Keep INNER JOIN when merging relations #27063

MaxLap · 2016-11-16T01:24:11Z

Doing Author.joins(:posts).merge(Post.joins(:comments)) does this
SELECT ... INNER JOIN posts ON... LEFT OUTER JOIN comments ON...
instead of doing
SELECT ... INNER JOIN posts ON... INNER JOIN comments ON....

This behavior is unexpected and makes little sense as, basically, doing
Post.joins(:comments) means I want posts that have comments. Turning
it to a LEFT JOIN means I want posts and join the comments data, if
any.

We can see this problem directly in the existing tests.
The test_relation_merging_with_merged_joins_as_symbols only does joins
from posts to comments to ratings while the ratings fixture isn't
loaded, but the count is non-zero.

The only thing I'm not sure about is if my fix should use make_outer_joins as it was before and as is used in walk or if it should use make_left_outer_joins which is used above in the same method.

rails-bot · 2016-11-16T01:24:12Z

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mijoharas · 2016-12-02T12:12:42Z

Is anyone available to review this PR? it fixes a bug that has stung me (and I assume other people). Thanks for the work @MaxLap

mijoharas · 2016-12-02T12:13:13Z

Follows from #26195

maclover7 · 2017-03-11T23:04:54Z

I believe this would also solve #28219

pixeltrix · 2017-03-12T09:26:25Z

From the discussion on #16140 and #12933 and that @sgrif seems to have looked at this behaviour in his comment on #12933 I reckon that this must be by design - at the time it was implemented we didn't have the left_joins method so it would be a way of getting one without writing raw SQL. As other people have pointed out you can use a hash inside joins to construct a inner join and also you can add a has_many :through association definition so it's not like there's no way to achieve what you want.

Personally, I've never used merge as it seems ill-defined conceptually what merging two relations should do - I prefer to use the more explicit query builder methods.

@MaxLap thanks for your PR but sorry I can't merge it.

MaxLap · 2017-03-14T16:27:56Z

Nobody can explain the rationale because there is none.

I agree that merge can feel ill defined in what it should do exactly, but the doc says Merges in the conditions from other, if other is an ActiveRecord::Relation [...]

You can use a joins for 3 reasons:

You want to have some kinds of duplicates
You want to add conditions using that joins
You want to ignore some data if it doesn't map to some data (by using an INNER JOIN isntead of LEFT JOIN)

Sounds to me like the last 2 choices are "conditions". So changing the INNER JOIN to LEFT JOIN seems like changing the condition, which is clearly unexpected.

To me, the point of merge is to allow code reuse. You have a scope on posts which uses a joins, and then you want to use that scope to find only author that have a matching posts, then you can do: Author.joins(:posts).merge(Post.some_scope). Making a has_many through will either need to duplicate the code, or manually do what merge should be doing to allow code reuse.

You can't pass this off as a feature that could create LEFT JOIN before the left_joins methods appeared:

If you want to have a LEFT JOIN as 2nd join, you can, instead of writing it in text, do this:
Author.joins(:posts).merge(Post.joins(:comments))
Which will do the "expected":
SELECT "authors".* FROM "authors" INNER JOIN "posts" ON "posts"."author_id" = "authors"."id" LEFT OUTER JOIN "comments" ON "comments"."post_id" = "posts"."id"
Note that, this only work when it is nested! Doing:
Author.merge(Author.joins(:posts))
will keep the classical join behavior:
SELECT "authors".* FROM "authors" INNER JOIN "posts" ON "posts"."author_id" = "authors"."id"

Honestly, I just noticed this bug while trying to make a PR which got refused in the mailing list and thought it was a pretty easy fix. I don't need this feature, i'm just trying to help others that landed on this bug.

mijoharas · 2017-03-14T16:47:13Z

Just my two cents. My reading of the situation was that no-one was sure if it was by design, and given that it does cause bugs without this fix, and no-one can find a case where it would be needed, shouldn't we change it?

pixeltrix · 2017-03-14T17:27:57Z

Honestly, I just noticed this bug while trying to make a PR which got refused in the mailing list and thought it was a pretty easy fix. I don't need this feature, i'm just trying to help others that landed on this bug.

@MaxLap and thanks for your contribution, but often someone's bug is another person's feature so we have to trade off both sides when we decide about whether to merge something. In this instance it's a definite change in behaviour that we can revisit when we've shipped 5.1 but I think it's too risky to do it now.

pixeltrix · 2017-03-14T17:33:30Z

Just my two cents. My reading of the situation was that no-one was sure if it was by design

@mijoharas from @thedarkone's comment:

@joanniclaborde it is by design, all "association" joins are a merged
(via scope.merge(relation)) as OUTER JOINS, I can't give you a rationale,
because I don't know it, but if I "fix" merge to use  INNER JOINS all kind
of ActiveRecord tests start failing.

If tests start failing that's a pretty good indication that something was by design - even for Rails 😉

MaxLap · 2017-03-14T17:42:40Z

My PR fixes this issue and only needs to change a single test: a broken test that relied on missing fixtures, which was introduced in the PR that added the joins handling to merge (the PR root of this problem).

