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
|
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
extend ActiveSupport::Concern
included do
cattr_accessor :lock_optimistically, :instance_writer => false
self.lock_optimistically = true
class << self
alias_method :locking_column=, :set_locking_column
end
end
def locking_enabled? #:nodoc:
self.class.locking_enabled?
end
private
def attributes_from_column_definition
result = super
# 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(attribute_names = @attributes.keys) #:nodoc:
return super 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
relation = self.class.unscoped
affected_rows = relation.where(
relation.table[self.class.primary_key].eq(quoted_id).and(
relation.table[self.class.locking_column].eq(quote_value(previous_value))
)
).arel.update(arel_attributes_values(false, false, attribute_names))
unless affected_rows == 1
raise ActiveRecord::StaleObjectError, "Attempted to update a stale object: #{self.class.name}"
end
affected_rows
# If something went wrong, revert the version.
rescue Exception
send(lock_col + '=', previous_value)
raise
end
end
def destroy #:nodoc:
return super unless locking_enabled?
unless new_record?
lock_col = self.class.locking_column
previous_value = send(lock_col).to_i
table = self.class.arel_table
predicate = table[self.class.primary_key].eq(id)
predicate = predicate.and(table[self.class.locking_column].eq(previous_value))
affected_rows = self.class.unscoped.where(predicate).delete_all
unless affected_rows == 1
raise ActiveRecord::StaleObjectError, "Attempted to delete a stale object: #{self.class.name}"
end
end
freeze
end
module ClassMethods
DEFAULT_LOCKING_COLUMN = 'lock_version'
# 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(id, counters)
counters = counters.merge(locking_column => 1) if locking_enabled?
super
end
end
end
end
end
|
