When are associations saved?

I have a tough time figuring out when associations are saved. I can of
course refer to the associations guide, or test from the console each
(which is what I do), but is there a general underlying thumb rule that
can use?

For example:

class Blog < ActiveRecord::Base
has_many :posts

class Post < ActiveRecord::Base
belongs_to :blog

blog = Blog.create(name:‘My Travel Blog’)
post = Post.create(name: ‘My first post’)
blog.posts << post

is post’s blog_id field already updated in the database? or do I need

call post.save? Will blog.save work as well?

Perhaps there is an underlying principle, such as (this is only an
example, it is not generally true
) - associations are never saved
explicit call to save on the model that has a foreign key field.

@blog = Blog.create(name:‘My First Blog’)
@post = @blog.posts.create(name: ‘My first post in My First Blog’)

post is now created and saved

@blog = Blog.find_by_name: ‘My First Blog’
@post = @blog.posts.create(name: ‘My second post in My First Blog’)

post is now created and saved

@blog = Blog.first
@post = @blog.posts.build(name: ‘My third post in my My First Blog’)

post is being built but has not yet been saved


post is now saved

Probably the easiest thing you can do is understand the types of methods
that are available to each object through their association.

Example (from rails console):

@blog = Blog.first
@blog.methods.sort.each do |method|
p method

Or, single line it for posts (just writing it different ways):

@blog.posts.methods.sort.map{ |method| p method }