class Project
  include Mongoid::Document
  has_and_belongs_to_many :team_members
end

class TeamMember
  include Mongoid::Document
  field :name, type: String
  has_and_belongs_to_many :projects
end

project = Project.create()
amy = TeamMember.create(name: 'amy')
steve = TeamMember.create(name: 'steve')

# Add Amy and Steve to the project
project.team_members = [amy, steve]
[project, amy, steve].each(&:reload)

# Data looks good
puts project.team_members.pluck(:name).inspect # => ['amy', 'steve']
puts amy.projects.count # => 1
puts steve.projects.count # => 1

# Remove Steve from the project, should just be Amy now
project.team_members = [amy]
[project, amy, steve].each(&:reload)

# Amy has no projects now!
puts project.team_members.pluck(:name).inspect # => ['amy']
puts amy.projects.count # => 0 Amy should still be on the project! Returns 1 in Mongoid 3
puts steve.projects.count # => 0