Implement support for converting db values to datastore values with db.Query.filter

lib/src/db/db.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of gcloud.db;
 
 /**
  * A function definition for transactional functions.
  *
  * The function will be given a [Transaction] object which can be used to make
@@ skipping 144 matching lines @@
    *
    * [comparisonObject] is the object for comparison.
    */
   void filter(String filterString, Object comparisonObject) {
     var parts = filterString.split(' ');
     if (parts.length != 2 || !_relationMapping.containsKey(parts[1])) {
       throw new ArgumentError(
           "Invalid filter string '$filterString'.");
     }
 
-    // TODO: do value transformation on [comparisonObject]
+    var name = parts[0];
+    var comparison = parts[1];
+    var propertyName = _convertToDatastoreName(name);
 
-    var propertyName = _convertToDatastoreName(parts[0]);
+    // This is for backwards compatibility: We allow [datastore.Key]s for now.
+    // TODO: We should remove the condition in a major version update of
+    // `package:gcloud`.
+    if (comparisonObject is! datastore.Key) {
+      var encoded = _db.modelDB.toDatastoreValue(_kind, name, comparisonObject);
+
+      // We encode Lists as repeated properties normally, and the encoding of
+      // `['abc']` will just be `'abc'` (see [ListProperty]).
+      // But for IN filters, we need to treat them as lists.
+      if (comparison == 'IN' &&
+          comparisonObject is List &&
+          comparisonObject.length == 1 &&
+          encoded is! List) {
+        encoded = [encoded];
+      }
+
+      comparisonObject = encoded;
+    }
     _filters.add(new datastore.Filter(
-        _relationMapping[parts[1]], propertyName, comparisonObject));
+        _relationMapping[comparison], propertyName, comparisonObject));
   }
 
   /**
    * Adds an order to this [Query].
    *
    * [orderString] has the form "-name" where 'name' is a fieldName of the model
    * and the optional '-' says whether the order is descending or ascending.
    */
   void order(String orderString) {
     // TODO: validate [orderString] (e.g. is name valid)
@@ skipping 217 matching lines @@
 
 Future<List<Model>> _lookupHelper(
     DatastoreDB db, List<Key> keys,
     {datastore.Transaction datastoreTransaction}) {
   var entityKeys = keys.map(db.modelDB.toDatastoreKey).toList();
   return db.datastore.lookup(entityKeys, transaction: datastoreTransaction)
       .then((List<datastore.Entity> entities) {
     return entities.map(db.modelDB.fromDatastoreEntity).toList();
   });
 }