aboutsummaryrefslogtreecommitdiffstats
path: root/activerecord/lib/active_record/locking/optimistic.rb
blob: 65f88cfdc76290b36972a3f51348e884c7b0f042 (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
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
    #
    # 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 :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?

          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 = connection.update(<<-end_sql, "#{self.class.name} Update with optimistic locking")
              UPDATE #{self.class.quoted_table_name}
              SET #{quoted_comma_pair_list(connection, attributes_with_quotes(false, false, attribute_names))}
              WHERE #{self.class.primary_key} = #{quote_value(id)}
              AND #{self.class.quoted_locking_column} = #{quote_value(previous_value)}
            end_sql

            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

      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