236 lines
9.4 KiB
Ruby
236 lines
9.4 KiB
Ruby
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require 'thread'
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module ActiveRecord
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# See ActiveRecord::Transactions::ClassMethods for documentation.
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module Transactions
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class TransactionError < ActiveRecordError # :nodoc:
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end
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def self.included(base)
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base.extend(ClassMethods)
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base.class_eval do
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[:destroy, :save, :save!].each do |method|
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alias_method_chain method, :transactions
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end
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end
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end
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# Transactions are protective blocks where SQL statements are only permanent
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# if they can all succeed as one atomic action. The classic example is a
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# transfer between two accounts where you can only have a deposit if the
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# withdrawal succeeded and vice versa. Transactions enforce the integrity of
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# the database and guard the data against program errors or database
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# break-downs. So basically you should use transaction blocks whenever you
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# have a number of statements that must be executed together or not at all.
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# Example:
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#
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# ActiveRecord::Base.transaction do
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# david.withdrawal(100)
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# mary.deposit(100)
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# end
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#
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# This example will only take money from David and give to Mary if neither
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# +withdrawal+ nor +deposit+ raises an exception. Exceptions will force a
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# ROLLBACK that returns the database to the state before the transaction was
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# begun. Be aware, though, that the objects will _not_ have their instance
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# data returned to their pre-transactional state.
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#
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# == Different Active Record classes in a single transaction
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#
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# Though the transaction class method is called on some Active Record class,
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# the objects within the transaction block need not all be instances of
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# that class. This is because transactions are per-database connection, not
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# per-model.
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#
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# In this example a <tt>Balance</tt> record is transactionally saved even
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# though <tt>transaction</tt> is called on the <tt>Account</tt> class:
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#
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# Account.transaction do
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# balance.save!
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# account.save!
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# end
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#
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# Note that the +transaction+ method is also available as a model instance
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# method. For example, you can also do this:
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#
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# balance.transaction do
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# balance.save!
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# account.save!
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# end
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#
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# == Transactions are not distributed across database connections
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#
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# A transaction acts on a single database connection. If you have
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# multiple class-specific databases, the transaction will not protect
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# interaction among them. One workaround is to begin a transaction
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# on each class whose models you alter:
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#
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# Student.transaction do
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# Course.transaction do
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# course.enroll(student)
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# student.units += course.units
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# end
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# end
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#
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# This is a poor solution, but full distributed transactions are beyond
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# the scope of Active Record.
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#
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# == Save and destroy are automatically wrapped in a transaction
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#
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# Both Base#save and Base#destroy come wrapped in a transaction that ensures
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# that whatever you do in validations or callbacks will happen under the
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# protected cover of a transaction. So you can use validations to check for
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# values that the transaction depends on or you can raise exceptions in the
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# callbacks to rollback, including <tt>after_*</tt> callbacks.
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#
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# == Exception handling and rolling back
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#
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# Also have in mind that exceptions thrown within a transaction block will
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# be propagated (after triggering the ROLLBACK), so you should be ready to
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# catch those in your application code.
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#
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# One exception is the ActiveRecord::Rollback exception, which will trigger
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# a ROLLBACK when raised, but not be re-raised by the transaction block.
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#
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# *Warning*: one should not catch ActiveRecord::StatementInvalid exceptions
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# inside a transaction block. StatementInvalid exceptions indicate that an
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# error occurred at the database level, for example when a unique constraint
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# is violated. On some database systems, such as PostgreSQL, database errors
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# inside a transaction causes the entire transaction to become unusable
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# until it's restarted from the beginning. Here is an example which
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# demonstrates the problem:
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#
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# # Suppose that we have a Number model with a unique column called 'i'.
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# Number.transaction do
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# Number.create(:i => 0)
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# begin
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# # This will raise a unique constraint error...
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# Number.create(:i => 0)
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# rescue ActiveRecord::StatementInvalid
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# # ...which we ignore.
