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
|
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 Records support optimistic locking if the field +lock_version+ 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 a 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 a 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
config_attribute :lock_optimistically, :global => true
self.lock_optimistically = true
end
def locking_enabled? #:nodoc:
self.class.locking_enabled?
end
private
def increment_lock
lock_col = self.class.locking_column
previous_lock_value = send(lock_col).to_i
send(lock_col + '=', previous_lock_value + 1)
end
def update(attribute_names = @attributes.keys) #:nodoc:
return super unless locking_enabled?
return 0 if attribute_names.empty?
lock_col = self.class.locking_column
previous_lock_value = send(lock_col).to_i
increment_lock
attribute_names += [lock_col]
attribute_names.uniq!
begin
relation = self.class.unscoped
stmt = relation.where(
relation.table[self.class.primary_key].eq(id).and(
relation.table[lock_col].eq(quote_value(previous_lock_value))
)
).arel.compile_update(arel_attributes_with_values_for_update(attribute_names))
affected_rows = connection.update stmt
unless affected_rows == 1
raise ActiveRecord::StaleObjectError.new(self, "update")
end
affected_rows
# If something went wrong, revert the version.
rescue Exception
send(lock_col + '=', previous_lock_value)
raise
end
end
def destroy #:nodoc:
return super unless locking_enabled?
if persisted?
table = self.class.arel_table
lock_col = self.class.locking_column
predicate = table[self.class.primary_key].eq(id).
and(table[lock_col].eq(send(lock_col).to_i))
affected_rows = self.class.unscoped.where(predicate).delete_all
unless affected_rows == 1
raise ActiveRecord::StaleObjectError.new(self, "destroy")
end
end
@destroyed = true
freeze
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)
@locking_column = value.to_s
end
# The version column used for optimistic locking. Defaults to +lock_version+.
def locking_column
reset_locking_column unless defined?(@locking_column)
@locking_column
end
# Quote the column name used for optimistic locking.
def quoted_locking_column
connection.quote_column_name(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
# If the locking column has no default value set,
# start the lock version at zero. Note we can't use
# <tt>locking_enabled?</tt> at this point as
# <tt>@attributes</tt> may not have been initialized yet.
def initialize_attributes(attributes) #:nodoc:
if attributes.key?(locking_column) && lock_optimistically
attributes[locking_column] ||= 0
end
attributes
end
end
end
end
end
|
