Fix that #exists? 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.

Rather than build a select distinct clause in #construct_limited_ids_condition,
I set #distinct! and pass just the columns into the select statement.
This requires introducing a #columns_for_distinct method to return the
select columns but not the statement itself.

1 files changed, 2 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/relation/finder_methods.rb b/activerecord/lib/active_record/relation/finder_methods.rb
index 72e9272cd7..ff825e52c1 100644
--- a/activerecord/lib/active_record/relation/finder_methods.rb
+++ b/activerecord/lib/active_record/relation/finder_methods.rb
@@ -245,9 +245,9 @@ module ActiveRecord
 
     def construct_limited_ids_condition(relation)
       orders = relation.order_values.map { |val| val.presence }.compact
-      values = @klass.connection.distinct("#{quoted_table_name}.#{primary_key}", orders)
+      values = @klass.connection.columns_for_distinct("#{quoted_table_name}.#{quoted_primary_key}", orders)
 
-      relation = relation.dup.select(values)
+      relation = relation.dup.select(values).distinct!
 
       id_rows = @klass.connection.select_all(relation.arel, 'SQL', relation.bind_values)
       ids_array = id_rows.map {|row| row[primary_key]}