Using `assign_attributes` saves `has_many through:` association immediately

Question

As far as I know, assign_attributes (unlike update_attributes) is not supposed to save the record or for that matter, any record.

So it quite startled me when I discovered that this is not true when supplying _ids for a has_many through: relation.

Consider the following example:

class GroupUser < ApplicationRecord
  belongs_to :group
  belongs_to :user
end

class Group < ApplicationRecord
  has_many :group_users
  has_many :users, through: :group_users
end

class User < ApplicationRecord
  has_many :group_users
  has_many :groups, through: :group_users

  validates :username, presence: true
end

So we have users and groups in an m-to-m relationship.

Group.create # Create group with ID 1
Group.create # Create group with ID 2

u = User.create(username: 'Johny')

# The following line inserts two `GroupUser` join objects, despite the fact 
# that we have called `assign_attributes` instead of `update_attributes` 
# and, equally disturbing, the user object is not even valid as we've 
# supplied an empty `username` attribute.
u.assign_attributes(username: '', group_ids: [1, 26])

The log as requested by a commenter:

irb(main):013:0> u.assign_attributes(username: '', group_ids: [1, 2])
  Group Load (0.2ms)  SELECT "groups".* FROM "groups" WHERE "groups"."id" IN (1, 2)
  Group Load (0.1ms)  SELECT "groups".* FROM "groups" INNER JOIN "group_users" ON "groups"."id" = "group_users"."group_id" WHERE "group_users"."user_id" = ?  [["user_id", 1]]
   (0.0ms)  begin transaction
  SQL (0.3ms)  INSERT INTO "group_users" ("group_id", "user_id", "created_at", "updated_at") VALUES (?, ?, ?, ?)  [["group_id", 1], ["user_id", 1], ["created_at", "2017-06-29 08:15:11.691941"], ["updated_at", "2017-06-29 08:15:11.691941"]]
  SQL (0.1ms)  INSERT INTO "group_users" ("group_id", "user_id", "created_at", "updated_at") VALUES (?, ?, ?, ?)  [["group_id", 2], ["user_id", 1], ["created_at", "2017-06-29 08:15:11.693984"], ["updated_at", "2017-06-29 08:15:11.693984"]]
   (2.5ms)  commit transaction
=> nil

I daresay that update_attributes and the _ids construct are mostly used for processing web forms - in this case a form that updates the user itself as well as its group association. So I think it is quite safe to say that the general assumption here is all or nothing, and not a partial save.

Am I using it wrong in some way?

Chelseachelsey

what makes you say that the record is saved? It is not, try u.valid? and u.save!. – Kurth

The fact that I can see the queries and that it is persisted in the database (for the join table, not the record itself of course). And valid? is false and save! raises, as expected, a validation error. – Chelseachelsey

You see queries? can you verify doing GroupUser.pluck(:group_id) and u.reload!, u.username? – Kurth

The user object is not saved of course, only the association (that's the whole point of this question). And I guarantee you that this is the case...I'm well able to verify that there are join records in the database ;) – Chelseachelsey

Please, post the log. – Gael

I don't see the point here at all, but I've added the log above. The user is not touched but the join entries are created. I'm very well able to judge whether something is in the DB, trust me on this. – Chelseachelsey

Read this: guides.rubyonrails.org/… and github.com/rails/rails/issues/17368 – Gael

it throws error if group_id is invalid – Kurth

Thank you @GokulM, this answers my question. – Chelseachelsey

Answer 1

@gokul-m suggests reading about the issue at https://github.com/rails/rails/issues/17368. One of the comments in there points to a temporary workaround: https://gist.github.com/sudoremo/4204e399e547ff7e3afdd0d89a5aaf3e

Morganstein

Yes, that's the solution i finally came up with. But I'd still like Rails to fix this - in my oppinion the current behavior is so unexpected that I would regard it as a bug. – Chelseachelsey

Answer 2

an example of my solution to this problem:

  ruby: 
  def assign_parameters(attributes, options = {})
    with_transaction_returning_status {self.assign_attributes(attributes, options)}
  end

Nedi

Please put your answer always in context instead of just pasting code. See here for more details. – Exoenzyme

Answer 3

You can handle validation with assign_attributes like so

@item.assign_attributes{ year: "2021", type: "bad" }.valid?

Rope

The call to assign_attributes already persisted the changes, so this is not a solution I'm afraid. – Chelseachelsey

Answer 4

Wrapping everything from assigning the values to the save in a transaction worked for me.

User.transaction do
 // assign variables
 // save changes
end

Unchristian