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POValidate presence of nested attributes
 

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    <p>How do I validate that a model has at least one associated model using nested attributes? This has been driving me crazy as I am sure that I am missing something simple. For example, I want to require that a List always has at least one Task.</p> <pre><code>class List &lt; ActiveRecord::Base has_many :tasks, :dependent =&gt; :destroy accepts_nested_attributes_for :tasks, :allow_destroy =&gt; true end class Task &lt; ActiveRecord::Base belongs_to :list end </code></pre> <p>I've tried many different options.</p> <p>1- adding a validation to lists:</p> <pre><code>def validate if self.tasks.length &lt; 1 self.errors[:base] &lt;&lt; "A list must have at least one task." end end </code></pre> <p>but this will still allow you to delete all the tasks of an <strong>existing</strong> list since when deleting tasks the validation of list happens before the tasks are destroyed.</p> <p>2- checking to see if any tasks are not marked for destruction in a before_save callback</p> <pre><code>before_save :check_tasks private #look for any task which won't be deleted def check_tasks for t in self.tasks return true if ! t.marked_for_destruction? end false end </code></pre> <p>For some reason I can't get it to ever delete a task with anything that iterates over a list's tasks. The same is true if I do this check in <code>def validate</code> instead of a callback</p> <p>3- requiring the presence of tasks <code>validates_presence_of :tasks</code>, but with this it won't ever delete any tasks</p>
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