Active Record Callbacks

7/31/2019 Active Record Callbacks

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Active Record Callbacks


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Use Cases of Callbacks / Observers

Force a Rollback (prevent the model from beingsaved) under certain circumstances

Track Activities, Log Changes, Send

Notifications Ensure the existance of associated objects

Inform associated models about changes

Save Information about the circumstances themodel was saved / created (e.g. date / time)

Trigger a state change

Process raw data into more usabale data (e.g.


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Overview

Creating an Object

before_validation

After_validation

before_save

around_save

before_create

around_create

after_create

after_save

Updating an Object before_validation after_validation before_save around_save before_update

around_update after_update after_save

Destroying an Object before_destroy around_destroy after_destroy

Initializing an Object after_initialize after_find


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Callback Chain

Stolen from the rails guide:


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Callback Chain


or: before_update

or: after_update


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Callback Chain


or: after_rollback

The only twocallbacks called afterthe transaction

or: before_update

or: after_update


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Callback Chain


or: after_rollback

The only twocallbacks called afterthe transaction

Additional to the callbacks in thesave / destroy workflow:

- after_find- after_initialize

=> after a object was found, and initiatedby a finder, while after_initialize is triggeredfor new_records as well.

or: before_update

or: after_update


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Transactions

Are a database feature

Wrapped by ActiveRecord::Transactions

Typical Example for Transaction stolen from Railsguides:

Read more about Transactions and the AR-Support for different Databases here:http://api.rubyonrails.org/classes/ActiveRecord/Trans
http://api.rubyonrails.org/classes/ActiveRecord/Transactions/ClassMethods.htmlhttp://api.rubyonrails.org/classes/ActiveRecord/Transactions/ClassMethods.html


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Rollback Transactions

All Exceptions thrown within an transactions arecaught, and trigger a rollback, before they arere-raised by the transaction block (catch them)

Exception: raising ActiveRecord::Rollbacktriggers a rollback, but is not re-raised

Catching Exceptions inside the transaction,means it doesn't trigger a callback. Which mightcause unwanted behaviour / trouble (e.g.AR::StatementInvalid)


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Callbacks and Transactions

Still true: All after-/beforevalidate-/save-/update-/create callbacks are runinside a transaction

Ways to cancel the transactions: Raise an exception ( be ready to catch it)

Return false ( even if it is unwanted, be sure tocheck the return value of the last method call Inyour)

Try to save or create an invalid record.

Being in a transaction means all changes I

apply to any other active record object, are


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Custom Callbacks

Note: If you include ActiveSupport::Callbacks this works with any kind of ruby class


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Observer

Allow to use the same callbacks as available inthe model itself

Main purpose is to free models from callbacks

that don't affect the model itself, e.g. sending anotification email.

Are also wrapped inside the transaction.


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Oberserver Examples


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Callback Classes

Similar to observers you can also pass a class(or instance of a class) to a callback


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More

If changing (associated) models in callbacks,never call .save or .create use build instead.Associated objects will be saved together withthe original model.

Callbacks can take conditions (:if, and :unless),taking a method (symbol), proc or string (notrecommended)

Never call .valid? Inside a after_validationcallback stack level too deep

Active Record Callbacks

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