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, 1 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/relation/finder_methods.rb b/activerecord/lib/active_record/relation/finder_methods.rb
index 08cfe4f70d..cdf18f8080 100644
--- a/activerecord/lib/active_record/relation/finder_methods.rb
+++ b/activerecord/lib/active_record/relation/finder_methods.rb
@@ -254,6 +254,7 @@ module ActiveRecord
       values = @klass.connection.distinct("#{@klass.connection.quote_table_name table_name}.#{primary_key}", orders)
 
       relation = relation.dup.select(values)
+      relation.uniq_value = nil
 
       id_rows = @klass.connection.select_all(relation.arel, 'SQL', relation.bind_values)
       ids_array = id_rows.map {|row| row[primary_key]}