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

Question

I am working on a website, where a user can have multiple projects and multpile users can contribute to a single project.
I have a project model
class Project < ActiveRecord::Base
#associations
has_and_belongs_to_many :users
end
and a users model
class User < ActiveRecord::Base
#associations
has_and_belongs_to_many :projects
end
and I have created a joins table by the name - :projects_users
everything works fine when I run the code on rails console.
but when I try to do the save thing in the controller, the data is not being saved in the joins table.
Code for the 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 to some error"
render 'new'
end
end
private
def project_params
params.require(:project).permit(:name,:description)
end
end

Answer 1

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 do |format|
if #question.save
format.html { redirect_to #question, notice: 'question was successfully 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 look like this
<%= form_for(#question) do |f| %>
<% if #question.errors.any? %>
<div id="error_explanation">
<h2><%= pluralize(#question.errors.count, "error") %> prohibited this question from being saved:</h2>
<ul>
<% #question.errors.full_messages.each do |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 do |builder| %>
<%= builder.label :content %>
<%= builder.text_area :content %>
<% end %>
<div class="actions">
<%= f.submit %>
</div>
<% end %>

Answer 2

just changed
#project.users << #user
to
#user.projects << #project
and it started working.Dont know the reason yet but its working