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

lib/src/db/model_db_impl.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;
 
 /// An implementation of [ModelDB] based on model class annotations.
 ///
 /// The two constructors will scan loaded dart libraries for classes with a
 /// [Kind] annotation.
@@ skipping 126 matching lines @@
           'The class $type was not associated with a kind.');
     }
     return kind;
   }
 
   /// Returns the name of the property corresponding to the kind [kind] and
   /// [fieldName].
   String fieldNameToPropertyName(String kind, String fieldName) {
     var modelDescription = _kind2ModelDesc[kind];
     if (modelDescription == null) {
-      throw new ArgumentError('The kind $kind is unknown.');
+      throw new ArgumentError('The kind "$kind" is unknown.');
     }
     return modelDescription.fieldNameToPropertyName(fieldName);
   }
 
+  /// Converts [value] according to the [Property] named [name] in [type].
+  Object toDatastoreValue(String kind, String fieldName, Object value) {
+    var modelDescription = _kind2ModelDesc[kind];
+    if (modelDescription == null) {
+      throw new ArgumentError('The kind "$kind" is unknown.');
+    }
+    return modelDescription.encodeField(this, fieldName, value);
+  }
 
   Iterable<_ModelDescription> get _modelDescriptions {
     return _modelDesc2Type.values;
   }
 
   Map<String, Property> _propertiesForModel(
       _ModelDescription modelDescription) {
     return _modelDesc2Properties[modelDescription];
   }
 
@@ skipping 274 matching lines @@
   }
 
   String fieldNameToPropertyName(String fieldName) {
     return _field2PropertyName[fieldName];
   }
 
   String propertyNameToFieldName(ModelDBImpl db, String propertySearchName) {
     return _property2FieldName[propertySearchName];
   }
 
-  Object encodeField(ModelDBImpl db, String fieldName, Object value) {
+  Object encodeField(ModelDBImpl db, String fieldName, Object value,
+                     {bool enforceFieldExists: true}) {
     Property property = db._propertiesForModel(this)[fieldName];
-    if (property != null) return property.encodeValue(db, value);
+    if (property != null) {
+      return property.encodeValue(db, value);
+    }
+    if (enforceFieldExists) {
+      throw new ArgumentError(
+          'A field named "$fieldName" does not exist in kind "$kind".');
+    }
     return null;
   }
 }
 
 // NOTE/TODO:
 // Currently expanded properties are only
 //   * decoded if there are no clashes in [usedNames]
 //   * encoded if there are no clashes in [usedNames]
 // We might want to throw an error if there are clashes, because otherwise
 //   - we may end up removing properties after a read-write cycle
@@ skipping 50 matching lines @@
 
   String propertyNameToFieldName(ModelDBImpl db, String propertyName) {
     String fieldName = super.propertyNameToFieldName(db, propertyName);
     // If the ModelDescription doesn't know about [propertyName], it's an
     // expanded property, where propertyName == fieldName.
     if (fieldName == null) fieldName = propertyName;
     return fieldName;
   }
 
   Object encodeField(ModelDBImpl db, String fieldName, Object value) {
-    Object primitiveValue = super.encodeField(db, fieldName, value);
+    Object primitiveValue = super.encodeField(db, fieldName, value,
+        enforceFieldExists: false);
     // If superclass can't encode field, we return value here (and assume
     // it's primitive)
     // NOTE: Implicit assumption:
     // If value != null then superclass will return != null.
     // TODO: Ensure [value] is primitive in this case.
     if (primitiveValue == null) primitiveValue = value;
     return primitiveValue;
   }
 }