transactions = 0
160: elsif transaction_open
161: decrement_open_transactions
162: begin
163: if open_transactions == 0
164: commit_db_transaction
165: commit_transaction_records
166: else
167: release_savepoint
168: save_point_records = @_current_transaction_records.pop
169: unless save_point_records.blank?
170: @_current_transaction_records.push([]) if @_current_transaction_records.empty?
171: @_current_transaction_records.last.concat(save_point_records)
172: end
173: end
174: rescue Exception => database_transaction_rollback
175: if open_transactions == 0
176: rollback_db_transaction
177: rollback_transaction_records(true)
178: else
179: rollback_to_savepoint
180: rollback_transaction_records(false)
181: end
182: raise
183: end
184: end
185: end
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