fix(model): allow empty arrays for bulkWrite

lib/helpers/getDefaultBulkwriteResult.js

@@ -0,0 +1,27 @@
+'use strict';
+function getDefaultBulkwriteResult() {
+  return {
+    result: {
+      ok: 1,
+      writeErrors: [],
+      writeConcernErrors: [],
+      insertedIds: [],
+      nInserted: 0,
+      nUpserted: 0,
+      nMatched: 0,
+      nModified: 0,
+      nRemoved: 0,
+      upserted: []
+    },
+    insertedCount: 0,
+    matchedCount: 0,
+    modifiedCount: 0,
+    deletedCount: 0,
+    upsertedCount: 0,
+    upsertedIds: {},
+    insertedIds: {},
+    n: 0
+  };
+}
+
+module.exports = getDefaultBulkwriteResult;

lib/model.js

@@ -31,6 +31,7 @@ const applyStatics = require('./helpers/model/applyStatics');
 const applyWriteConcern = require('./helpers/schema/applyWriteConcern');
 const assignVals = require('./helpers/populate/assignVals');
 const castBulkWrite = require('./helpers/model/castBulkWrite');
+const getDefaultBulkwriteResult = require('./helpers/getDefaultBulkwriteResult');
 const discriminator = require('./helpers/model/discriminator');
 const each = require('./helpers/each');
 const getDiscriminatorByValue = require('./helpers/discriminator/getDiscriminatorByValue');
@@ -3493,14 +3494,17 @@ Model.bulkWrite = function(ops, options, callback) {
   const validations = ops.map(op => castBulkWrite(this, op, options));
 
   callback = this.$handleCallbackError(callback);
-
   return promiseOrCallback(callback, cb => {
     cb = this.$wrapCallback(cb);
     each(validations, (fn, cb) => fn(cb), error => {
       if (error) {
         return cb(error);
       }
 
+      if (ops.length === 0) {
+        return cb(null, getDefaultBulkwriteResult());
+      }
+
       this.collection.bulkWrite(ops, options, (error, res) => {
         if (error) {
           return cb(error);

test/model.test.js

@@ -6799,10 +6799,44 @@ describe('Model', function() {
       assert.equal(typeof users[0].updatedAt, 'number');
       assert.equal(typeof users[1].updatedAt, 'number');
 
-      // not-lean queries casts to number even if stored on DB as a date
+      // not-lean queries cast to number even if stored on DB as a date
       assert.equal(users[0] instanceof User, false);
       assert.equal(users[1] instanceof User, false);
     });
   });
 
+  it('Model#bulkWrite(...) does not throw an error when provided an empty array (gh-9131)', function() {
+    return co(function*() {
+      const userSchema = new Schema();
+      const User = db.model('User', userSchema);
+
+      const res = yield User.bulkWrite([]);
+
+      assert.deepEqual(
+        res,
+        {
+          result: {
+            ok: 1,
+            writeErrors: [],
+            writeConcernErrors: [],
+            insertedIds: [],
+            nInserted: 0,
+            nUpserted: 0,
+            nMatched: 0,
+            nModified: 0,
+            nRemoved: 0,
+            upserted: []
+          },
+          insertedCount: 0,
+          matchedCount: 0,
+          modifiedCount: 0,
+          deletedCount: 0,
+          upsertedCount: 0,
+          upsertedIds: {},
+          insertedIds: {},
+          n: 0
+        }
+      );
+    });
+  });
 });