Fix Migration 145 on MSSQL if varchar is changed to nvarchar (#12445)

* Handle MSSQL constraints

Signed-off-by: Andrew Thornton <art27@cantab.net>

* Update models/migrations/v145.go

* Prevent hang with Sync2

* Agh it's uniques not indexes

* Let us try a different way of finding constraints on the language column

Signed-off-by: Andrew Thornton <art27@cantab.net>

* minor simplification

Signed-off-by: Andrew Thornton <art27@cantab.net>

* Try drop index instead?

Signed-off-by: Andrew Thornton <art27@cantab.net>

models/migrations/v145.go

@@ -47,9 +47,31 @@ func increaseLanguageField(x *xorm.Engine) error {
 			return err
 		}
 	case setting.Database.UseMSSQL:
+		// Yet again MSSQL just has to be awkward.
+		// Here we have to drop the constraints first and then rebuild them
+		constraints := make([]string, 0)
+		if err := sess.SQL(`SELECT i.name AS Name
+			FROM sys.indexes i INNER JOIN sys.index_columns ic
+      			ON i.index_id = ic.index_id AND i.object_id = ic.object_id
+   			INNER JOIN sys.tables AS t 
+      			ON t.object_id = i.object_id
+			INNER JOIN sys.columns c
+				ON t.object_id = c.object_id AND ic.column_id = c.column_id
+			WHERE t.name = 'language_stat' AND c.name = 'language'`).Find(&constraints); err != nil {
+			return fmt.Errorf("Find constraints: %v", err)
+		}
+		for _, constraint := range constraints {
+			if _, err := sess.Exec(fmt.Sprintf("DROP INDEX [%s] ON `language_stat`", constraint)); err != nil {
+				return fmt.Errorf("Drop table `language_stat` constraint `%s`: %v", constraint, err)
+			}
+		}
 		if _, err := sess.Exec(fmt.Sprintf("ALTER TABLE language_stat ALTER COLUMN language %s", sqlType)); err != nil {
 			return err
 		}
+		// Finally restore the constraint
+		if err := sess.CreateUniques(new(LanguageStat)); err != nil {
+			return err
+		}
 	case setting.Database.UsePostgreSQL:
 		if _, err := sess.Exec(fmt.Sprintf("ALTER TABLE language_stat ALTER COLUMN language TYPE %s", sqlType)); err != nil {
 			return err