Don't preserve SELECT columns on COUNT

The COUNT clause of a finder_sql relationship is being rewritten from
COUNT(*) to COUNT(table_name.*). This does not appear to be valid syntax
in MySQL:

```
mysql> SELECT COUNT( table_name.* ) FROM `table_name`;
ERROR 1064 (42000): You have an error in your SQL syntax; check the
manual that corresponds to your MySQL server version for the right
syntax to use near '* ) FROM `table_name`' at line 1
```

This fixes the bug, as well as adding tests so we don't re-introduce
it in the future.

Fixes #3956.

1 files changed, 2 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/associations/collection_association.rb b/activerecord/lib/active_record/associations/collection_association.rb
index b15df4f308..babbb12e99 100644
--- a/activerecord/lib/active_record/associations/collection_association.rb
+++ b/activerecord/lib/active_record/associations/collection_association.rb
@@ -373,7 +373,8 @@ module ActiveRecord
             # replace the SELECT clause with COUNT(SELECTS), preserving any hints within /* ... */
             interpolate(options[:finder_sql]).sub(/SELECT\b(\/\*.*?\*\/ )?(.*)\bFROM\b/im) do
               count_with = $2.to_s
-              count_with = '*' if count_with.blank? || count_with =~ /,/
+              p count_with
+              count_with = '*' if count_with.blank? || count_with =~ /,/ || count_with =~ /\.\*/
               "SELECT #{$1}COUNT(#{count_with}) FROM"
             end
           end