Add `columns_for_distinct` for MySQL 5.7 with ONLY_FULL_GROUP_BY

In MySQL 5.7.5 and up, ONLY_FULL_GROUP_BY affects handling of queries
that use DISTINCT and ORDER BY. It requires the ORDER BY columns in the
select list for distinct queries, and requires that the ORDER BY include
the distinct column.

See https://dev.mysql.com/doc/refman/5.7/en/group-by-handling.html

1 files changed, 44 insertions, 0 deletions

diff --git a/activerecord/test/cases/adapters/mysql2/mysql2_adapter_test.rb b/activerecord/test/cases/adapters/mysql2/mysql2_adapter_test.rb
new file mode 100644
index 0000000000..4efd728754
--- /dev/null
+++ b/activerecord/test/cases/adapters/mysql2/mysql2_adapter_test.rb
@@ -0,0 +1,44 @@
+require "cases/helper"
+
+class Mysql2AdapterTest < ActiveRecord::Mysql2TestCase
+  def setup
+    @conn = ActiveRecord::Base.connection
+  end
+
+  def test_columns_for_distinct_zero_orders
+    assert_equal "posts.id",
+      @conn.columns_for_distinct("posts.id", [])
+  end
+
+  def test_columns_for_distinct_one_order
+    assert_equal "posts.id, posts.created_at AS alias_0",
+      @conn.columns_for_distinct("posts.id", ["posts.created_at desc"])
+  end
+
+  def test_columns_for_distinct_few_orders
+    assert_equal "posts.id, posts.created_at AS alias_0, posts.position AS alias_1",
+      @conn.columns_for_distinct("posts.id", ["posts.created_at desc", "posts.position asc"])
+  end
+
+  def test_columns_for_distinct_with_case
+    assert_equal(
+      'posts.id, CASE WHEN author.is_active THEN UPPER(author.name) ELSE UPPER(author.email) END AS alias_0',
+      @conn.columns_for_distinct('posts.id',
+        ["CASE WHEN author.is_active THEN UPPER(author.name) ELSE UPPER(author.email) END"])
+    )
+  end
+
+  def test_columns_for_distinct_blank_not_nil_orders
+    assert_equal "posts.id, posts.created_at AS alias_0",
+      @conn.columns_for_distinct("posts.id", ["posts.created_at desc", "", "   "])
+  end
+
+  def test_columns_for_distinct_with_arel_order
+    order = Object.new
+    def order.to_sql
+      "posts.created_at desc"
+    end
+    assert_equal "posts.id, posts.created_at AS alias_0",
+      @conn.columns_for_distinct("posts.id", [order])
+  end
+end