Связь многие ко многим без танцев с бубном!

Many-to-many Dance-off!

I've noticed there's a bit of confusion about the differences between the two ways to create many-to-many relationships using Rails associations. That confusion is understandable, since has_many :through is still pretty new there isn't much written about it. has_and_belongs_to_many is the old, established player and most stuff out there assumes that's what you use for a many-to-many relationship. In fact, a lot of people don't seem to grasp that there is a difference at all!

As we all learned from watching classic movies, the best way to tell the difference between two prospective choices is to have a dance-off. You get to see everyone's moves (which of course are an accurate reflection of inner character), nobody has to die, and the hummable tune makes it a shoe-in for an Oscar nomination. Well, it's either that or do one of those boring "compare and contrast" essays they taught us about in sixth grade English class.

So who are the players we have to choose between? Let's take a quick look at them before the music starts and we get to see their moves.

Join Table: Simple Associations

Table:

create_table "dancers_movies", :id => false do |t|

t.column "dancer_id", :integer, :null => false

t.column "movie_id", :integer, :null => false

end

Models:

class Dancer < ActiveRecord::Base

has_and_belongs_to_many :movies

end

class Movie < ActiveRecord::Base

has_and_belongs_to_many :dancers

end

has_and_belongs_to_many associations are simple to set up. The join table has only foreign keys for the models being joined - no primary key or other attributes. (Other attributes were supported using push_with_attributes for a while, but that feature has been deprecated.) There is no model class for the join table.

Join Model: Rich Associations

Table:

create_table "appearances", do |t|

t.column "dancer_id", :integer, :null => false

t.column "movie_id", :integer, :null => false

t.column "character_name", :string

t.column "dance_numbers", :integer

end

Models:

class Appearance < ActiveRecord::Base