Make date time values comparable

Fixes #58

moor/lib/src/dsl/columns.dart

@@ -22,7 +22,8 @@ abstract class TextColumn extends Column<String, StringType> {
 
 /// A column that stores a [DateTime]. Times will be stored as unix timestamp
 /// and will thus have a second accuracy.
-abstract class DateTimeColumn extends Column<DateTime, DateTimeType> {}
+abstract class DateTimeColumn extends Column<DateTime, DateTimeType>
+    implements DateTimeExpression {}
 
 /// A column that stores arbitrary blobs of data as a [Uint8List].
 abstract class BlobColumn extends Column<Uint8List, BlobType> {}

moor/lib/src/runtime/expressions/comparable.dart

@@ -7,6 +7,9 @@ abstract class IntExpression extends Expression<int, IntType>
 abstract class DoubleExpression extends Expression<double, RealType>
     implements ComparableExpr<double, RealType> {}
 
+abstract class DateTimeExpression extends Expression<DateTime, DateTimeType>
+    implements ComparableExpr<DateTime, DateTimeType> {}
+
 mixin ComparableExpr<DT, ST extends SqlType<DT>> on Expression<DT, ST> {
   /// Returns an expression that is true if this expression is strictly bigger
   /// than the other expression.

moor/lib/src/runtime/expressions/datetimes.dart

@@ -35,13 +35,19 @@ Expression<int, IntType> second(Expression<DateTime, DateTimeType> date) =>
 
 /// A sql expression that evaluates to the current date represented as a unix
 /// timestamp. The hour, minute and second fields will be set to 0.
-const Expression<DateTime, DateTimeType> currentDate =
-    CustomExpression("strftime('%s', CURRENT_DATE)");
+const DateTimeExpression currentDate =
+    _CustomDateTimeExpression("strftime('%s', CURRENT_DATE)");
 
 /// A sql expression that evaluates to the current date and time, similar to
 /// [DateTime.now]. Timestamps are stored with a second accuracy.
-const Expression<DateTime, DateTimeType> currentDateAndTime =
-    CustomExpression("strftime('%s', CURRENT_TIMESTAMP)");
+const DateTimeExpression currentDateAndTime =
+    _CustomDateTimeExpression("strftime('%s', CURRENT_TIMESTAMP)");
+
+class _CustomDateTimeExpression extends CustomExpression<DateTime, DateTimeType>
+    with ComparableExpr
+    implements DateTimeExpression {
+  const _CustomDateTimeExpression(String content) : super(content);
+}
 
 /// Expression that extracts components out of a date time by using the builtin
 /// sqlite function "strftime" and casting the result to an integer.

moor/lib/src/runtime/structure/columns.dart

@@ -201,6 +201,7 @@ class GeneratedIntColumn extends GeneratedColumn<int, IntType>
 }
 
 class GeneratedDateTimeColumn extends GeneratedColumn<DateTime, DateTimeType>
+    with ComparableExpr
     implements DateTimeColumn {
   GeneratedDateTimeColumn(
     String $name,

moor/test/columns/datetime_test.dart

@@ -15,4 +15,11 @@ void main() {
     expect(nullableQuery.sql, equals('name INTEGER NULL'));
     expect(nonNullQuery.sql, equals('name INTEGER NOT NULL'));
   });
+
+  test('can compare', () {
+    final ctx = GenerationContext(null, null);
+    nonNull.isSmallerThan(currentDateAndTime).writeInto(ctx);
+
+    expect(ctx.sql, 'name < strftime(\'%s\', CURRENT_TIMESTAMP)');
+  });
 }