1 files changed, 89 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/locking/optimistic.rb b/activerecord/lib/active_record/locking/optimistic.rb
new file mode 100644
index 0000000000..7bae573924
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+++ b/activerecord/lib/active_record/locking/optimistic.rb
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+module ActiveRecord
+  module Locking
+    # 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:
+        super
+        base.extend ClassMethods
+
+        base.cattr_accessor :lock_optimistically
+        base.lock_optimistically = true
+
+        base.alias_method_chain :update, :lock
+        class << base
+          alias_method :locking_column=, :set_locking_column
+        end
+      end
+
+      def locking_enabled? #:nodoc:
+        lock_optimistically && respond_to?(self.class.locking_column)
+      end
+
+      def update_with_lock #:nodoc:
+        return update_without_lock unless locking_enabled?
+
+        lock_col = self.class.locking_column
+        previous_value = send(lock_col)
+        send(lock_col + '=', previous_value + 1)
+
+        affected_rows = connection.update(<<-end_sql, "#{self.class.name} Update with optimistic locking")
+          UPDATE #{self.class.table_name}
+          SET #{quoted_comma_pair_list(connection, attributes_with_quotes(false))}
+          WHERE #{self.class.primary_key} = #{quote(id)}
+          AND #{self.class.quoted_locking_column} = #{quote(previous_value)}
+        end_sql
+
+        unless affected_rows == 1
+          raise ActiveRecord::StaleObjectError, "Attempted to update a stale object"
+        end
+
+        return true
+      end
+
+      module ClassMethods
+        DEFAULT_LOCKING_COLUMN = 'lock_version'
+
+        # 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
+      end
+    end
+  end
+end