param is missing or the value is empty

Question

I have two models: Boards and Topics. I want to be able to add Topics to Boards. My nested resources are:

resources :boards do 
    resources :topics
end

My 'boards#show' action:

def show 
    @board = Board.find(params[:id])
    @new_topics = Topic.all
end

which lists all posts and has a link_to:

<ul>
    <%@new_topics.each do |i|%>
        <li><%=i.title%> <%=link_to "Add", board_topic_path(@board,i), :method=> :put%></li>
    <%end%>
</ul>

I'm also using strong_params for my Boards and Topics controller as follows:

boards_controller:

def update
    @board = Board.find(params[:board_id])
    @topic = Topic.find(params[:id])        

    if @board.update(board_params)
        flash[:notice] = "Added!"
        @board.topics << @topic
        redirect_to boards_path

    else
        flash[:alert] = "Problem!"
        redirect_to boards_path
    end

end

...
private

def board_params
    params.require(:board).permit(:name,:description)
end

topics_controller:

...
private

def topic_params
    params.require(:topic).permit(:title,:body,:user_id)
end

the error message I'm getting: param is missing or the value is empty: topic.

Answer 1

In a RESTful situation as yours, with that link you should be hitting the update action of TopicsController with two params: board_id and id.

Try this instead:

# boards_controller.rb
def update
   @board = Board.find(params[:id])
   @topic = Topic.find(params[:topic_id])        

if @board.update(board_params)
    flash[:notice] = "Added!"
    @board.topics << @topic
    redirect_to boards_path

else
    flash[:alert] = "Problem!"
    redirect_to boards_path
end

end

# In the view 
<%=link_to "Add", board_path(@board, topic_id: i.id), :method=> :put%>

Still, this is still off from any convention, as you are not updating a whole topic. You probably want to use an extra action to add a topic to a board, using the PATCH verb.

Answer 2

I believe that your design is wrong.

Starting from the beginning, I would say that you have a business model Board that references one or more Topics and a Topic that is referenced by one or more Boards. So, logically you have something like this:

So, these are two independent resources, that they have a many-to-many relationship.

My model with Rails would have been:

# routes
resources :boards
resources :topics

In other words, topics are not nested resource of boards. If it were, this would mean that the topics of a board would die when the board would die. Which is not your case here, as far as I understand.

Now, since the relationship is many-to-many, then you will need a 3rd table to hold your associations (table boards and table topics are not enough). Read this on Rails Guides.

Briefly:

class Board
  has_and_belongs_to_many :topics
end

class Topic
  has_and_belongs_to_many :boards
end

Now, if you want to add topics to boards on your UI, then you need to have a form to edit the board. This form, besides the others, needs to have a multiple select box with the topics that would be added to the board. Then on your boards_controller#update method the param[:board] would have an attribute topic_ids[] which will automatically be used to associate the particular/selected topics to the board that you are editing. Rails does that automatically.

Note I am not inclined to be using has_and_belongs_to_many Rails association. It has a lot of limitations. You can always design your own table that will hold the many-to-many association and other extra attributes that your business model will require. For example, for each topic that is attached to a board, you might want to hold the subject, or the author. I do not know. In that case a more custom model might be needed:

class Board
  has_many :board_topics
  has_many :topics, through: :board_topics
end

class Topic
  has_many :board_topics
  has_many :boards, through: :board_topics
end

class BoardTopic
  belongs_to :topic, inverse_of: :board_topics
  belongs_to :board, inverse_of: :board_topics
  .... add other attributes that give real business value to this association ....
 end