Save changes to a has_many association ONLY when you successfully save the parent object?

Suppose each Project has_many Tasks.

If I do

some_project.tasks = list_of_tasks
some_project.save

The project's tasks get updated even if the save fails. If list_of_tasks开发者_高级运维 consists of new records, the project's tasks get deleted even if the save fails! WHOA!

If the save fails, the project should have the same tasks it had before I started messing with it. How do I get this behavior and why isn't it the default?

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Enclose the statements in a transaction:

Project.transaction do
  p.tasks = task_list
  p.save!
end

The save! method throws an exception upon error, which rolls back any changes done to task list.

You can read the documentation if you want to dive a bit more deeply on the subject.

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I believe that accepts_nested_attributes_for() will provide the behavior you want:

class Project < ActiveRecord::Base
  accepts_nested_attributes_for :tasks
end

This should wrap everything inside a transaction. You then need to build the form that populates the tasks accordingly. The method tasks_attributes in your Project model is called instead of the tasks method. See the API for more information.

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You might find that the AutosaveAssociation feature does what you want.

class Project < ActiveRecord::Base
  has_many :tasks, :autosave => true
end

This should wrap the save in a transaction automatically.

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Before calling #save you can ask if some_project#valid? . That helps the issue if the save fails due to some_project being an invalid record, but is not a comprehensive solution.

As for the Rails' default behavior, it makes sense. Saying some_project.tasks = list_of_tasks is like saying "remove all existing tasks from some_project and assign these new ones". You are dropping the reference to the existing association Array and assigning a brand new one. This is reflected by ActiveRecord in the DB.