When I use $push in mongodb, the expected outcome turns out differently. It actually wraps the object I want to push in an array. This is problematic because I'd have to map over the result just to extract it. Any help would be greatly appreciated.

My Mongo Query

const pushAction = {
    $push: {
      cart: {
        id: itemId,
        quantity: quantity
      }
    }
  }

  // Add item to user's cart
  User.update({_id: userId}, pushAction, (err, success) => {
    if (err) {
      res.status(422).json({'error': 'There was a problem adding the item to your cart.'});
    }
    if (success) {
      // Find user and return the cart
      User.findOne({_id: userId}, {cart: 1}, (err, user) => {
        res.status(200).json({'message': 'The item was successfully added to your cart.', cart: user.cart});
      })
    }
  });

User Schema

// Define User Model
const userSchema = new Schema({
  firstName: {
    type: Schema.Types.String,
    required: true
  },
  lastName: {
    type: Schema.Types.String,
    required: true
  },
  password: {
    type: Schema.Types.String,
    required: true
  },
  email: {
    type: Schema.Types.String,
    required: true
  },
  cart: {
    type: Schema.Types.Array
  },
  dateCreated: {
    type: Schema.Types.Date,
    default: Date.now,
    required: true
  },
  dateUpdated: [
    {
      date: {
        type: Schema.Types.Date
      },
      details: {
        type: Schema.Types.ObjectId
      }
    }
  ],
  verified: {
    type: Schema.Types.Boolean,
    required: true
  },
  role: {
    type: Schema.Types.String,
    default: ROLES_BASIC_USER
  }
});

Expected Outcome

"cart" : [ 
    {
        "id" : "587b6b69799ad7ff650edbb5",
        "quantity" : 1
    }, 
    {
        "id" : "587b6b69799ad7ff650edbb5",
        "quantity" : 1
    }, 
    {
        "id" : "587b6b69799ad7ff650edbb5",
        "quantity" : 1
    }
],

Actual Result

"cart" : [ 
        [ 
            {
                "id" : "587b6b69799ad7ff650edbb5",
                "quantity" : 1
            }
        ], 
        [ 
            {
                "id" : "587b6b69799ad7ff650edbb5",
                "quantity" : 1
            }
        ], 
        [ 
            {
                "id" : "587b6b69799ad7ff650edbb5",
                "quantity" : 1
            }
        ]
    ]