There are many ways to fix bugs, the use of double quotes around "fix" in that comment make it sound like it was just a quick change that may not have been as complete or thorough as it needed. For example, if the change also made #includes use a INNER JOIN, of course all hell would break lose.

MaxLap · 2017-03-14T17:46:24Z

And also, as a previous comment of mine mentions, Author.merge(Author.joins(:posts)) will leave it as a INNER JOIN, so not all "association" joins are merged as OUTER JOIN. Only the nested ones.

pixeltrix · 2017-03-14T17:52:57Z

@MaxLap it's still a change in long-standing behavior after we've shipped a beta release, I promise to look again as soon as we branch for 5.1.

MaxLap · 2017-03-14T18:18:06Z

@pixeltrix Thank you!

meinac · 2017-03-17T10:06:52Z

I would love to see this behaviour change, thanks @MaxLap.

rafaelfranca · 2017-06-20T17:15:12Z

activerecord/lib/active_record/associations/join_dependency.rb

          if join_root.match? oj.join_root
            walk join_root, oj.join_root
          else
            oj.join_root.children.flat_map { |child|
-              make_outer_joins oj.join_root, child
+              if join_type == Arel::Nodes::OuterJoin
+                make_left_outer_joins oj.join_root, child


Why using make_left_outer_joins instead of the previous make_outer_joins?

This is a question I asked in the initial message of the pull request:

The only thing I'm not sure about is if my fix should use make_outer_joins as it was before and as is used in walk or if it should use make_left_outer_joins which is used above in the same method.

Since I'm doing basically the same code as what is done at line 110, I used the same one as is used there. The code of those 2 methods is similar, except for one using table_aliases_for, which is very obcsure to me.

So I leave that one last choice up to someone more experienced for those internals.

Note, this has been changed on HEAD, the 2 methods were replaced by a single one, but it has a parameter to specify if aliasing is to be done or not. Once the choice is made, i'll rebase with the chosen way.

I think it is fine to keep it the same as in the line 110. Could you please rebase. I'll merge it to master. I'm not going to backport since even that it is a bug it may cause behavior change in a stable branch for some people.

@rafaelfranca The upgrade guide for whichever version this is included in probably needs to warn people about this loudly. It could lead to a lot of stuff being excluded from queries where it was previously included.

This is a good point. Usually we don't start the upgrade guide until before the release but I agree we should warn about this.

rafaelfranca · 2017-06-20T20:53:28Z

activerecord/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   Merging two relations which have joins no longer transforms the joins of
+    the merged relation into LEFT OUTER JOIN.
+


Can you add a new line here with: TODO: Add to the Rails 5.2 upgrade guide

Doing `Author.joins(:posts).merge(Post.joins(:comments))` does this `SELECT ... INNER JOIN posts ON... LEFT OUTER JOIN comments ON...` instead of doing `SELECT ... INNER JOIN posts ON... INNER JOIN comments ON...`. This behavior is unexpected and makes little sense as, basically, doing `Post.joins(:comments)` means I want posts that have comments. Turning it to a LEFT JOIN means I want posts and join the comments data, if any. We can see this problem directly in the existing tests. The test_relation_merging_with_merged_joins_as_symbols only does joins from posts to comments to ratings while the ratings fixture isn't loaded, but the count is non-zero.

MaxLap · 2017-06-21T00:51:48Z

Rebased, added the message to CHANGELOG for the update guide. I also clarified in the CHANGELOG what actually changed.

rails-bot assigned pixeltrix Nov 16, 2016

maclover7 added the activerecord label Nov 16, 2016

pixeltrix closed this Mar 12, 2017

pixeltrix reopened this Mar 14, 2017

pixeltrix added the needs feedback label Mar 14, 2017

rafaelfranca reviewed Jun 20, 2017

View reviewed changes

MaxLap force-pushed the merge_keep_inner_join branch from 31b2d61 to 486b825 Compare June 21, 2017 00:46

MaxLap force-pushed the merge_keep_inner_join branch from 486b825 to 249ddd0 Compare June 21, 2017 00:47

rafaelfranca merged commit 0c32bbd into rails:master Jun 21, 2017

MaxLap mentioned this pull request Oct 4, 2017

INNER JOIN becomes LEFT OUTER JOIN when merging relations #16140

Closed

chalofa mentioned this pull request Jun 6, 2018

Child joins should be aliased when merging relations #33066

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Keep INNER JOIN when merging relations #27063

Keep INNER JOIN when merging relations #27063

MaxLap commented Nov 16, 2016

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mijoharas commented Dec 2, 2016

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MaxLap commented Jun 21, 2017

		@@ -1,3 +1,8 @@
		* Merging two relations which have joins no longer transforms the joins of
		the merged relation into LEFT OUTER JOIN.

Keep INNER JOIN when merging relations #27063

Keep INNER JOIN when merging relations #27063

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