Rails association= return value and behavior - ruby-on-rails

Question

The guide does not say what return value would be for association= methods. For example the has_one association=
For the simple case, it returns the assigned object. However this is only when assignment succeeds.
Sometimes association= would persist the change in database immediately, for example a persisted record setting the has_one association.
How does association= react to assignment failure? (Can I tell if it fails?)
Is there a bang! version in which failure raises exception?

Answer 1

How does association= react to assignment failure? (Can I tell if it fails?)
It can't fail. Whatever you assign, it will either work as expected:
Behind the scenes, this means extracting the primary key from this
object and setting the associated object's foreign key to the same
value.
or will save the association as a string representation of passed in object, if the object is "invalid".
Is there a bang! version in which failure raises exception?
Nope, there is not.

Answer 2

The association= should not be able to fail. It is a simple assignment to a attribute on your attribute. There are no validations called by this method and the connection doesn't get persisted in the database until you call save.
The return value of assignments is the value you pass to it.

Answer 3

http://guides.rubyonrails.org/association_basics.html#has-one-association-reference-when-are-objects-saved-questionmark
So another part of the guide does talk about the return behavior for association assignment.
If association assignment fails, it returns false.
There is no bang version of this.
Update
Behaviors around :has_many/has_one through seems to be different.
Demo repository: https://github.com/lulalalalistia/association-assignment-demo
In the demo I seeded some data in first commit, and hard code validation error in second commit. Demo is using rails 4.2
has_many through
class Boss < ActiveRecord::Base
has_many :room_ownerships, as: :owner
has_many :rooms, through: :room_ownerships
end
When I add a room, exception is raised:
irb(main):008:0> b.rooms << Room.first
Boss Load (0.2ms) SELECT "bosses".* FROM "bosses" ORDER BY "bosses"."id" ASC LIMIT 1
Room Load (0.1ms) SELECT "rooms".* FROM "rooms" ORDER BY "rooms"."id" ASC LIMIT 1
(0.1ms) begin transaction
(0.1ms) rollback transaction
ActiveRecord::RecordInvalid: Validation failed: foo
irb(main):014:0> b.rooms
=> #<ActiveRecord::Associations::CollectionProxy []>
has_one through
class Employee < ActiveRecord::Base
has_one :room_ownership, as: :owner
has_one :room, through: :room_ownership
end
When I add a room I don't get exception:
irb(main):021:0> e.room = Room.first
Room Load (0.2ms) SELECT "rooms".* FROM "rooms" ORDER BY "rooms"."id" ASC LIMIT 1
RoomOwnership Load (0.1ms) SELECT "room_ownerships".* FROM "room_ownerships" WHERE "room_ownerships"."owner_id" = ? AND "room_ownerships"."owner_type" = ? LIMIT 1 [["owner_id", 1], ["owner_type", "Employee"]]
(0.1ms) begin transaction
(0.1ms) rollback transaction
=> #<Room id: 1, created_at: "2016-10-03 02:32:33", updated_at: "2016-10-03 02:32:33">
irb(main):022:0> e.room
=> #<Room id: 1, created_at: "2016-10-03 02:32:33", updated_at: "2016-10-03 02:32:33">
This makes it difficult to see whether the assignment succeeds or not.