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                       

@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