Rails: how to create a form to create a new has_many relationship

Question

I have a User and a Project models that are linked with a has_and_belongs_to_many relationship (each users can be a part of several projects, and they can be the creator of several projects). I want to create a form to add a user to a project. The user would need to enter the username of another user to add him. How can I do that?

Here is what I've tried:

<%= form_for(@project) do |f| %>
    <%= f.label :username, "Username" %>
    <%= f.text_field :users %>
    <%= f.submit "Add" %>
<% end %>

Then in the controller I would do something like this:

def add_user
    @project = Project.find(params[:id])
    user = User.find_by(username: params[:username])
    user.projects << @project
    user.save
    redirect_to @project
  end

The issue is that the text field for the user does not do what I thought it would, it output the @project.users inspect ( #<ActiveRecord::Associations::CollectionProxy::ActiveRecord_Associations_CollectionProxy_User:0x007fc3e98e8d70> ).

I've see the nested forms but I don't think it's appropriate as the user and the project are already created. I'm using rails 4.

Answer 1

The nested forms stuff isn't just for creating new records, you can use it to add to join models, too

I would look at accepts_nested_attributes_for

---

Accepts Nested Attributes For

I would personally try this:

 #app/controllers/projects_controller.rb
 def new
      @project = Project.new
      @project.projects_users.build
 end

 def create
      @project = Project.new(project_params)
      @project.save
 end

 def edit
      @project = Project.find(params[:id])
 end

 def update
     @project = Project.find(params[:id])
     @project.save
 end

 private
 def project_params
     params.require(:project).permit(:project, :params, projects_users_attributes: [:user_id])
 end

#app/models/project.rb
Class Project < ActiveRecord::Base
    accepts_nested_attributes_for :projects_users
end

#app/views/projects/edit.html.erb
<%= form_for(@project) do |f| %>
    <%= f.fields_for :projects_users %>
        <%= f.label :username, "User ID" %>
        <%= f.text_field :user_id %> #-> can change to <select> later
    <% end %>
    <%= f.submit "Add" %>
<% end %>

Answer 2

Take a look at the cocoon gem. It provides some nested form stuff that would be hard to solve with rails of the shelf functionality. I have used it recently to do something very similar (find or create on M2M relation)

https://github.com/nathanvda/cocoon