Throw on unsupported fields projection

packages/minimongo/minimongo_tests.js

@@ -1116,6 +1116,18 @@ Tinytest.add("minimongo - fetch with fields", function (test) {
     if (!i) return;
     test.isTrue(x.i === arr[i-1].i + 1);
   });
+
+  // Temporary unsupported operators
+  // queries are taken from MongoDB docs examples
+  test.throws(function () {
+    c.find({}, { fields: { 'grades.$': 1 } });
+  });
+  test.throws(function () {
+    c.find({}, { fields: { grades: { $elemMatch: { mean: 70 } } } });
+  });
+  test.throws(function () {
+    c.find({}, { fields: { grades: { $slice: [20, 10] } } });
+  });
 });
 
 Tinytest.add("minimongo - fetch with projection, subarrays", function (test) {

packages/minimongo/projection.js

@@ -6,6 +6,11 @@
 //                       according to projection rules. Doesn't retain subfields
 //                       of passed argument.
 LocalCollection._compileProjection = function (fields) {
+  // XXX: $-operators are not supported in fields projections yet
+  if (! LocalCollection._supportedProjection(fields))
+    throw MinimongoError("Minimongo doesn't support fields projections "
+                         + "with $-operators yet");
+
   var _idProjection = _.isUndefined(fields._id) ? true : fields._id;
   var details = projectionDetails(fields);
 
@@ -159,3 +164,12 @@ pathsToTree = function (paths, newLeafFn, conflictFn, tree) {
   return tree;
 };
 
+LocalCollection._supportedProjection = function (fields) {
+  return _.all(fields, function (val, keyPath) {
+    if (_.contains(keyPath.split('.'), '$'))
+      return false;
+    return !_.isObject(val) ||
+      (!_.has(val, '$slice') && !_.has(val, '$elemMatch'));
+  });
+};
+