diff --git a/lib/indexes.js b/lib/indexes.js
index d39ece0..8271dbf 100644
--- a/lib/indexes.js
+++ b/lib/indexes.js
@@ -1,9 +1,18 @@
-var BinarySearchTree = require('binary-search-tree').BinarySearchTree;
+var BinarySearchTree = require('binary-search-tree').BinarySearchTree
+  , model = require('./model')
+  ;
+
+/**
+ * We can't use the one in model here since it doesn't work for arrays
+ */
+function checkValueEquality (a, b) {
+  return model.compareThings(a, b) === 0;
+}
 
 
 /**
  * Create a new index
- * @param {String} options.fieldName On which field should the index apply
+ * @param {String} options.fieldName On which field should the index apply (can use dot notation to index on sub fields)
  * @param {Datastore} options.datastore Datastore on which the index is created
  * @param {Boolean} options.unique Optional, enforce a unique constraint (default: false)
  * @param {Boolean} options.sparse Optional, allow a sparse index (we can have documents for which fieldName is undefined) (default: false)
@@ -14,10 +23,28 @@ function Index (options) {
   this.unique = options.unique || false;
   this.sparse = options.unique || false;
 
-  if (this.sparse) { this.fieldUndefined = []; }   // Will store all elements for which the indexed field is not defined
+  if (this.sparse) { this.unindexedDocs = []; }
+
+  this.tree = new BinarySearchTree({ unique: this.unique, compareKeys: model.compareThings, checkValueEquality: checkValueEquality });
 }
 
 
+/**
+ * Insert a new document in the index (synchronously)
+ */
+Index.prototype.insert = function (doc) {
+  var key = model.getDotValue(doc, this.fieldName);
+
+  // We don't index documents that don't contain the field if the index is sparse
+  if (key === undefined && this.sparse) {
+    this.unindexedDocs.push(doc);
+    return;
+  }
+
+  this.tree.insert(key, doc);
+};
+
+
 
 
 // Interface
diff --git a/lib/model.js b/lib/model.js
index e9f6a80..29b4161 100644
--- a/lib/model.js
+++ b/lib/model.js
@@ -156,16 +156,21 @@ function compareArrays (a, b) {
 
 
 /**
- * Compare things
+ * Compare { things U undefined }
  * Things are defined as any native types (string, number, boolean, null, date) and objects
+ * We need to compare with undefined as it will be used in indexes
  * In the case of objects and arrays, we compare the serialized versions
- * If two objects dont have the same type, the (arbitrary) type hierarchy is: null, number, strings, boolean, dates, arrays, objects
+ * If two objects dont have the same type, the (arbitrary) type hierarchy is: undefined, null, number, strings, boolean, dates, arrays, objects
  * Return -1 if a < b, 1 if a > b and 0 if a = b (note that equality here is NOT the same as defined in areThingsEqual!)
  */
 function compareThings (a, b) {
   var aKeys, bKeys, comp, i;
 
-  // null is less than everything except null
+  // undefined
+  if (a === undefined) { return b === undefined ? 0 : -1; }
+  if (b === undefined) { return a === undefined ? 0 : 1; }
+
+  // null
   if (a === null) { return b === null ? 0 : -1; }
   if (b === null) { return a === null ? 0 : 1; }
 
diff --git a/test/model.test.js b/test/model.test.js
index 23c27e7..e1d243b 100644
--- a/test/model.test.js
+++ b/test/model.test.js
@@ -331,7 +331,18 @@ describe('Model', function () {
 
   describe('Comparing things', function () {
 
-    it('null is less than everything except null', function () {
+    it('undefined is the smallest', function () {
+      var otherStuff = [null, "string", "", -1, 0, 5.3, 12, true, false, new Date(12345), {}, { hello: 'world' }, [], ['quite', 5]];
+
+      model.compareThings(undefined, undefined).should.equal(0);
+
+      otherStuff.forEach(function (stuff) {
+        model.compareThings(undefined, stuff).should.equal(-1);
+        model.compareThings(stuff, undefined).should.equal(1);
+      });
+    });
+
+    it('Then null', function () {
       var otherStuff = ["string", "", -1, 0, 5.3, 12, true, false, new Date(12345), {}, { hello: 'world' }, [], ['quite', 5]];
 
       model.compareThings(null, null).should.equal(0);
@@ -342,7 +353,7 @@ describe('Model', function () {
       });
     });
 
-    it('After null, numbers are the smallest', function () {
+    it('Then numbers', function () {
       var otherStuff = ["string", "", true, false, new Date(4312), {}, { hello: 'world' }, [], ['quite', 5]]
         , numbers = [-12, 0, 12, 5.7];