diff --git a/packages/mongo-livedata/doc_fetcher.js b/packages/mongo-livedata/doc_fetcher.js
new file mode 100644
index 0000000000..11051b6129
--- /dev/null
+++ b/packages/mongo-livedata/doc_fetcher.js
@@ -0,0 +1,60 @@
+var Future = Npm.require('fibers/future');
+
+DocFetcher = function (mongoConnection) {
+  var self = this;
+  self._mongoConnection = mongoConnection;
+  // Map from cache key -> [Future]
+  self._futuresForCacheKey = {};
+};
+
+_.extend(DocFetcher.prototype, {
+  // Fetches document "id" from collectionName, returning it or null if not
+  // found. Throws other errors. Can yield.
+  //
+  // If you make multiple calls to fetch() with the same cacheKey (a string),
+  // DocFetcher may assume that they all return the same document. (It does
+  // not check to see if collectionName/id match.)
+  fetch: function (collectionName, id, cacheKey) {
+    var self = this;
+
+    check(collectionName, String);
+    // id is some sort of scalar
+    check(cacheKey, String);
+
+    // If there's already an in-progress fetch for this cache key, yield until
+    // it's done and return whatever it returns.
+    if (_.has(self._futuresForCacheKey, cacheKey)) {
+      var f = new Future;
+      self._futuresForCacheKey.push(f);
+      return f.wait();
+    }
+
+    var futures = self._futuresForCacheKey[cacheKey] = [];
+
+    try {
+      var doc = self._mongoConnection.findOne(
+        collectionName, {_id: id}) || null;
+      // Return doc to all fibers that are blocking on us. Note that this array
+      // can continue to grow during calls to Future.return.
+      while (!_.isEmpty(futures)) {
+        // Clone the document so that the various calls to fetch don't return
+        // objects that are intertwingled with each other. Clone before popping
+        // the future, so that if clone throws, the error gets thrown to the
+        // next future instead of that fiber hanging.
+        var clonedDoc = EJSON.clone(doc);
+        futures.pop().return(clonedDoc);
+      }
+    } catch (e) {
+      while (!_.isEmpty(futures)) {
+        futures.pop().throw(e);
+      }
+      throw e;
+    } finally {
+      // XXX consider keeping the doc around for a period of time before
+      // removing from the cache
+      delete self._futuresForCacheKey[cacheKey];
+    }
+
+    return doc;
+  }
+});
diff --git a/packages/mongo-livedata/id_map.js b/packages/mongo-livedata/id_map.js
index 160ee505c4..57ee9cd8d9 100644
--- a/packages/mongo-livedata/id_map.js
+++ b/packages/mongo-livedata/id_map.js
@@ -1,36 +1,36 @@
 IdMap = function () {
   var self = this;
-  self.map = {};
+  self._map = {};
 };
 
 _.extend(IdMap.prototype, {
   get: function (id) {
     var self = this;
     var key = LocalCollection._idStringify(id);
-    return self.map[key];
+    return self._map[key];
   },
   set: function (id, value) {
     var self = this;
     var key = LocalCollection._idStringify(id);
-    self.map[key] = value;
+    self._map[key] = value;
   },
   remove: function (id) {
     var self = this;
     var key = LocalCollection._idStringify(id);
-    delete self.map[key];
+    delete self._map[key];
   },
   has: function (id) {
     var self = this;
     var key = LocalCollection._idStringify(id);
-    return _.has(self.map, key);
+    return _.has(self._map, key);
   },
   // XXX used?
   setDefault: function (id, def) {
     var self = this;
     var key = LocalCollection._idStringify(id);
-    if (_.has(self.map, key))
-      return self.map[key];
-    self.map[key] = def;
+    if (_.has(self._map, key))
+      return self._map[key];
+    self._map[key] = def;
     return def;
   }
 });
diff --git a/packages/mongo-livedata/package.js b/packages/mongo-livedata/package.js
index 3827dbb929..5389cd1157 100644
--- a/packages/mongo-livedata/package.js
+++ b/packages/mongo-livedata/package.js
@@ -38,7 +38,7 @@ Package.on_use(function (api) {
   // For tests only.
   api.export('MongoTest', 'server');
 
-  api.add_files('id_map.js', 'server');
+  api.add_files(['id_map.js', 'doc_fetcher.js'], 'server');
   api.add_files('mongo_driver.js', 'server');
   api.add_files('local_collection_driver.js', ['client', 'server']);
   api.add_files('remote_collection_driver.js', 'server');