transactionブロックの中で登録・更新処理を行う場合は、saveやupdateではなく、save!, update!を使用する。 | on GitHub. ". That is, do not execute statements like 'CREATE TABLE' inside such blocks. For more information about deadlocks in Rails, I recommend this amazing post from Brightbox. When transaction is finished and tries to release the savepoint it created earlier, a database error will occur because the savepoint has already been automatically released. `save` to persist the changes, or `reload` to discard them end You can start a transaction and acquire the lock in one go by calling with_lock with a block. It’s a little old but most (if not all of it) still holds true today. Rails transactions are a way to ensure that a set of database operations will only occur if all of them succeed.
If anything goes wrong, the database rolls back to the beginning of the sub-transaction without rolling back the parent transaction. This way if any error happens inside the transaction block the entire operations will be roll-backed to the previous state. account2.lock!
ActiveRecord::StatementInvalid exceptions indicate that an error occurred at the database level, for example when a unique constraint is violated.
Shortcut for after_commit :hook, on: :destroy. For example, if you try to update the index of a search engine in after_save the indexer won't see the updated record. Transactions reset the state of records through a process called a rollback. Exceptions will force a ROLLBACK that returns the database to the state before the transaction began. But will still be tied to the same database connection (see the bonus tips for a little more insight on this). Our examples will… See the ConnectionAdapters::DatabaseStatements#transaction API docs. Additionally, Rails already wraps the #save and #destroy methods in a transaction, therefore a transaction is never needed when updating a single record. We tried to do an invalid operation! In this case, you only want Ted to receive money if John loses the same money. The classic example is a transfer between two accounts where you can only have a deposit if the withdrawal succeeded and vice versa.
Since these exceptions are captured in transaction blocks, the parent block does not see it and the real transaction is committed. On some database systems, such as PostgreSQL, database errors inside a transaction cause the entire transaction to become unusable until it's restarted from the beginning. Be aware, though, that the objects will not have their instance data returned to their pre-transactional state.
Since we are using the ! So basically you should use transaction blocks whenever you have a number of statements that must be executed together or not at all. And as long as the transaction block is running this one database connection is open. Our examples will demonstrate it in the most useful scenario for transactions: money transfers. In this example a balance record is transactionally saved even though transaction is called on the Account class: The transaction method is also available as a model instance method. This error is thrown when for some reason the changes you want to make in the records would turn them invalid. Wraps the passed block in a transaction, locking the object before yielding. Shortcut for after_commit :hook, on: :create. So you can use validations to check for values that the transaction depends on or you can raise exceptions in the callbacks to rollback, including after_* callbacks. Reason is the ActiveRecord::Rollback exception in the nested block does not issue a ROLLBACK. method to lock one record by id. So try to do as little as needed inside the transaction block, otherwise, you will be blocking a database connection for more time than you should. Otherwise they will rollback to the previous state of data. methods from ActiveRecord, we should expect some errors to occur. Reloads the record to obtain the requested lock.
In order to get a ROLLBACK for the nested transaction you may ask for a real sub-transaction by passing requires_new: true. The following example demonstrates the problem: Note that âTRUNCATEâ is also a MySQL DDL statement! Because of this, Active Record emulates nested transactions by using savepoints on MySQL and PostgreSQL. Here is an example which demonstrates the problem: One should restart the entire transaction if an ActiveRecord::StatementInvalid occurred. account2.balance += 100 account2.save! after_rollback callbacks are called on every record saved or destroyed within a transaction immediately after the transaction or savepoint is rolled back. Example: You can start a transaction and acquire the lock in one go by calling with_lock with a block. account1.balance -= 100 account1.save! By default, this makes all database statements in the nested transaction block become part of the parent transaction. Rails transactions are a way to ensure that a set of database operations will only occur if all of them succeed.
# File activerecord/lib/active_record/locking/pessimistic.rb, line 63. Both #save and #destroy come wrapped in a transaction that ensures that whatever you do in validations or callbacks will happen under its protected cover.
In this case, each transaction will happen and rollback independently of one another.
The block is called from within a transaction, the object is already locked. Active Record Transactions. Though the transaction class method is called on some Active Record class, the objects within the transaction block need not all be instances of that class. If you're on MySQL, then do not use Data Definition Language (DDL) operations in nested transactions blocks that are emulated with savepoints. transaction calls can be nested. Locking a record with unpersisted changes is not supported. As a consequence changes to the database are not seen outside your connection until the operation is complete. SQLite3 version >= '3.6.8' also supports it. Let’s have a look at the code below: This code would start locking every single user in your database until the whole transaction finishes.
If you have multiple class-specific databases, the transaction will not protect interaction among them. show Locking::Pessimistic provides support for row-level locking using SELECT ⦠FOR UPDATE and other lock types. after_commit callbacks are called on every record saved or destroyed within a transaction immediately after the transaction is committed. You can pass the SQL locking clause as argument (see lock!). In this case, you only want Ted to receive money if John loses the same money. We take care of this rescuing from the RecordInvalid error and printing out a friendly message to our users. One thing to take note is that the ActiveRecord::Rollback error does not actually raise an error, it is just used to trigger the rollback from the inside of a transaction. This is why we use the .update! Rails transactions are tied to one database connection. Sometimes we want to cancel the transaction manually. Like in the following example: In this scenario, we raise an error according to the business logic of our application. One exception is the ActiveRecord::Rollback exception, which will trigger a ROLLBACK when raised, but not be re-raised by the transaction block. But we will learn how to treat that in the next topic.