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
|
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 ActiveRecord::StaleObjectError is thrown if that has occurred and the update is ignored.
#
# Check out ActiveRecord::Locking::Pessimistic for an alternative.
#
# == Usage
#
# Active Records support optimistic locking if the field <tt>lock_version</tt> 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.
#
# You must ensure that your database schema defaults the lock_version column to 0.
#
# This behavior can be turned off by setting <tt>ActiveRecord::Base.lock_optimistically = false</tt>.
# To override the name of the lock_version column, invoke the <tt>set_locking_column</tt> method.
# This method uses the same syntax as <tt>set_table_name</tt>
module Optimistic
def self.included(base) #:nodoc:
base.extend ClassMethods
base.cattr_accessor :lock_optimistically, :instance_writer => false
base.lock_optimistically = true
base.alias_method_chain :update, :lock
base.alias_method_chain :destroy, :lock
base.alias_method_chain :attributes_from_column_definition, :lock
class << base
alias_method :locking_column=, :set_locking_column
end
end
def locking_enabled? #:nodoc:
self.class.locking_enabled?
end
private
def attributes_from_column_definition_with_lock
result = attributes_from_column_definition_without_lock
# If the locking column has no default value set,
# start the lock version at zero. Note we can't use
# locking_enabled? at this point as @attributes may
# not have been initialized yet
if lock_optimistically && result.include?(self.class.locking_column)
result[self.class.locking_column] ||= 0
end
return result
end
def update_with_lock(attribute_names = @attributes.keys) #:nodoc:
return update_without_lock(attribute_names) unless locking_enabled?
return 0 if attribute_names.empty?
lock_col = self.class.locking_column
previous_value = send(lock_col).to_i
send(lock_col + '=', previous_value + 1)
attribute_names += [lock_col]
attribute_names.uniq!
begin
affected_rows = table.where(
table[self.class.primary_key].eq(quoted_id).and(
table[self.class.locking_column].eq(quote_value(previous_value))
)
).update(arel_attributes_values(false, false, attribute_names))
unless affected_rows == 1
raise ActiveRecord::StaleObjectError, "Attempted to update a stale object"
end
affected_rows
# If something went wrong, revert the version.
rescue Exception
send(lock_col + '=', previous_value)
raise
end
end
def destroy_with_lock #:nodoc:
return destroy_without_lock unless locking_enabled?
unless new_record?
lock_col = self.class.locking_column
previous_value = send(lock_col).to_i
affected_rows = table.where(
table[self.class.primary_key].eq(quoted_id).and(
table[self.class.locking_column].eq(quote_value(previous_value))
)
).delete
unless affected_rows == 1
raise ActiveRecord::StaleObjectError, "Attempted to delete a stale object"
end
end
freeze
end
module ClassMethods
DEFAULT_LOCKING_COLUMN = 'lock_version'
def self.extended(base)
class <<base
alias_method_chain :update_counters, :lock
end
end
# Is optimistic locking enabled for this table? 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 set_locking_column(value = nil, &block)
define_attr_method :locking_column, value, &block
value
end
# The version column used for optimistic locking. Defaults to +lock_version+.
def locking_column
reset_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
set_locking_column DEFAULT_LOCKING_COLUMN
end
# Make sure the lock version column gets updated when counters are
# updated.
def update_counters_with_lock(id, counters)
counters = counters.merge(locking_column => 1) if locking_enabled?
update_counters_without_lock(id, counters)
end
end
end
end
end
|