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
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
|
# frozen_string_literal: true
module ActiveRecord
module Locking
# == What is Optimistic Locking
#
# Optimistic locking allows multiple users to access the same record for edits, and assumes a minimum of
# conflicts with the data. It does this by checking whether another process has made changes to a record since
# it was opened, an <tt>ActiveRecord::StaleObjectError</tt> exception is thrown if that has occurred
# and the update is ignored.
#
# Check out <tt>ActiveRecord::Locking::Pessimistic</tt> for an alternative.
#
# == Usage
#
# Active Record supports optimistic locking if the +lock_version+ field is present. Each update to the
# record increments the +lock_version+ column and the locking facilities ensure that records instantiated twice
# will let the last one saved raise a +StaleObjectError+ if the first was also updated. Example:
#
# p1 = Person.find(1)
# p2 = Person.find(1)
#
# p1.first_name = "Michael"
# p1.save
#
# p2.first_name = "should fail"
# p2.save # Raises an ActiveRecord::StaleObjectError
#
# Optimistic locking will also check for stale data when objects are destroyed. Example:
#
# p1 = Person.find(1)
# p2 = Person.find(1)
#
# p1.first_name = "Michael"
# p1.save
#
# p2.destroy # Raises an ActiveRecord::StaleObjectError
#
# You're then responsible for dealing with the conflict by rescuing the exception and either rolling back, merging,
# or otherwise apply the business logic needed to resolve the conflict.
#
# This locking mechanism will function inside a single Ruby process. To make it work across all
# web requests, the recommended approach is to add +lock_version+ as a hidden field to your form.
#
# This behavior can be turned off by setting <tt>ActiveRecord::Base.lock_optimistically = false</tt>.
# To override the name of the +lock_version+ column, set the <tt>locking_column</tt> class attribute:
#
# class Person < ActiveRecord::Base
# self.locking_column = :lock_person
# end
#
module Optimistic
extend ActiveSupport::Concern
included do
class_attribute :lock_optimistically, instance_writer: false, default: true
end
def locking_enabled? #:nodoc:
self.class.locking_enabled?
end
private
def _create_record(attribute_names = self.attribute_names)
if locking_enabled?
# We always want to persist the locking version, even if we don't detect
# a change from the default, since the database might have no default
attribute_names |= [self.class.locking_column]
end
super
end
def _touch_row(attribute_names, time)
@_touch_attr_names << self.class.locking_column if locking_enabled?
super
end
def _update_row(attribute_names, attempted_action = "update")
return super unless locking_enabled?
begin
locking_column = self.class.locking_column
previous_lock_value = read_attribute_before_type_cast(locking_column)
attribute_names << locking_column
self[locking_column] += 1
affected_rows = self.class._update_record(
attributes_with_values(attribute_names),
@primary_key => id_in_database,
locking_column => previous_lock_value
)
if affected_rows != 1
raise ActiveRecord::StaleObjectError.new(self, attempted_action)
end
affected_rows
# If something went wrong, revert the locking_column value.
rescue Exception
self[locking_column] = previous_lock_value.to_i
raise
end
end
def destroy_row
return super unless locking_enabled?
locking_column = self.class.locking_column
affected_rows = self.class._delete_record(
@primary_key => id_in_database,
locking_column => read_attribute_before_type_cast(locking_column)
)
if affected_rows != 1
raise ActiveRecord::StaleObjectError.new(self, "destroy")
end
affected_rows
end
module ClassMethods
DEFAULT_LOCKING_COLUMN = "lock_version"
# Returns true if the +lock_optimistically+ flag is set to true
# (which it is, by default) and the table includes the
# +locking_column+ column (defaults to +lock_version+).
def locking_enabled?
lock_optimistically && columns_hash[locking_column]
end
# Set the column to use for optimistic locking. Defaults to +lock_version+.
def locking_column=(value)
reload_schema_from_cache
@locking_column = value.to_s
end
# The version column used for optimistic locking. Defaults to +lock_version+.
def locking_column
@locking_column = DEFAULT_LOCKING_COLUMN unless defined?(@locking_column)
@locking_column
end
# Reset the column used for optimistic locking back to the +lock_version+ default.
def reset_locking_column
self.locking_column = DEFAULT_LOCKING_COLUMN
end
# Make sure the lock version column gets updated when counters are
# updated.
def update_counters(id, counters)
counters = counters.merge(locking_column => 1) if locking_enabled?
super
end
private
# We need to apply this decorator here, rather than on module inclusion. The closure
# created by the matcher would otherwise evaluate for `ActiveRecord::Base`, not the
# sub class being decorated. As such, changes to `lock_optimistically`, or
# `locking_column` would not be picked up.
def inherited(subclass)
subclass.class_eval do
is_lock_column = ->(name, _) { lock_optimistically && name == locking_column }
decorate_matching_attribute_types(is_lock_column, "_optimistic_locking") do |type|
LockingType.new(type)
end
end
super
end
end
end
# In de/serialize we change `nil` to 0, so that we can allow passing
# `nil` values to `lock_version`, and not result in `ActiveRecord::StaleObjectError`
# during update record.
class LockingType < DelegateClass(Type::Value) # :nodoc:
def deserialize(value)
super.to_i
end
def serialize(value)
super.to_i
end
def init_with(coder)
__setobj__(coder["subtype"])
end
def encode_with(coder)
coder["subtype"] = __getobj__
end
end
end
end
|
