added Datastore.count with documentation and tests

README.md

@@ -164,6 +164,19 @@ db.findOne({ _id: 'id1' }, function (err, doc) {
 });
 ```
 
+#### Counting objects
+Similar to `find`, you can count the number of matching documents by calling `count`. For example
+
+```javascript
+// Using same datastore as above
+
+// Count all planets in the solar system
+db.count({ system: 'solar' }, function (err, count) {
+  // count equals to 3
+  // If no document is found, count is 0
+});
+```
+
 #### Operators ($lt, $lte, $gt, $gte, $in, $nin, $ne, $exists, $regex)
 The syntax is `{ field: { $op: value } }` where `$op` is any comparison operator:  
 

lib/datastore.js

@@ -272,6 +272,35 @@ Datastore.prototype.insert = function () {
   this.executor.push({ this: this, fn: this._insert, arguments: arguments });
 };
 
+/**
+ * Count all documents matching the query
+ * @param {Object} query MongoDB-style query
+ *
+ * @api private Use count
+ */
+Datastore.prototype._count = function(query, callback) {
+  var res = 0
+    , self = this
+    , candidates = this.getCandidates(query)
+    , i
+    ;
+
+  try {
+    for (i = 0; i < candidates.length; i += 1) {
+      if (model.match(candidates[i], query)) {
+        res++;
+      }
+    }
+  } catch (err) {
+    return callback(err);
+  }
+
+  return callback(null, res);
+}
+
+Datastore.prototype.count = function() {
+    this.executor.push({this: this, fn: this._count, arguments: arguments });
+}
 
 /**
  * Find all documents matching the query

test/db.test.js

@@ -484,6 +484,90 @@ describe('Database', function () {
 
   });   // ==== End of 'Find' ==== //
 
+  describe('Count', function() {
+
+    it('Count all documents if an empty query is used', function (done) {
+      async.waterfall([
+      function (cb) {
+        d.insert({ somedata: 'ok' }, function (err) {
+          d.insert({ somedata: 'another', plus: 'additional data' }, function (err) {
+            d.insert({ somedata: 'again' }, function (err) { return cb(err); });
+          });
+        });
+      }
+      , function (cb) {   // Test with empty object
+        d.count({}, function (err, docs) {
+          assert.isNull(err);
+          docs.should.equal(3);
+          return cb();
+        });
+      }
+      ], done);
+    });
+
+    it('Count all documents matching a basic query', function (done) {
+      async.waterfall([
+      function (cb) {
+        d.insert({ somedata: 'ok' }, function (err) {
+          d.insert({ somedata: 'again', plus: 'additional data' }, function (err) {
+            d.insert({ somedata: 'again' }, function (err) { return cb(err); });
+          });
+        });
+      }
+      , function (cb) {   // Test with query that will return docs
+        d.count({ somedata: 'again' }, function (err, docs) {
+          assert.isNull(err);
+          docs.should.equal(2);
+          return cb();
+        });
+      }
+      , function (cb) {   // Test with query that doesn't match anything
+        d.count({ somedata: 'nope' }, function (err, docs) {
+          assert.isNull(err);
+          docs.should.equal(0);
+          return cb();
+        });
+      }
+      ], done);
+    });
+
+    it('Array fields match if any element matches', function (done) {
+      d.insert({ fruits: ['pear', 'apple', 'banana'] }, function (err, doc1) {
+        d.insert({ fruits: ['coconut', 'orange', 'pear'] }, function (err, doc2) {
+          d.insert({ fruits: ['banana'] }, function (err, doc3) {
+            d.count({ fruits: 'pear' }, function (err, docs) {
+              assert.isNull(err);
+              docs.should.equal(2);
+
+              d.count({ fruits: 'banana' }, function (err, docs) {
+                assert.isNull(err);
+                docs.should.equal(2);
+
+                d.count({ fruits: 'doesntexist' }, function (err, docs) {
+                  assert.isNull(err);
+                  docs.should.equal(0);
+
+                  done();
+                });
+              });
+            });
+          });
+        });
+      });
+    });
+
+    it('Returns an error if the query is not well formed', function (done) {
+      d.insert({ hello: 'world' }, function () {
+        d.count({ $or: { hello: 'world' } }, function (err, docs) {
+          assert.isDefined(err);
+          assert.isUndefined(docs);
+
+	  done();
+        });
+      });
+    });
+
+  }); 
 
   describe('Update', function () {