Resolving Rails Runtime Errors: A Guide to Handling Unpersisted Changes with with_lock

UpdatedJanuary 14, 2025

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Resolving Rails Runtime Errors: A Guide to Handling Unpersisted Changes with with_lock

Lynn Chang

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Lynn's Dev Blog

20 posts

Introduction

In Ruby on Rails applications, handling database transactions efficiently is crucial to maintaining data integrity. One common issue developers face is dealing with runtime errors caused by unpersisted changes when using the with_lock method. This guide will help you understand and resolve these errors effectively.

How to Reproduce

class Invoice < ApplicationRecord before_save :sync_monetary_columns # Existing code... private def sync_monetary_columns return unless amount_changed? with_lock do self.amount_decimal = amount.to_f end end end

invoice = Invoice.last invoice.amount # 1000 invoice.amount = 500 invoice.save! # RuntimeError: Locking a record with unpersisted changes is not supported. # Use `save` to persist the changes, or `reload` to discard them explicitly.

Understanding how with_lock Works

with_lock is a Rails method that provides pessimistic locking, which is a database-level lock that prevents multiple processes from modifying the same record simultaneously. Here's how it works:

When with_lock is called, Rails:

Starts a database transaction
Reloads the record with SELECT ... FOR UPDATE
Executes your block of code
Commits the transaction

The FOR UPDATE lock means:

Other transactions must wait to read or modify this record
Prevents race conditions where multiple processes might try to update the same data
Lock is held until the transaction commits or rolls back

def with_lock transaction do # Reloads the object and obtains lock: self.reload(lock: true) # SELECT * FROM table WHERE id = 123 FOR UPDATE # Now yields to your block: yield # Transaction commits after your block completes end end

How to Fix This Error

To fix the runtime error caused by unpersisted changes, here’s my key implementation details:

Cache Values Before with_lock:

with_lock internally calls reload, which would clear any unpersisted changes. (Source code from Rails)
Caching preserves the values we need to modify.

Reload Only Persisted Records:

For persisted records: with_lock executes reload(lock: true) using the record's ID. When a record has unpersisted changes, this causes the RuntimeError.
For new records: No ID exists yet, so reload cannot trigger the error.

This solution ensures proper handling of both new and persisted records while maintaining data integrity during the locking process.

class Invoice < ApplicationRecord before_save :sync_monetary_columns # Existing code... private def sync_monetary_columns return unless amount_changed? # Cache values new_amount = amount.to_of reload if persisted? with_lock do self.amount_decimal = new_amount end end end

Resolving Rails Runtime Errors: A Guide to Handling Unpersisted Changes with with_lock

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What’s the relationship between require, $LOAD_PATH and $LOADED_FEATURES in Ruby?

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Introduction

Environment

Error message

How to Reproduce

Understanding how `with_lock` Works

How to Fix This Error

Conclusion

Reference

Command Palette

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More from this blog

Introduction

Environment

Error message

How to Reproduce

Understanding how with_lock Works

How to Fix This Error

Conclusion

Reference

Understanding how `with_lock` Works