aboutsummaryrefslogtreecommitdiffstats
path: root/activerecord/lib/active_record/locking/optimistic.rb
blob: 6265be8681a478dbd899a1936f9b86bab69cbf2d (plain) (blame)
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
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.
    #
    # 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
      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, self.class.columns_hash[lock_col]))
              )
            ).arel.compile_update(arel_attributes_values(false, false, 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?

          destroy_associations

          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

        def locking_column=(value)
          @original_locking_column = @locking_column if defined?(@locking_column)
          @locking_column          = value.to_s
        end

        # Set the column to use for optimistic locking. Defaults to +lock_version+.
        def set_locking_column(value = nil, &block)
          deprecated_property_setter :locking_column, value, block
        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

        def original_locking_column #:nodoc:
          deprecated_original_property_getter :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, options = {}) #:nodoc:
          if attributes.key?(locking_column) && lock_optimistically
            attributes[locking_column] ||= 0
          end

          attributes
        end
      end
    end
  end
end