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# end
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#
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# # On PostgreSQL, the transaction is now unusable. The following
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# # statement will cause a PostgreSQL error, even though the unique
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# # constraint is no longer violated:
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# Number.create(:i => 1)
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# # => "PGError: ERROR: current transaction is aborted, commands
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# # ignored until end of transaction block"
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# end
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#
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# One should restart the entire transaction if a StatementError occurred.
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#
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# == Nested transactions
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#
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# #transaction calls can be nested. By default, this makes all database
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# statements in the nested transaction block become part of the parent
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# transaction. For example:
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#
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# User.transaction do
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# User.create(:username => 'Kotori')
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# User.transaction do
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# User.create(:username => 'Nemu')
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# raise ActiveRecord::Rollback
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# end
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# end
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#
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# User.find(:all) # => empty
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#
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# It is also possible to requires a sub-transaction by passing
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# <tt>:requires_new => true</tt>. If anything goes wrong, the
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# database rolls back to the beginning of the sub-transaction
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# without rolling back the parent transaction. For example:
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#
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# User.transaction do
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# User.create(:username => 'Kotori')
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# User.transaction(:requires_new => true) do
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# User.create(:username => 'Nemu')
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# raise ActiveRecord::Rollback
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# end
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# end
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#
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# User.find(:all) # => Returns only Kotori
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#
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# Most databases don't support true nested transactions. At the time of
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# writing, the only database that we're aware of that supports true nested
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# transactions, is MS-SQL. Because of this, Active Record emulates nested
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# transactions by using savepoints. See
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# http://dev.mysql.com/doc/refman/5.0/en/savepoints.html
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# for more information about savepoints.
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#
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# === Caveats
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#
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# If you're on MySQL, then do not use DDL operations in nested transactions
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# blocks that are emulated with savepoints. That is, do not execute statements
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# like 'CREATE TABLE' inside such blocks. This is because MySQL automatically
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# releases all savepoints upon executing a DDL operation. When #transaction
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# is finished and tries to release the savepoint it created earlier, a
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# database error will occur because the savepoint has already been
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# automatically released. The following example demonstrates the problem:
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#
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# Model.connection.transaction do # BEGIN
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# Model.connection.transaction(:requires_new => true) do # CREATE SAVEPOINT active_record_1
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# Model.connection.create_table(...) # active_record_1 now automatically released
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# end # RELEASE savepoint active_record_1
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# # ^^^^ BOOM! database error!
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# end
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module ClassMethods
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# See ActiveRecord::Transactions::ClassMethods for detailed documentation.
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def transaction(options = {}, &block)
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# See the ConnectionAdapters::DatabaseStatements#transaction API docs.
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connection.transaction(options, &block)
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end
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end
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# See ActiveRecord::Transactions::ClassMethods for detailed documentation.
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def transaction(&block)
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self.class.transaction(&block)
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end
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def destroy_with_transactions #:nodoc:
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with_transaction_returning_status(:destroy_without_transactions)
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end
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def save_with_transactions(perform_validation = true) #:nodoc:
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rollback_active_record_state! { with_transaction_returning_status(:save_without_transactions, perform_validation) }
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end
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def save_with_transactions! #:nodoc:
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rollback_active_record_state! { self.class.transaction { save_without_transactions! } }
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end
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# Reset id and @new_record if the transaction rolls back.
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def rollback_active_record_state!
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id_present = has_attribute?(self.class.primary_key)
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previous_id = id
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previous_new_record = new_record?
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yield
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rescue Exception
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@new_record = previous_new_record
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if id_present
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self.id = previous_id
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else
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@attributes.delete(self.class.primary_key)
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@attributes_cache.delete(self.class.primary_key)
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end
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raise
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end
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# Executes +method+ within a transaction and captures its return value as a
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# status flag. If the status is true the transaction is committed, otherwise
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# a ROLLBACK is issued. In any case the status flag is returned.
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#
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# This method is available within the context of an ActiveRecord::Base
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# instance.
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def with_transaction_returning_status(method, *args)
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status = nil
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self.class.transaction do
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status = send(method, *args)
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raise ActiveRecord::Rollback unless status
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end
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status
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end
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end
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end
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