Exist queries are retryable (#1294)

Author: aidanharan

lib/active_record/connection_adapters/sqlserver/core_ext/finder_methods.rb

@@ -33,7 +33,7 @@ def _construct_relation_for_exists(conditions)
               end
               # End of monkey-patch
             else
-              relation = except(:select, :distinct, :order)._select!(::ActiveRecord::FinderMethods::ONE_AS_ONE).limit!(1)
+              relation = except(:select, :distinct, :order)._select!(Arel.sql(::ActiveRecord::FinderMethods::ONE_AS_ONE, retryable: true)).limit!(1)
             end
 
             case conditions

test/cases/coerced_tests.rb

@@ -2803,3 +2803,14 @@ def test_with_recursive_coerced
   end
 end
 
+module ActiveRecord
+  class AdapterConnectionTest < ActiveRecord::TestCase
+    # Original method defined for core adapters.
+    undef_method :raw_transaction_open?
+    def raw_transaction_open?(connection)
+      connection.instance_variable_get(:@raw_connection).query("SELECT @@trancount").to_a[0][0] > 0
+    rescue
+      false
+    end
+  end
+end