Can use with

lib/model.js

@@ -264,6 +264,9 @@ lastStepModifierFunctions.$unset = function (obj, field, value) {
 
 /**
  * Push an element to the end of an array field
+ * Optional modifier $each instead of value to push several values
+ * Optional modifier $slice to slice the resulting array, see https://docs.mongodb.org/manual/reference/operator/update/slice/
+ * Différeence with MongoDB: if $slice is specified and not $each, we act as if value is an empty array
  */
 lastStepModifierFunctions.$push = function (obj, field, value) {
   // Create the array if it doesn't exist
@@ -271,32 +274,33 @@ 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.$slice && value.$each === undefined) {
+    value.$each = [];
+  }
+
   if (value !== null && typeof value === 'object' && value.$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 $each"); }
+    if (Object.keys(value).length >= 3 || (Object.keys(value).length === 2 && value.$slice === undefined)) { throw new Error("Can only use $slice in cunjunction with $each when $push to array"); }
     if (!util.isArray(value.$each)) { throw new Error("$each requires an array value"); }
 
     value.$each.forEach(function (v) {
       obj[field].push(v);
     });
 
-    var slice = parseInt(value.$slice, 10);
-    if (!isNaN(slice)) {
-      if (slice === 0) {
+    if (value.$slice === undefined || typeof value.$slice !== 'number') { return; }
+
+    if (value.$slice === 0) {
       obj[field] = [];
     } else {
       var start, end, n = obj[field].length;
-        if (slice < 0) {
-          start = Math.max(0, n + slice);
+      if (value.$slice < 0) {
+        start = Math.max(0, n + value.$slice);
         end = n;
-        } else if (slice > 0) {
+      } else if (value.$slice > 0) {
         start = 0;
-          end = Math.min(n, slice);
+        end = Math.min(n, value.$slice);
       }
       obj[field] = obj[field].slice(start, end);
     }
-    }
   } else {
     obj[field].push(value);
   }

test/model.test.js

@@ -520,7 +520,7 @@ describe('Model', function () {
         }).should.throw();
       });
 
-      it('Can use the $slice modifier to limits the number of array elements', function () {
+      it('Can use the $slice modifier to limit the number of array elements', function () {
         var obj = { arr: ['hello'] }
           , modified;
 
@@ -547,6 +547,18 @@ describe('Model', function () {
 
         modified = model.modify(obj, { $push: { arr: { $each: [], $slice: 1 } } });
         assert.deepEqual(modified, { arr: ['hello'] });
+
+        // $each not specified, but $slice is
+        modified = model.modify(obj, { $push: { arr: { $slice: 1 } } });
+        assert.deepEqual(modified, { arr: ['hello'] });
+
+        (function () {
+          modified = model.modify(obj, { $push: { arr: { $slice: 1, unauthorized: true } } });
+        }).should.throw();
+
+        (function () {
+          modified = model.modify(obj, { $push: { arr: { $each: [], unauthorized: true } } });
+        }).should.throw();
       });
 
     });   // End of '$push modifier'