# band.rb
class Band
  include Mongoid::Document

  field :n, type: String, as: :name

  # This seems to be necessary in order to have short relation
  # columns with aliases (similar to the above def)
  has_and_belongs_to_many :tags,
    foreign_key: :t_ids,
    inverse_of: 'b',
    inverse_class_name: 'Tag'
end

# tag.rb
class Tag
  include Mongoid::Document

  field :n, type: String, as: :name

  has_and_belongs_to_many :bands,
    foreign_key: :b_ids,
    inverse_of: 't',
    inverse_class_name: 'Band'
end

b1 = Band.create name: 'Foo'
# MOPED: 127.0.0.1:27017 INSERT       database=development collection=bands documents=[{"_id"=>"5220dfe44984b96f87000001", "n"=>"Foo"}] flags=[] (0.2840ms)
# => #<Band _id: 5220dfe44984b96f87000001, n(name): "Foo", t_ids: nil>

b2 = Band.create name: 'Bar'
# MOPED: 127.0.0.1:27017 INSERT       database=development collection=bands documents=[{"_id"=>"5220dfe74984b96f87000002", "n"=>"Bar"}] flags=[] (0.1979ms)
# => #<Band _id: 5220dfe74984b96f87000002, n(name): "Bar", t_ids: nil>

t1 = Tag.create name: 'Rock'
# MOPED: 127.0.0.1:27017 INSERT       database=development collection=tags documents=[{"_id"=>"5220e00f4984b96f87000003", "n"=>"Rock"}] flags=[] (0.2429ms)
# => #<Tag _id: 5220e00f4984b96f87000003, n(name): "Rock", b_ids: nil>

t2 = Tag.create name: 'Soul'
# MOPED: 127.0.0.1:27017 INSERT       database=development collection=tags documents=[{"_id"=>"5220e0174984b96f87000004", "n"=>"Soul"}] flags=[] (0.2277ms)
# => #<Tag _id: 5220e0174984b96f87000004, n(name): "Soul", b_ids: nil>


# In this instance, adding a tag to a band runs updates for **both**
b1.tags << t1
# MOPED: 127.0.0.1:27017 UPDATE       database=development collection=bands selector={"_id"=>"5220dfe44984b96f87000001"} update={"$addToSet"=>{"t_ids"=>{"$each"=>["5220e00f4984b96f87000003"]}}} flags=[] (0.3152ms)
# MOPED: 127.0.0.1:27017 UPDATE       database=development collection=tags selector={"_id"=>"5220e00f4984b96f87000003"} update={"$addToSet"=>{"b_ids"=>{"$each"=>["5220dfe44984b96f87000001"]}}} flags=[] (0.2391ms)
# => [#<Tag _id: 5220e00f4984b96f87000003, n(name): "Rock", b_ids: ["5220dfe44984b96f87000001"]>]


# ISSUE: Adding a band to a tag, only runs an update for **the tag only**
t2.bands << b2
# MOPED: 127.0.0.1:27017 UPDATE       database=development collection=tags selector={"_id"=>"5220e0174984b96f87000004"} update={"$addToSet"=>{"b_ids"=>{"$each"=>["5220dfe74984b96f87000002"]}}} flags=[] (0.2179ms)
# => [#<Band _id: 5220dfe74984b96f87000002, n(name): "Bar", t_ids: nil>]