Add functions to extract individual fields from a datetime

sally/lib/src/dsl/columns.dart

@@ -21,9 +21,7 @@ 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> {}
 
 class ColumnBuilder<Builder, ResultColumn> {
   /// By default, the field name will be used as the column name, e.g.

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

@@ -0,0 +1,32 @@
+import 'package:sally/sally.dart';
+import 'package:sally/src/runtime/components/component.dart';
+import 'package:sally/src/runtime/expressions/expression.dart';
+
+Expression<IntType> year(Expression<DateTimeType> date) =>
+    _StrftimeSingleFieldExpression('%Y', date);
+Expression<IntType> month(Expression<DateTimeType> date) =>
+    _StrftimeSingleFieldExpression('%m', date);
+Expression<IntType> day(Expression<DateTimeType> date) =>
+    _StrftimeSingleFieldExpression('%d', date);
+Expression<IntType> hour(Expression<DateTimeType> date) =>
+    _StrftimeSingleFieldExpression('%H', date);
+Expression<IntType> minute(Expression<DateTimeType> date) =>
+    _StrftimeSingleFieldExpression('%M', date);
+Expression<IntType> second(Expression<DateTimeType> date) =>
+    _StrftimeSingleFieldExpression('%S', date);
+
+/// Expression that extracts components out of a date time by using the builtin
+/// sqlite function "strftime" and casting the result to an integer.
+class _StrftimeSingleFieldExpression extends Expression<IntType> {
+  final String format;
+  final Expression<DateTimeType> date;
+
+  _StrftimeSingleFieldExpression(this.format, this.date);
+
+  @override
+  void writeInto(GenerationContext context) {
+    context.buffer.write('CAST(strftime("$format", ');
+    date.writeInto(context);
+    context.buffer.write(', "unixepoch") AS INTEGER)');
+  }
+}

sally/lib/src/runtime/expressions/user_api.dart

@@ -1,2 +1,3 @@
 export 'bools.dart' show and, or, not;
+export 'datetimes.dart';
 export 'variables.dart';

sally/test/expressions/datetime_expression_test.dart

@@ -0,0 +1,28 @@
+import 'package:sally/sally.dart';
+import 'package:sally/src/runtime/components/component.dart';
+import 'package:sally/src/runtime/expressions/expression.dart';
+import 'package:test_api/test_api.dart';
+
+typedef Expression<IntType> _Extractor(Expression<DateTimeType> d);
+
+/// Tests the top level [year], [month], ..., [second] methods
+void main() {
+  final expectedResults = <_Extractor, String>{
+    year: 'CAST(strftime("%Y", column, "unixepoch") AS INTEGER)',
+    month: 'CAST(strftime("%m", column, "unixepoch") AS INTEGER)',
+    day: 'CAST(strftime("%d", column, "unixepoch") AS INTEGER)',
+    hour: 'CAST(strftime("%H", column, "unixepoch") AS INTEGER)',
+    minute: 'CAST(strftime("%M", column, "unixepoch") AS INTEGER)',
+    second: 'CAST(strftime("%S", column, "unixepoch") AS INTEGER)',
+  };
+  final column = GeneratedDateTimeColumn('column', false);
+
+  expectedResults.forEach((key, value) {
+    test('should extract field', () {
+      final ctx = GenerationContext(null);
+      key(column).writeInto(ctx);
+
+      expect(ctx.sql, value);
+    });
+  });
+}

sally/test/schema_test.dart

@@ -46,7 +46,8 @@ void main() {
     });
 
     test('adds columns', () async {
-      await Migrator(db, mockQueryExecutor).addColumn(db.users, db.users.isAwesome);
+      await Migrator(db, mockQueryExecutor)
+          .addColumn(db.users, db.users.isAwesome);
 
       verify(mockQueryExecutor.call('ALTER TABLE users ADD COLUMN '
           'is_awesome BOOLEAN NOT NULL CHECK (is_awesome in (0, 1));'));

sally/test/types/datetime_test.dart

@@ -3,14 +3,16 @@ import 'package:test_api/test_api.dart';
 
 const _exampleUnixSqlite = 1550172560;
 const _exampleUnixMillis = 1550172560000;
-final _exampleDateTime = DateTime.fromMillisecondsSinceEpoch(_exampleUnixMillis);
+final _exampleDateTime =
+    DateTime.fromMillisecondsSinceEpoch(_exampleUnixMillis);
 
 void main() {
   final type = const DateTimeType();
 
   group('DateTimes', () {
     test('can be read from unix stamps returned by sql', () {
-      expect(type.mapFromDatabaseResponse(_exampleUnixSqlite), _exampleDateTime);
+      expect(
+          type.mapFromDatabaseResponse(_exampleUnixSqlite), _exampleDateTime);
     });
 
     test('can read null value from sql', () {
@@ -18,7 +20,8 @@ void main() {
     });
 
     test('can be mapped to sql constants', () {
-      expect(type.mapToSqlConstant(_exampleDateTime), _exampleUnixSqlite.toString());
+      expect(type.mapToSqlConstant(_exampleDateTime),
+          _exampleUnixSqlite.toString());
     });
 
     test('can be mapped to variables', () {