project.rb

class Project
  include Mongoid::Document
  include Mongoid::Timestamps

  embeds_many :histories
  has_many :invitations

  field :name, type: String
  validates_presence_of :name

  has_and_belongs_to_many :users
end

projects_controller.rb

# work

  def create
    @project = Project.new(params[:project])
    if @project.save
      @project.users << current_user
      redirect_to project_path(@project), :notice => 'Project created'
    else
      render 'new'
    end
  end


# no work

  def create
    @project = current_user.projects.build(params[:project])
    if @project.save
      redirect_to project_path(@project), :notice => 'Project created'
    else
      render 'new'
    end
  end

user.rb

class User
  include Mongoid::Document
  # Include default devise modules. Others available are:
  # :token_authenticatable, :encryptable, :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable, :recoverable, :rememberable, :trackable, :validatable

   field :name
   validates_presence_of :name
   validates_uniqueness_of :name, :email, :case_sensitive => false
   attr_accessible :name, :email, :password, :password_confirmation, :remember_me

   has_and_belongs_to_many :projects
end                       

ruby-1.9.2-p290 :049 > u = User.last
 => #<User _id: 4e80aeb894fed428d7000006, _type: nil, email: "jose@fourmach.com", encrypted_password: "$2a$10$x293r.HK19do3f2H113nT.vdFZcrHRPvvPL6p5cBXoBzkMNasJB4O", remember_created_at: nil, reset_password_token: nil, reset_password_sent_at: nil, sign_in_count: 1, current_sign_in_at: 2011-09-26 16:56:24 UTC, last_sign_in_at: 2011-09-26 16:56:24 UTC, current_sign_in_ip: "127.0.0.1", last_sign_in_ip: "127.0.0.1", name: "joselito", project_ids: []> 
ruby-1.9.2-p290 :050 > u.projects
 => [] 
ruby-1.9.2-p290 :051 > p = u.projects.build(:name => 'test')
 => #<Project _id: 4e80dbd594fed4360f000005, _type: nil, created_at: nil, updated_at: nil, user_ids: [BSON::ObjectId('4e80aeb894fed428d7000006')], name: "test"> 
ruby-1.9.2-p290 :052 > u.save
 => true 
ruby-1.9.2-p290 :053 > 
ruby-1.9.2-p290 :054 >   
ruby-1.9.2-p290 :055 >   
ruby-1.9.2-p290 :056 >   u
 => #<User _id: 4e80aeb894fed428d7000006, _type: nil, email: "jose@fourmach.com", encrypted_password: "$2a$10$x293r.HK19do3f2H113nT.vdFZcrHRPvvPL6p5cBXoBzkMNasJB4O", remember_created_at: nil, reset_password_token: nil, reset_password_sent_at: nil, sign_in_count: 1, current_sign_in_at: 2011-09-26 16:56:24 UTC, last_sign_in_at: 2011-09-26 16:56:24 UTC, current_sign_in_ip: "127.0.0.1", last_sign_in_ip: "127.0.0.1", name: "joselito", project_ids: [BSON::ObjectId('4e80dbd594fed4360f000005')]> 
ruby-1.9.2-p290 :057 > u.reload
 => #<User _id: 4e80aeb894fed428d7000006, _type: nil, email: "jose@fourmach.com", encrypted_password: "$2a$10$x293r.HK19do3f2H113nT.vdFZcrHRPvvPL6p5cBXoBzkMNasJB4O", remember_created_at: nil, reset_password_token: nil, reset_password_sent_at: nil, sign_in_count: 1, current_sign_in_at: 2011-09-26 16:56:24 UTC, last_sign_in_at: 2011-09-26 16:56:24 UTC, current_sign_in_ip: "127.0.0.1", last_sign_in_ip: "127.0.0.1", name: "joselito", project_ids: [BSON::ObjectId('4e80dbd594fed4360f000005')]> 
ruby-1.9.2-p290 :058 > u
 => #<User _id: 4e80aeb894fed428d7000006, _type: nil, email: "jose@fourmach.com", encrypted_password: "$2a$10$x293r.HK19do3f2H113nT.vdFZcrHRPvvPL6p5cBXoBzkMNasJB4O", remember_created_at: nil, reset_password_token: nil, reset_password_sent_at: nil, sign_in_count: 1, current_sign_in_at: 2011-09-26 16:56:24 UTC, last_sign_in_at: 2011-09-26 16:56:24 UTC, current_sign_in_ip: "127.0.0.1", last_sign_in_ip: "127.0.0.1", name: "joselito", project_ids: [BSON::ObjectId('4e80dbd594fed4360f000005')]> 
ruby-1.9.2-p290 :059 > u.projects
 => [] 
ruby-1.9.2-p290 :060 > p
 => #<Project _id: 4e80dbd594fed4360f000005, _type: nil, created_at: nil, updated_at: nil, user_ids: [BSON::ObjectId('4e80aeb894fed428d7000006')], name: "test"> 
ruby-1.9.2-p290 :062 > p.users
 => [#<User _id: 4e80aeb894fed428d7000006, _type: nil, email: "jose@fourmach.com", encrypted_password: "$2a$10$x293r.HK19do3f2H113nT.vdFZcrHRPvvPL6p5cBXoBzkMNasJB4O", remember_created_at: nil, reset_password_token: nil, reset_password_sent_at: nil, sign_in_count: 1, current_sign_in_at: 2011-09-26 16:56:24 UTC, last_sign_in_at: 2011-09-26 16:56:24 UTC, current_sign_in_ip: "127.0.0.1", last_sign_in_ip: "127.0.0.1", name: "joselito", project_ids: [BSON::ObjectId('4e80dbd594fed4360f000005')]>] 
ruby-1.9.2-p290 :063 > p.reload
Mongoid::Errors::DocumentNotFound: No se encontró ningún documento para la clase Project con los id(s) 4e80dbd594fed4360f000005.
    from /Users/ceritium/.rvm/gems/ruby-1.9.2-p290@scrum/gems/mongoid-2.2.1/lib/mongoid/document.rb:149:in `reload'
    from (irb):63
    from /Users/ceritium/.rvm/gems/ruby-1.9.2-p290@scrum/gems/railties-3.1.0/lib/rails/commands/console.rb:45:in `start'
    from /Users/ceritium/.rvm/gems/ruby-1.9.2-p290@scrum/gems/railties-3.1.0/lib/rails/commands/console.rb:8:in `start'
    from /Users/ceritium/.rvm/gems/ruby-1.9.2-p290@scrum/gems/railties-3.1.0/lib/rails/commands.rb:40:in `<top (required)>'
    from script/rails:6:in `require'
    from script/rails:6:in `<main>'

Exactly, project is not being marked as dirtyand thus is not saved automatically. As to why, I don't know :)

This should work:

def create
    @project = current_user.projects.new(params[:project])
    if @project.save && current_user.save
      redirect_to project_path(@project), :notice => 'Project created'
    else
      render 'new'
    end

@Nerian But if for some reason @project.save works but current_user.save blows up, you will end with a corrupted relationship. I think that is safer to go with something like

def create
  @project = current_user.create!(params[:project])
  redirect_to @project, :notice => 'Project created'
rescue Mongoid::Errors::Validations
  # You can check for sanity here
  render 'new'
end