In Ruby on Rails, if you have a has_many association between two models and you want to raise a validation error on the parent model if any of the associated child models are not valid, you can use the validates_associated method. This method ensures that the associated child models are valid before saving the parent model.

Here's how you can implement it:

Let's assume you have a Parent model that has many Child models:

ruby
# app/models/parent.rb class Parent < ApplicationRecord has_many :children validates_associated :children end # app/models/child.rb class Child < ApplicationRecord belongs_to :parent validates :name, presence: true end

In this example, we have defined a simple validation for the Child model, where the name attribute must be present.

Now, when you save a Parent model that has associated Child models, Rails will automatically validate the associated children and raise a validation error on the Parent model if any of the children are not valid.

Here's an example in a Rails controller:

ruby
class ParentsController < ApplicationController def create @parent = Parent.new(parent_params) if @parent.save # Parent and associated children are valid redirect_to @parent else # Validation error occurred due to invalid children render :new end end # Other controller actions and private methods... private def parent_params params.require(:parent).permit(:name, children_attributes: [:id, :name, :_destroy]) end end

In the controller's create action, we attempt to save the Parent model. If any of the associated children are not valid (e.g., missing a name), the save method on the Parent model will return false, and the validation error will be raised on the Parent model. We can then handle the error and show appropriate messages or render the form again for the user to correct the input.

By using validates_associated :children, Rails will automatically handle the validation of the associated children, ensuring that they are valid before saving the parent model.

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