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Why is Rails ActiveRelation.update_all updating a different set of records?

Andy Zhao (he/him) on January 23, 2019

I've got a weird case of ActiveRecord_Relation magic doing something unintended. Given the following: comment = Comment.find(params[:id]) # sele...

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rhymes • Jan 24 '19 • Edited

You probably hit a limitation of ActiveRecord :-)

union is not part of ActiveRecord, which might be the reason why update_all is not able to work with it.

There's a lot of magic in it :D

PostgreSQL itself is able to update rows from unions if you project the same exact row types from both queries:

PracticalDeveloper_development> select count(*) from comments;
+---------+
| count   |
|---------|
| 30      |
+---------+
PracticalDeveloper_development> update comments set updated_at = now() where id in (select id from comments where id < 10 union select id from comments where id > 15);
UPDATE 24

In conclusion: I don't know what's going on 🧐

Andy Zhao (he/him) • Jan 24 '19

Haha yeah the magic is certainly mystifying me. Guess I'll put on my wizard robes and dive into the code.

rhymes • Jan 24 '19

Let us know if you find anything :D

Andy Zhao (he/him) • Jan 23 '19 • Edited

Hmm interesting, I'll give this a try when I get a chance.

Re: what I'm trying to do. I'm trying to combine two Active record queries and then do a third where query to get only the commenter's comments. The final chained query should produce a single SQL query, and then I want to update_all a single collection/relation of records.

The where is called on the ActiveRecord relation and not an individual ActiveRecord record, which is how I can chain the where after doing union.

Philip Hallstrom • Jan 23 '19 • Edited

What is the SQL generated by related_comments.count. Count will change the query sometimes if Rails decides it doesn't need the join/includes. I'm guessing it's dropping your union -- or simply not doing it correctly. And that's what is leading to the difference in rows affected.

edit: however your second solution suggests i'm wrong about that. still, would be curious to see the SQL if you just run the query to select them and for the count.

Andy Zhao (he/him) • Jan 23 '19

Here's the SQL for related_comments.count:

(22.0ms)  SELECT COUNT(*) FROM ( (SELECT "comments".* FROM "comments"
WHERE (("comments"."ancestry" LIKE '12/%' OR "comments"."ancestry" = '12')
OR "comments"."id" = 12)) UNION (SELECT "comments".* FROM "comments" WHERE
1=0 ORDER BY COALESCE("comments"."ancestry", '') ASC) ) "comments" WHERE
"comments"."user_id" = $1  [["user_id", 11]] [sql_query]

And for the select:

(4.1ms)  SELECT "comments".* FROM ( (SELECT "comments".* FROM "comments"
WHERE (("comments"."ancestry" LIKE '12/%' OR "comments"."ancestry" = '12')
OR "comments"."id" = 12)) UNION (SELECT "comments".* FROM "comments" WHERE
1=0 ORDER BY COALESCE("comments"."ancestry", '') ASC) ) "comments" WHERE
"comments"."user_id" = $1  [["user_id", 11]] [sql_query]

FWIW the code I'm using does not run related_comments.count; that was to demonstrate how many records should be updated.

Corey Alexander • Jan 26 '19

I don't know what the magic/issue is with your first example, but I have an idea of how to make your workaround take a single query. I can't seem to get an example like this working with union(and on Rails 5.2 it gives a deprecation warning, about removing the delegation down into AREL) so I can't test this well myself.

But if you do NOT pluck the id, and simply pass a relation into the where clause for id it will do a sub-select for the update. I tried it with a simpler case and got the update_all to use a sub_select. So I think you should be able to do this in one query with:

comment = Comment.find(params[:id])

related_comments = comment.where(something: params[:something)
  .union(comment.where(something_else: params[:something_else]))
  .where(user_id: comment.user_id)


Comment.where(id: related_comments).update_all(receive_notifications: true)

Andy Zhao (he/him) • Jan 23 '19

Ahh gotcha :) I'll keep note of that for next time!

Andy Zhao (he/him) • Jan 23 '19

You're totally right about not being able to call .where on comment; the example I used is wrong. I'm actually doing this:

comment.subtree.union(comment.ancestors).where(user_id: comment.user_id)

.subtree and .ancestors are methods from a gem, and I initially thought putting that in the example would be confusing. My workaround-example made it more confusing. 🙃

Andy Zhao (he/him) • Jan 23 '19

The Ancestry gem. It's what we (dev.to) use for the comment tree.

Andy Zhao (he/him) • Jan 24 '19

@acflint running .or gives me an error since the built in subtree and ancestors methods have some extra SQL in there. :(

comment.subtree.or(c.ancestors)
#=> ArgumentError: Relation passed to #or must be
#=> structurally compatible. Incompatible values:
#=> [:order, :reordering]

Ben Halpern • Jan 23 '19

What happens if you tack .all onto the first related_comments = query?

Andy Zhao (he/him) • Jan 23 '19

.update_all still returns 12 :/