Build vs Create in has many through relationship - ruby-on-rails

Question

I wonder if anyone can help me to understand about the difference between build and create in Has Many Through (HMT) relationship?
From my understanding after reading through stackoverflow and google, create is essentially build + save. However, it seems like create does more than just build + save. It also somehow save into the join table of the HMT relationship as shown below.
I created 3 models: user, wiki, and collaborator.
class User < ActiveRecord::Base
has_many :wikis, through: :collaborators
has_many :collaborators
end
class Wiki < ActiveRecord::Base
has_many :users, through: :collaborators
has_many :collaborators
end
class Collaborator < ActiveRecord::Base
belongs_to :user
belongs_to :wiki
end
Then I tested the build and create behavior in rails console:
$rails c
Loading development environment (Rails 4.0.10)
> user = User.first #create instance user
User Load SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT 1
=> #<User id: 1, name: "Jayzz55">
> (user.wikis.build(title:"hello",body:"world")).save #build and save wiki
SQL INSERT INTO "wikis" ("body", "created_at", "title", "updated_at") VALUES(?,?,?,?) [["body","world"], ["ccreated_at, Sat, 08 Nov 2014 01:39:36 UTC +00:00], ["title","hello"], ["updated_at",Sat, 08 Nov 2014 01:39:36 UTC +00:00]]
=> true
>wiki1 = Wiki.first #create instance wiki called wiki1
=> #<Wiki id:1, title:"hello",body:"world">
>user.wikis
=> #<Wiki id:1, title:"hello",body:"world">
>user.wikis.count
=> 0
>wiki1.users
=> []
>Collaborator.all
=> []
> user.wikis.create(title:"second title",body:"another one")
begin transaction
SQL INSERT INTO "wikis" ("body", "created_at", "title", "updated_at") VALUES (?, ?, ?, ?)[0m [["body", "another one"], ["created_at", Sat, 08 Nov 2014 01:59:39 UTC +00:00], ["title", "second title"], ["updated_at", Sat, 08 Nov 2014 01:59:39 UTC +00:00]]
SQL INSERT INTO "collaborators" ("created_at", "updated_at", "user_id", "wiki_id") VALUES (?, ?, ?, ?) [["created_at", Sat, 08 Nov 2014 01:59:39 UTC +00:00], ["updated_at", Sat, 08 Nov 2014 01:59:39 UTC +00:00], ["user_id", 1], ["wiki_id", 2]]
=> #<Wiki id:2, title:"second title",body:"another one>
>user.wikis
=> [#<Wiki id: 1, title:"hello",body:"world">, #<Wiki id:2, title:"second title",body:"another one>]
>user.wikis.count
=> 1
>wiki2.users
=>#<User id: 1, name: "Jayzz55">
>Collaborator.all
Collaborator Load SELECT "collaborators".* FROM "collaborators"
=> #<Collaborator id:`, user_id:1, wiki_id: 2>
Case wiki1 : build and then save it
The data is not saved into the join table. calling user.wikis does return the object wiki, but running user.wikis.count return 0. Furthermore, running wiki1.users doesn't return the object user. Checking the join table Collaborator returns empty array.
Case wiki2 : create
The data is saved into the join table. calling user.wikis does return the object wiki. Running user.wikis.count return 1. Furthermore, running wiki1.users does return the object user. Checking the join table Collaborator, it does show the relationship clearly mapped.
Seems like Create is not simply just build + new. I'm curious about this behavior and hopefully someone can share their knowledge on this.

Answer 1

I believe that if you had in your first case instead written:
user.wikis.build(title:"hello",body:"world")
user.save
... you would find that ActiveRecord does the "full" save that create also does. The way you've written it, you're asking ActiveRecord to save the newly created Wiki instance, but not the association. So it doesn't create the record in the join table that's required.

Answer 2

Build + Save is equivalent to Create in any relationship