rails - left shift "<<" operator saves record automatically - ruby-on-rails

Question

Need help understanding this code, as what to my knowledge I know "<<" append to a collection but here it saves the record correctly, how come it does without calling .save method?
#user.rb
has_many :saved_properties, through: :property_saves, source: :property
#users_controller.rb
def update
if #user.saved_properties << Property.find(params[:saved_property_id])
render plain: "Property saved"
end

Answer 1

In the has_many documentation it says:
Adds one or more objects to the collection by setting their foreign
keys to the collection's primary key. Note that this operation
instantly fires update SQL without waiting for the save or update
call on the parent object, unless the parent object is a new record.

Answer 2

Maybe looking at the source code will help you. This is my trail of searches based on the << method in activerecord:
def <<(*records)
proxy_association.concat(records) && self
end
rails/collection_proxy.rb at 5053d5251fb8c03e666f1f8b765464ec33e3066e · rails/rails · GitHub
def concat(*records)
records = records.flatten
if owner.new_record?
load_target
concat_records(records)
else
transaction { concat_records(records) }
end
end
rails/collection_association.rb at 5053d5251fb8c03e666f1f8b765464ec33e3066e · rails/rails · GitHub
def concat_records(records, should_raise = false)
result = true
records.each do |record|
raise_on_type_mismatch!(record)
add_to_target(record) do |rec|
result &&= insert_record(rec, true, should_raise) unless owner.new_record?
end
end
result && records
end
rails/collection_association.rb at 5053d5251fb8c03e666f1f8b765464ec33e3066e · rails/rails · GitHub
def insert_record(record, validate = true, raise = false)
set_owner_attributes(record)
set_inverse_instance(record)
if raise
record.save!(validate: validate)
else
record.save(validate: validate)
end
end
https://github.com/rails/rails/blob/5053d5251fb8c03e666f1f8b765464ec33e3066e/activerecord/lib/active_record/associations/has_many_association.rb#L32
def insert_record(record, validate = true, raise = false)
ensure_not_nested
if record.new_record? || record.has_changes_to_save?
if raise
record.save!(validate: validate)
else
return unless record.save(validate: validate)
end
end
save_through_record(record)
record
end
https://github.com/rails/rails/blob/5053d5251fb8c03e666f1f8b765464ec33e3066e/activerecord/lib/active_record/associations/has_many_through_association.rb#L38
As you can see, in the end, it does call the save method.
Disclaimer: I'm not that familiar with Rails souce code, but you have interesting question.

Answer 3

In a has_many relationship the link information is saved in the target record. This means that << would have to modify that record in order to add it to the set.
Perhaps intending convenience, ActiveRecord automatically saves these for you when making an assignment if the assignment was successful. The exception is for new records, the record they're being associated with doesn't have any identifier so that has to be delayed. They are saved when the record they're associated with is finally created.
This can be a little confusing, perhaps unexpected, but it's actually the thing you'd want to happen 99% of the time. If you don't want that to happen you should manipulate the linkage manually:
property = Property.find(params[:saved_property_id])
property.user = #user
property.save!
That's basically equivalent but a lot more verbose.