data is not saved in joins table

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