Add support for the array update operator.

See https://docs.mongodb.org/manual/reference/operator/update/slice/ for details.

lib/model.js

@@ -272,12 +272,31 @@ lastStepModifierFunctions.$push = function (obj, field, value) {
   if (!util.isArray(obj[field])) { throw new Error("Can't $push an element on non-array values"); }
 
   if (value !== null && typeof value === 'object' && value.$each) {
-    if (Object.keys(value).length > 1) { throw new Error("Can't use another field in conjunction with $each"); }
+    var keys = Object.keys(value);
+    if (keys.length > 1 && keys.sort()[1] !== '$slice') {
+      throw new Error("Can't use another field in conjunction with $each"); }
     if (!util.isArray(value.$each)) { throw new Error("$each requires an array value"); }
 
     value.$each.forEach(function (v) {
       obj[field].push(v);
     });
+
+    var slice = parseInt(value.$slice, 10);
+    if (!isNaN(slice)) {
+      if (slice === 0) {
+        obj[field] = [];
+      } else {
+        var start, end, n = obj[field].length;
+        if (slice < 0) {
+          start = Math.max(0, n + slice);
+          end = n;
+        } else if (slice > 0) {
+          start = 0;
+          end = Math.min(n, slice);
+        }
+        obj[field] = obj[field].slice(start, end);
+      }
+    }
   } else {
     obj[field].push(value);
   }

test/model.test.js

@@ -520,6 +520,35 @@ describe('Model', function () {
         }).should.throw();
       });
 
+      it('Can use the $slice modifier to limits the number of array elements', function () {
+        var obj = { arr: ['hello'] }
+          , modified;
+
+        modified = model.modify(obj, { $push: { arr: { $each: ['world', 'earth', 'everything'], $slice: 1 } } });
+        assert.deepEqual(modified, { arr: ['hello'] });
+
+        modified = model.modify(obj, { $push: { arr: { $each: ['world', 'earth', 'everything'], $slice: -1 } } });
+        assert.deepEqual(modified, { arr: ['everything'] });
+
+        modified = model.modify(obj, { $push: { arr: { $each: ['world', 'earth', 'everything'], $slice: 0 } } });
+        assert.deepEqual(modified, { arr: [] });
+
+        modified = model.modify(obj, { $push: { arr: { $each: ['world', 'earth', 'everything'], $slice: 2 } } });
+        assert.deepEqual(modified, { arr: ['hello', 'world'] });
+
+        modified = model.modify(obj, { $push: { arr: { $each: ['world', 'earth', 'everything'], $slice: -2 } } });
+        assert.deepEqual(modified, { arr: ['earth', 'everything'] });
+
+        modified = model.modify(obj, { $push: { arr: { $each: ['world', 'earth', 'everything'], $slice: -20 } } });
+        assert.deepEqual(modified, { arr: ['hello', 'world', 'earth', 'everything'] });
+
+        modified = model.modify(obj, { $push: { arr: { $each: ['world', 'earth', 'everything'], $slice: 20 } } });
+        assert.deepEqual(modified, { arr: ['hello', 'world', 'earth', 'everything'] });
+
+        modified = model.modify(obj, { $push: { arr: { $each: [], $slice: 1 } } });
+        assert.deepEqual(modified, { arr: ['hello'] });
+      });
+
     });   // End of '$push modifier'
 
     describe('$addToSet modifier', function () {