Warn when distinct is used with group_by

lib/DBIx/Class/ResultSet.pm

@@ -2896,7 +2896,12 @@ sub _resolved_attrs {
   # generate the distinct induced group_by early, as prefetch will be carried via a
   # subquery (since a group_by is present)
   if (delete $attrs->{distinct}) {
-    $attrs->{group_by} ||= [ grep { !ref($_) || (ref($_) ne 'HASH') } @{$attrs->{select}} ];
+    if ($attrs->{group_by}) {
+      carp ("Useless use of distinct on a grouped resultset ('distinct' is ignored when a 'group_by' is present)");
+    }
+    else {
+      $attrs->{group_by} = [ grep { !ref($_) || (ref($_) ne 'HASH') } @{$attrs->{select}} ];
+    }
   }
 
   $attrs->{collapse} ||= {};
@@ -3515,7 +3520,8 @@ done.
 
 =back
 
-Set to 1 to group by all columns.
+Set to 1 to group by all columns. If the resultset already has a group_by
+attribute, this setting is ignored and an appropriate warning is issued.
 
 =head2 where
 

t/60core.t

@@ -3,6 +3,7 @@ use warnings;
 
 use Test::More;
 use Test::Exception;
+use Test::Warn;
 use lib qw(t/lib);
 use DBICTest;
 use DBIC::SqlMakerTest;
@@ -35,10 +36,10 @@ ok($art->update, 'Update run');
 my %not_dirty = $art->get_dirty_columns();
 is(scalar(keys(%not_dirty)), 0, 'Nothing is dirty');
 
-eval {
+throws_ok ( sub {
   my $ret = $art->make_column_dirty('name2');
-};
-ok(defined($@), 'Failed to make non-existent column dirty');
+}, qr/No such column 'name2'/, 'Failed to make non-existent column dirty');
+
 $art->make_column_dirty('name');
 my %fake_dirty = $art->get_dirty_columns();
 is(scalar(keys(%fake_dirty)), 1, '1 fake dirty column');
@@ -221,9 +222,9 @@ SKIP: {
     isa_ok($tdata{'last_updated_on'}, 'DateTime', 'inflated accessored column');
 }
 
-eval { $schema->class("Track")->load_components('DoesNotExist'); };
-
-ok $@, $@;
+throws_ok (sub {
+  $schema->class("Track")->load_components('DoesNotExist');
+}, qr!Can't locate DBIx/Class/DoesNotExist.pm!, 'exception on nonexisting component');
 
 is($schema->class("Artist")->field_name_for->{name}, 'artist name', 'mk_classdata usage ok');
 
@@ -238,6 +239,13 @@ my $collapsed_or_rs = $or_rs->search ({}, { distinct => 1 }); # induce collapse
 is ($collapsed_or_rs->all, 4, 'Collapsed joined search with OR returned correct number of rows');
 is ($collapsed_or_rs->count, 4, 'Collapsed search count with OR ok');
 
+# make sure sure distinct on a grouped rs is warned about
+my $cd_rs = $schema->resultset ('CD')
+              ->search ({}, { distinct => 1, group_by => 'title' });
+warnings_exist (sub {
+  $cd_rs->next;
+}, qr/Useless use of distinct/, 'UUoD warning');
+
 {
   my $tcount = $schema->resultset('Track')->search(
     {},