From 74c9373d3ab26975671b2560101d97ede5a006f5 Mon Sep 17 00:00:00 2001
From: Slava Kim <slava@meteor.com>
Date: Mon, 11 Nov 2013 11:43:06 -0800
Subject: [PATCH] Define projection function contract: returned doc shouldn't
 retain anything from the passed argument.

---
 packages/minimongo/minimongo_tests.js | 31 +++++++++++++++++++++++++++
 packages/minimongo/projection.js      |  6 ++++++
 2 files changed, 37 insertions(+)

diff --git a/packages/minimongo/minimongo_tests.js b/packages/minimongo/minimongo_tests.js
index 3f2ce855fa..43dbbf45e9 100644
--- a/packages/minimongo/minimongo_tests.js
+++ b/packages/minimongo/minimongo_tests.js
@@ -1151,6 +1151,37 @@ Tinytest.add("minimongo - fetch with projection, subarrays", function (test) {
                     {a: [ [ { c: 2 }, { c: 4 } ], { c: 5 }, [ { c: 9 } ] ] });
 });
 
+Tinytest.add("minimongo - fetch with projection, deep copy", function (test) {
+  // Compiled fields projection defines the contract: returned document doesn't
+  // retain anything from the passed argument.
+  var doc = {
+    a: { x: 42 },
+    b: {
+      y: { z: 33 }
+    },
+    c: "asdf"
+  };
+
+  var fields = {
+    'a': 1,
+    'b.y': 1
+  };
+
+  var projectionFn = LocalCollection._compileProjection(fields);
+  var filteredDoc = projectionFn(doc);
+  doc.a.x++;
+  doc.b.y.z--;
+  test.equal(filteredDoc.a.x, 42, "projection returning deep copy - including");
+  test.equal(filteredDoc.b.y.z, 33, "projection returning deep copy - including");
+
+  fields = { c: 0 };
+  projectionFn = LocalCollection._compileProjection(fields);
+  filteredDoc = projectionFn(doc);
+
+  doc.a.x = 5;
+  test.equal(filteredDoc.a.x, 43, "projection returning deep copy - excluding");
+});
+
 Tinytest.add("minimongo - observe ordered with projection", function (test) {
   // These tests are copy-paste from "minimongo -observe ordered",
   // slightly modified to test projection
diff --git a/packages/minimongo/projection.js b/packages/minimongo/projection.js
index 3230fc6b19..73a10c59cb 100644
--- a/packages/minimongo/projection.js
+++ b/packages/minimongo/projection.js
@@ -1,4 +1,10 @@
 // Knows how to compile a fields projection to a predicate function.
+// @returns - Function: a closure that filters out an object according to the
+//            fields projection rules:
+//            @param obj - Object: MongoDB-styled document
+//            @returns - Object: a document with the fields filtered out
+//                       according to projection rules. Doesn't retain subfields
+//                       of passed argument.
 LocalCollection._compileProjection = function (fields) {
   var _idProjection = _.isUndefined(fields._id) ? true : fields._id;
   var details = projectionDetails(fields);