.slice() before .populate() causes data loss (with test)

[aforty]

What is the current behavior?
If .slice() is called before .populate() then data of the original object is lost.

Reproduce
Add this test to test/model.populate.test.js and run mocha. I took the test "populating an array of refs and fetching many" and added the line assert.equal(blogposts[0].title, 'Woot');

  it('populating an array of refs, slicing, and fetching many', function(done) {
    var db = start(),
        BlogPost = db.model('RefBlogPost', posts),
        User = db.model('RefUser', users);

    User.create({
      name: 'Fan 1',
      email: 'fan1@learnboost.com'
    }, function(err, fan1) {
      assert.ifError(err);

      User.create({
        name: 'Fan 2',
        email: 'fan2@learnboost.com'
      }, function(err, fan2) {
        assert.ifError(err);

        BlogPost.create({
          title: 'Woot',
          fans: [fan1, fan2]
        }, function(err, post1) {
          assert.ifError(err);

          BlogPost.create({
            title: 'Woot',
            fans: [fan2, fan1]
          }, function(err, post2) {
            assert.ifError(err);

            BlogPost
            .find({_id: {$in: [post1._id, post2._id]}})
            .slice('fans', [0, 5])
            .populate('fans')
            .exec(function(err, blogposts) {
              db.close();
              assert.ifError(err);

              assert.equal(blogposts[0].title, 'Woot');

              assert.equal(blogposts[0].fans[0].name, 'Fan 1');
              assert.equal(blogposts[0].fans[0].email, 'fan1@learnboost.com');
              assert.equal(blogposts[0].fans[1].name, 'Fan 2');
              assert.equal(blogposts[0].fans[1].email, 'fan2@learnboost.com');

              assert.equal(blogposts[1].fans[0].name, 'Fan 2');
              assert.equal(blogposts[1].fans[0].email, 'fan2@learnboost.com');
              assert.equal(blogposts[1].fans[1].name, 'Fan 1');
              assert.equal(blogposts[1].fans[1].email, 'fan1@learnboost.com');
              done();
            });
          });
        });
      });
    });
  });

What is the expected behavior?
The above test should pass.

Please mention your node.js, mongoose and MongoDB version.
node 8.3.0
mongod 3.2.11
mongoose 4.12.4

[vkarpov15]

Thanks for the detailed repro instructions, will investigate ASAP 👍

[aforty]

Thanks for the quick fix! 🍻

[adijes]

This bug still reproduce when populating one of schema fields.

Reproduce
Add this test to test/model.populate.test.js and run mocha. It will fail
on the line assert.equal(blogposts[0].title, 'Test 1');

    it('populating schema field, slicing, and fetching', function(done) {
      var BlogPost = db.model('blog-post', new Schema({
        title: String,
        user: { type: ObjectId, ref: 'user' },
        fans: [{ type: ObjectId}]
      }));
      var User = db.model('user', new Schema({ name: String }));

      User.create([{ name: 'Fan 1' }], function(error, fans) {
        assert.ifError(error);
        var posts = [
          { title: 'Test 1', user: fans[0]._id, fans: [fans[0]._id] },
        ];
        BlogPost.create(posts, function(error) {
          assert.ifError(error);
          BlogPost.
            find({}).
            slice('fans', [0, 2]).
            populate('user').
            exec(function(err, blogposts) {
              assert.ifError(error);

              assert.equal(blogposts[0].user.name, 'Fan 1');
              assert.equal(blogposts[0].title, 'Test 1');
              done();
            });
        });
      });
    });

What is the expected behaviour?
The test mentioned should pass

Please mention your node.js, mongoose and MongoDB version.
node 8.4.0
mongod 3.4.1
mongoose 4.12.6

This test passed in mongoose 4.11.13, and since version 4.11.14 stopped working