1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
|
require 'active_record/vendor/simple.rb'
Transaction::Simple.send(:remove_method, :transaction)
require 'thread'
module ActiveRecord
module Transactions # :nodoc:
TRANSACTION_MUTEX = Mutex.new
class TransactionError < ActiveRecordError # :nodoc:
end
def self.append_features(base)
super
base.extend(ClassMethods)
base.class_eval do
alias_method :destroy_without_transactions, :destroy
alias_method :destroy, :destroy_with_transactions
alias_method :save_without_transactions, :save
alias_method :save, :save_with_transactions
end
end
# Transactions are protective blocks where SQL statements are only permanent if they can all succeed as one atomic action.
# The classic example is a transfer between two accounts where you can only have a deposit if the withdrawal succeeded and
# vice versa. Transactions enforce the integrity of the database and guard the data against program errors or database break-downs.
# So basically you should use transaction blocks whenever you have a number of statements that must be executed together or
# not at all. Example:
#
# transaction do
# david.withdrawal(100)
# mary.deposit(100)
# end
#
# This example will only take money from David and give to Mary if neither +withdrawal+ nor +deposit+ raises an exception.
# Exceptions will force a ROLLBACK that returns the database to the state before the transaction was begun. Be aware, though,
# that the objects by default will _not_ have their instance data returned to their pre-transactional state.
#
# == Transactions are not distributed across database connections
#
# A transaction acts on a single database connection. If you have
# multiple class-specific databases, the transaction will not protect
# interaction among them. One workaround is to begin a transaction
# on each class whose models you alter:
#
# Student.transaction do
# Course.transaction do
# course.enroll(student)
# student.units += course.units
# end
# end
#
# This is a poor solution, but full distributed transactions are beyond
# the scope of Active Record.
#
# == Save and destroy are automatically wrapped in a transaction
#
# Both Base#save and Base#destroy come wrapped in a transaction that ensures that whatever you do in validations or callbacks
# will happen under the protected cover of a transaction. So you can use validations to check for values that the transaction
# depend on or you can raise exceptions in the callbacks to rollback.
#
# == Object-level transactions
#
# You can enable object-level transactions for Active Record objects, though. You do this by naming each of the Active Records
# that you want to enable object-level transactions for, like this:
#
# Account.transaction(david, mary) do
# david.withdrawal(100)
# mary.deposit(100)
# end
#
# If the transaction fails, David and Mary will be returned to their pre-transactional state. No money will have changed hands in
# neither object nor database.
#
# == Exception handling
#
# Also have in mind that exceptions thrown within a transaction block will be propagated (after triggering the ROLLBACK), so you
# should be ready to catch those in your application code.
#
# Tribute: Object-level transactions are implemented by Transaction::Simple by Austin Ziegler.
module ClassMethods
def transaction(*objects, &block)
previous_handler = trap('TERM') { raise TransactionError, "Transaction aborted" }
lock_mutex
begin
objects.each { |o| o.extend(Transaction::Simple) }
objects.each { |o| o.start_transaction }
result = connection.transaction(Thread.current['start_db_transaction'], &block)
objects.each { |o| o.commit_transaction }
return result
rescue Exception => object_transaction_rollback
objects.each { |o| o.abort_transaction }
raise
ensure
unlock_mutex
trap('TERM', previous_handler)
end
end
def lock_mutex#:nodoc:
Thread.current['open_transactions'] ||= 0
TRANSACTION_MUTEX.lock if Thread.current['open_transactions'] == 0
Thread.current['start_db_transaction'] = (Thread.current['open_transactions'] == 0)
Thread.current['open_transactions'] += 1
end
def unlock_mutex#:nodoc:
Thread.current['open_transactions'] -= 1
TRANSACTION_MUTEX.unlock if Thread.current['open_transactions'] == 0
end
end
def transaction(*objects, &block)
self.class.transaction(*objects, &block)
end
def destroy_with_transactions #:nodoc:
transaction { destroy_without_transactions }
end
def save_with_transactions(perform_validation = true) #:nodoc:
transaction { save_without_transactions(perform_validation) }
end
end
end
|
