Support SQL Server unique indexes

database/migrations/add_ldap_columns_to_users_table.php.stub

@@ -1,5 +1,6 @@
 <?php
 
+use Illuminate\Support\Facades\DB;
 use Illuminate\Support\Facades\Schema;
 use Illuminate\Database\Schema\Blueprint;
 use Illuminate\Database\Migrations\Migration;
@@ -8,22 +9,56 @@ class AddLdapColumnsToUsersTable extends Migration
 {
     /**
      * Run the migrations.
+     * 
+     * @return void
      */
     public function up()
     {
-        Schema::table('users', function (Blueprint $table) {
-            $table->string('guid')->unique()->nullable();
+        $driver = Schema::getConnection()->getDriverName();
+
+        Schema::table('users', function (Blueprint $table) use ($driver) {
+            $table->string('guid')->nullable();
             $table->string('domain')->nullable();
+
+            if ($driver !== 'sqlsrv') {
+                $table->unique('guid');
+            }
         });
+
+        if ($driver === 'sqlsrv') {
+            DB::statement(
+                $this->compileUniqueSqlServerIndexStatement('users', 'guid')
+            );
+        }
     }
 
     /**
      * Reverse the migrations.
+     * 
+     * @return void
      */
     public function down()
     {
         Schema::table('users', function (Blueprint $table) {
             $table->dropColumn(['guid', 'domain']);
         });
     }
+
+    /**
+     * Compile a compatible "unique" SQL Server index constraint.
+     * 
+     * @param string $table
+     * @param string $column 
+     * 
+     * @return string 
+     */
+    protected function compileUniqueSqlServerIndexStatement($table, $column)
+    {
+        return sprintf('create unique index %s on %s (%s) where %s is not null',
+            implode('_', [$table, $column, 'unique']),
+            $table,
+            $column,
+            $column
+        );
+    }
 }