ruby on rails - data is not saved in joins table -

i working on website, user can have multiple projects , multpile users can contribute single project. have project model

class project < activerecord::base   #associations   has_and_belongs_to_many :users end 

and users model

class user < activerecord::base   #associations   has_and_belongs_to_many :projects end 

and have created joins table name - :projects_users works fine when run code on rails console. when try save thing in controller, data not being saved in joins table. code controller

please help

class projectscontroller < applicationcontroller   def new    @project = project.new  end   def create    @user = user.find(session[:user_id])    @project = project.new(project_params)         if @project.save        @project.users << @user        redirect_to @project      else         flash[:error] = "project has not been created due error"        render 'new'      end  end    private    def project_params     params.require(:project).permit(:name,:description)   end end 

try using nestes_attributes_for

class answer < activerecord::base     belongs_to :question end   class question < activerecord::base     has_many :answers     accepts_nested_attributes_for :answers, allow_destroy: true end 

controlller

def new     @question = question.new     @question.answers.build end  def create     @question = question.new(question_params)     respond_to |format|         if @question.save             format.html { redirect_to @question, notice: 'question created.' }             format.json { render action: 'show', status: :created, location: @question }         else             format.html { render action: 'new' }             format.json { render json: @question.errors, status: :unprocessable_entity }         end     end end  def question_params     params.require(:question).permit(:name, :description, answers_attributes:[:content, :id, :question_id]) end         

you form should this

<%= form_for(@question) |f| %>   <% if @question.errors.any? %>     <div id="error_explanation">       <h2><%= pluralize(@question.errors.count, "error") %> prohibited question being saved:</h2>        <ul>       <% @question.errors.full_messages.each |msg| %>         <li><%= msg %></li>       <% end %>       </ul>     </div>   <% end %>    <div class="field">     <%= f.label :name %><br>     <%= f.text_field :name %>   </div>    <div class="field">     <%= f.label :description %><br>     <%= f.text_field :description %>   </div>    <%= f.fields_for :answer |builder| %>      <%= builder.label :content %>      <%= builder.text_area :content %>   <% end %>   <div class="actions">     <%= f.submit %>   </div> <% end %>