Support random ordering

drift/lib/src/runtime/query_builder/components/order_by.dart

@@ -6,26 +6,33 @@ enum OrderingMode {
   asc,
 
   /// Descending ordering mode (highest items first)
-  desc
+  desc,
+
+  /// Random ordering mode
+  random,
 }
 
 const _modeToString = {
   OrderingMode.asc: 'ASC',
   OrderingMode.desc: 'DESC',
+  OrderingMode.random: 'RANDOM()',
 };
 
 /// A single term in a [OrderBy] clause. The priority of this term is determined
 /// by its position in [OrderBy.terms].
 class OrderingTerm extends Component {
   /// The expression after which the ordering should happen
-  final Expression expression;
+  final Expression? _expression;
 
   /// The ordering mode (ascending or descending).
   final OrderingMode mode;
 
   /// Creates an ordering term by the [expression] and the [mode] (defaults to
   /// ascending).
-  OrderingTerm({required this.expression, this.mode = OrderingMode.asc});
+  ///
+  /// When [mode] is [OrderingMode.random], the [expression] will be ignored
+  OrderingTerm({required Expression expression, this.mode = OrderingMode.asc})
+      : _expression = expression;
 
   /// Creates an ordering term that sorts for ascending values of [expression].
   factory OrderingTerm.asc(Expression expression) {
@@ -37,11 +44,19 @@ class OrderingTerm extends Component {
     return OrderingTerm(expression: expression, mode: OrderingMode.desc);
   }
 
+  /// Creates an ordering term  to get a number of random rows
+  /// using sqlite random function.
+  OrderingTerm.random()
+      : _expression = null,
+        mode = OrderingMode.random;
+
   @override
   void writeInto(GenerationContext context) {
-    expression.writeInto(context);
-    context.writeWhitespace();
+    if (mode != OrderingMode.random) {
+      _expression!.writeInto(context);
+    }
     context.buffer.write(_modeToString[mode]);
+    context.writeWhitespace();
   }
 }