r4664@asus: jeremy | 2006-06-19 18:55:36 -0700

 Use the #lock method to obtain a row lock on a single record. Simply reloads the record with :lock => true.


git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@4462 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

2 files changed, 166 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
--- /dev/null
+++ b/activerecord/lib/active_record/locking/optimistic.rb
@@ -0,0 +1,89 @@
+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
diff --git a/activerecord/lib/active_record/locking/pessimistic.rb b/activerecord/lib/active_record/locking/pessimistic.rb
new file mode 100644
index 0000000000..caad760742
--- /dev/null
+++ b/activerecord/lib/active_record/locking/pessimistic.rb
@@ -0,0 +1,77 @@
+# Copyright (c) 2006 Shugo Maeda <shugo@ruby-lang.org>
+#
+# Permission is hereby granted, free of charge, to any person obtaining
+# a copy of this software and associated documentation files (the
+# "Software"), to deal in the Software without restriction, including
+# without limitation the rights to use, copy, modify, merge, publish,
+# distribute, sublicense, and/or sell copies of the Software, and to
+# permit persons to whom the Software is furnished to do so, subject
+# to the following conditions:
+#
+# The above copyright notice and this permission notice shall be
+# included in all copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+# IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR
+# ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF
+# CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+# WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+
+module ActiveRecord
+  module Locking
+    # Locking::Pessimistic provides support for row-level locking using
+    # SELECT ... FOR UPDATE and other lock types.
+    #
+    # Pass :lock => true to ActiveRecord::Base.find to obtain an exclusive
+    # lock on the selected rows:
+    #   # select * from accounts where id=1 for update
+    #   Account.find(1, :lock => true)
+    #
+    # Pass :lock => 'some locking clause' to give a database-specific locking clause
+    # of your own such as 'LOCK IN SHARE MODE' or 'FOR UPDATE NOWAIT'.
+    #
+    # Example:
+    #   Account.transaction do
+    #     # select * from accounts where name = 'shugo' limit 1 for update
+    #     shugo = Account.find(:first, :conditions => "name = 'shugo'", :lock => true)
+    #     yuko = Account.find(:first, :conditions => "name = 'yuko'", :lock => true)
+    #     shugo.balance -= 100
+    #     shugo.save!
+    #     yuko.balance += 100
+    #     yuko.save!
+    #   end
+    #
+    # You can also use ActiveRecord::Base#lock! method to lock one record by id.
+    # This may be better if you don't need to lock every row. Example:
+    #   Account.transaction do
+    #     # select * from accounts where ...
+    #     accounts = Account.find(:all, :conditions => ...)
+    #     account1 = accounts.detect { |account| ... }
+    #     account2 = accounts.detect { |account| ... }
+    #     # select * from accounts where id=? for update
+    #     account1.lock!
+    #     account2.lock!
+    #     account1.balance -= 100
+    #     account1.save!
+    #     account2.balance += 100
+    #     account2.save!
+    #   end
+    #
+    # Database-specific information on row locking:
+    #   MySQL: http://dev.mysql.com/doc/refman/5.1/en/innodb-locking-reads.html
+    #   PostgreSQL: http://www.postgresql.org/docs/8.1/interactive/sql-select.html#SQL-FOR-UPDATE-SHARE
+    module Pessimistic
+      # Obtain a row lock on this record. Reloads the record to obtain the requested
+      # lock. Pass an SQL locking clause to append the end of the SELECT statement
+      # or pass true for "FOR UPDATE" (the default, an exclusive row lock).  Returns
+      # the locked record.
+      def lock!(lock = true)
+        reload(:lock => lock) unless new_record?
+        self
+      end
+    end
+  end
+end