Rails associations. How do they work?

Question

What is the exact purpose of associations? I understand what the relationships mean and when to use each type for example:

belongs_to, has_many, has_one , has_and_belongs_to_many, ect

but i dont quite understand what purpose they serve in terms of how the connect things within rails. Any input would be appreciated. Thanks!

Answer 1

What you're calling "associations" I would call "macros". That is, the belongs_to , has_many etc. macros are simply class methods being called on your ActiveRecord objects which, when called, define a bunch of functionality based on the association name.

So, what you're asking is: What functionality do these macro methods define? The answer for that lies within the Rails documentation for each of these methods:

• has_many
• belongs_to
• has_one

And, even more, you should read the overall documentation on ActiveRecord::Associations::ClassMethods .

But, in short, these macros define methods with names based on the association names you pass into them. So, for example:

belongs_to :my_object

Will define, as a greatly-simplified example:

def my_object
  MyObject.find_by_id(my_object_id)
end

So it's basically like metaprogramming your objects to have the methods needed to find the other, associated objects, update their collections, and so on.