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
130
|
require 'thread'
module ActiveRecord
module Transactions # :nodoc:
class TransactionError < ActiveRecordError # :nodoc:
end
def self.included(base)
base.extend(ClassMethods)
base.class_eval do
[:destroy, :save, :save!].each do |method|
alias_method_chain method, :transactions
end
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 will _not_ have their instance data returned to their pre-transactional state.
#
# == Different Active Record classes in a single transaction
#
# Though the transaction class method is called on some Active Record class,
# the objects within the transaction block need not all be instances of
# that class.
# In this example a <tt>Balance</tt> record is transactionally saved even
# though <tt>transaction</tt> is called on the <tt>Account</tt> class:
#
# Account.transaction do
# balance.save!
# account.save!
# end
#
# == 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
# depends on or you can raise exceptions in the callbacks to rollback.
#
# == 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. One exception is the ActiveRecord::Rollback exception, which will
# trigger a ROLLBACK when raised, but not be re-raised by the transaction block.
module ClassMethods
def transaction(&block)
increment_open_transactions
begin
connection.transaction(Thread.current['start_db_transaction'], &block)
ensure
decrement_open_transactions
end
end
private
def increment_open_transactions #:nodoc:
open = Thread.current['open_transactions'] ||= 0
Thread.current['start_db_transaction'] = open.zero?
Thread.current['open_transactions'] = open + 1
end
def decrement_open_transactions #:nodoc:
Thread.current['open_transactions'] -= 1
end
end
def transaction(&block)
self.class.transaction(&block)
end
def destroy_with_transactions #:nodoc:
transaction { destroy_without_transactions }
end
def save_with_transactions(perform_validation = true) #:nodoc:
rollback_active_record_state! { transaction { save_without_transactions(perform_validation) } }
end
def save_with_transactions! #:nodoc:
rollback_active_record_state! { transaction { save_without_transactions! } }
end
# Reset id and @new_record if the transaction rolls back.
def rollback_active_record_state!
id_present = has_attribute?(self.class.primary_key)
previous_id = id
previous_new_record = new_record?
yield
rescue Exception
@new_record = previous_new_record
if id_present
self.id = previous_id
else
@attributes.delete(self.class.primary_key)
@attributes_cache.delete(self.class.primary_key)
end
raise
end
end
end
|
