Active Record in Java

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Active Record
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class AddLockVersionToTimesheets < ActiveRecord::Migration def selfup add_column :timesheets, :lock_version, :integer, :default => 0 end def selfdown remove_column :timesheets, :lock_version end end
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Simply adding that lock_version column changes Active Record s behavior Now if the same record is loaded as two different model instances and saved differently, the first instance will win the update, and the second one will cause an ActiveRecord::StaleObjectError to be raised We can illustrate optimistic locking behavior with a simple spec:
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describe Timesheet do it "should lock optimistically" do t1 = Timesheetcreate t2 = Timesheetfind(t1id) t1rate = 250 t2rate = 175 t1saveshould be_true expect { t2save }to raise_error(ActiveRecord::StaleObjectError) end end
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5: Working with Active Record
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The spec passes, because calling save on the second instance raises the expected ActiveRecord::StaleObjectError exception Note that the save method (without the bang) returns false and does not raise exceptions if the save fails due to validation, but other problems such as locking in this case, can indeed cause it to raise exceptions To use a database column named something other than lock_version change the setting using set_locking_column To make the change globally, add the following line to your config/applicationrb:
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configactive_recordset_locking_column = 'alternate_lock_version'
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Like other Active Record settings, you can also change it on a per-model basis with a declaration in your model class:
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class Timesheet < ActiveRecord::Base set_locking_column 'alternate_lock_version' end
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Handling StaleObjectError Now of course, after adding optimistic locking, you don t want to just leave it at that, or the end user who is on the losing end of the collision would simply see an application error screen You should try to handle the StaleObjectError as gracefully as possible Depending on the criticality of the data being updated, you might want to invest time into crafting a user-friendly solution that somehow preserves the changes that the loser was trying to make At minimum, if the data for the update is easily re-creatable, let the user know why their update failed with controller code that looks something like the following:
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def update timesheet = Timesheetfind(params[:id]) timesheetupdate_attributes(params[:timesheet]) # redirect somewhere rescue ActiveRecord::StaleObjectError flash[:error] = "Timesheet was modified while you were editing it" redirect_to [:edit, timesheet] end
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There are some advantages to optimistic locking It doesn t require any special feature in the database, and it is fairly easy to implement As you saw in the example, very little code is required to handle the StaleObjectError The main disadvantages to optimistic locking are that update operations are a bit slower because the lock version must be checked, and the potential for bad user experience, since they don t find out about the failure until after they ve potentially lost data
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55 Database Locking
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552 Pessimistic Locking
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Pessimistic locking requires special database support (built into the major databases) and locks down specific database rows during an update operation It prevents another user from reading data that is about to be updated, in order to prevent them from working with stale data Pessimistic locking works in conjunction with transactions as in the following example:
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Timesheettransaction do t = Timesheetlockfirst tapproved = true tsave! end
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It s also possible to call lock! on an existing model instance, which simply calls reload(:lock => true) under the covers You wouldn t want to do that on an instance with attribute changes since it would cause them to be discarded by the reload If you decide you don t want the lock anymore, you can pass false to the lock! method Pessimistic locking takes place at the database level The SELECT statement generated by Active Record will have a FOR UPDATE (or similar) clause added to it, causing all other connections to be blocked from access to the rows returned by the select statement The lock is released once the transaction is committed There are theoretically situations (Rails process goes boom mid-transaction !) where the lock would not be released until the connection is terminated or times out
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