class User
  include Mongoid::Document
  field :email, type: String
  has_and_belongs_to_many :projects
end

class Project
  include Mongoid::Document
  field :name, type: String
  has_and_belongs_to_many :users
end

# try to call like this (user is a valid object of class User):
project = user.projects.find {|p| p.name == "some_name" } # project will always be 'nil'

# however, 'find_all' gives back the expected array
projects = user.projects.find_all {|p| p.name == "some_name" }