Firstly you should have set accepts_nested_attributes_for
in your models like this
class Store < ActiveRecord::Base
has_many :products
accepts_nested_attributes_for :products
end
class Product < ActiveRecord::Base
has_many :product_fields
belongs_to :store
accepts_nested_attributes_for :product_fields
end
class ProductField < ActiveRecord::Base
belongs_to :products
end
Secondly, your store_params
should look like this
def store_params
params.require(:store).permit(:name, :description, :user_id, products_attributes: [:id, :type, { product_fields_attributes: [:type, :content ] } ])
end