Backport a super-simplified version of #6792, fixing

that #exists? and others can produce invalid SQL: "SELECT DISTINCT DISTINCT"

The combination of a :uniq => true association and the #distinct call
in #construct_limited_ids_condition combine to create invalid SQL, because
we're explicitly selecting DISTINCT, and also sending #distinct on to AREL,
via the relation#distinct_value.

Where #6792 was the forever fix, this is the minimal fix. Instead of
properly indicating the distinctness of the query through #uniq_value alone,
we use a literal select statement and set #uniq_value to always be falsey

1 files changed, 7 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index e679d4f781..f8c75c0fba 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,5 +1,12 @@
 ## unreleased ##
 
+*   Fix that under some conditions, Active Record could produce invalid SQL of the sort:
+    "SELECT DISTINCT DISTINCT".
+
+    Backport of #6792.
+
+    *Ben Woosley*
+
 *   Require `ActiveRecord::Base` in railtie hooks for rake_tasks, console and runner to
     avoid circular constant loading issues.