No method error

I am working on a project in rails 4 where a user can have multiple projects and many people are associated with a single project. ie. Projects and users share a many to many relationship.

I created a new table by using this migration code -

class CreateProjectsUsersJoin < ActiveRecord::Migration
  def change
    create_table :projects_users_joins, :id => false do |t|

        t.integer :user_id
        t.integer :project_id
    end

    add_index :projects_users_joins, ["user_id","project_id"]
  end
end

and added associations in the following way -

class Project < ActiveRecord::Base

    validates :name , :presence => true
    validates :description, :presence => true


    #associations
    has_and_belongs_to_many :users

end


class User < ActiveRecord::Base

    include CarrierWave::MiniMagick

    #removes the white spaces before validating the data 


    before_validation :strip_whitespace, :only => [:name,:email]

    #data validations
    validates :email, :presence =>true,  :uniqueness => {case_sensitive: false}, :format => { :with=> /([a-zA-Z0-9_\-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|(([a-zA-Z0-9\-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)/, :message => "please enter a valid e-mail" }
    validates :name, :presence=>true
    validates :password ,:presence =>true, :confirmation=> true #, :length =>{ :minimum=>6, :maximum=>30}, :format=>{:with=>/(?=.*\d)(?=.*[a-z])(?=.*[A-Z]).{6,30}/}

    #for the image
    mount_uploader :image, ImageUploader

    #for the password
    has_secure_password


    #associations
    has_many :issues
    has_many :comments
    has_and_belongs_to_many :projects

end


    #squishes the data(removes heading and trailing  white spaces and replaces multiple space by songle)
def strip_whitespace
    self.email = self.email.squish
    self.name = self.name.squish
end

when I run the following commands on rails console I get a no method error - Please help

pro = Project.find(1)  (works)
me = User.find(27) (works)
pro.users << me  (throws a no method defined error)

Please help