In Ruby on Rails, fields_for is a helper method that allows you to build form fields for associated records in a one-to-many or has-many relationship. When you use fields_for within a form, it iterates through the associated records and generates form fields for each associated record.

Here's how you can use fields_for with a has-many relationship in Rails:

Let's assume you have two models: Post and Comment, where a Post has many Comments.

  1. Controller: In your controller, make sure to build the associated records (Comment instances) for the Post instance in the new or edit action, so the form can generate fields for them.

    ruby
    class PostsController < ApplicationController def new @post = Post.new # Build associated comments for the post 3.times { @post.comments.build } end # Other actions... end
  2. View: In your view, use the form_for method to create the main form for the Post model. Then, use fields_for to generate fields for the associated Comment models.

    html+erb
    <%= form_for @post do |f| %> <!-- Post form fields --> <%= f.text_field :title %> <%= f.text_area :content %> <!-- Comments form fields --> <%= f.fields_for :comments do |comment_fields| %> <%= comment_fields.text_area :body %> <% end %> <%= f.submit "Submit" %> <% end %>

    In this example, the fields_for :comments block iterates through the associated Comment records of the @post object and generates a text area field for each Comment's body attribute.

  3. Strong Parameters: In your controller, ensure you permit the nested attributes for the associated model (Comment in this case) using strong parameters.

    ruby
    class PostsController < ApplicationController def create @post = Post.new(post_params) # ... end # ... private def post_params params.require(:post).permit(:title, :content, comments_attributes: [:id, :body]) end end

    The comments_attributes key in the post_params method permits the body attribute of the associated Comment model to be saved.

With this setup, when you submit the form, it will create a new Post record and its associated Comment records based on the fields entered in the form.

Remember to adjust the names and attributes according to your specific models and requirements.

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