如何从 MongoDB 的数组中找到并显示特定的 object？

Question

In my app, I am having trouble writing a mongo script to find and display the service logs to one vehicle. For example, I have two users with multiple vehicles. Each vehicle has multiple service logs. With the queries I wrote, I'm not having much luck displaying only the serviceLogs objects from the second user with _id:"2" from the vehicle with _id:14 . Below is the sample data.

db.collection.insert(
{_id:"1",vehicles:[
{_id:11,make:"Mazda",model:"CX5",serviceLogs:[
{_id:110, "miles" : 7567,"service" : "Wipers replacement"}
]}]});

db.collection.insert(
{_id:"2",vehicles:[
{_id:12,make:"Mazda",model:"CX5",serviceLogs:[]},
{_id:13,make:"Chevy",model:"Sonic",serviceLogs:[]},
{_id:14,make:"Toyota",model:"Tacoma",serviceLogs:[
{_id:111, "miles" : 2134,"service" : "Brake pads replacement"},
{_id:112, "miles" : 5678,"service" : "VSS replacement"}
]}]});

Below are the queries I tried, I've tinkered around with them but not getting any luck.

db.collection.find({"_id": "2", vehicles: {$elemMatch: {make: "Toyota", model: "Tacoma"}}} ).pretty();
db.collection.find({"_id": "2", "vehicles.$.serviceLogs": {$elemMatch: {serviceLogs:{id: 111}}}})
db.collection.find({"_id": "2", vehicles: {$elemMatch: {serviceLogs:{id: 111}}}})

Expected results:

{
  "_id" : 111.0,
  "miles" : 2134.0,
  "service" : "Brake pads replacement"
}, 
{
  "_id" : 112.0,
  "miles" : 5678.0,
  "service" : "VSS replacement"
}

Any suggestions would be greatly appreciated, thanks.

Answer 1

Try this:

db.collection.find({"_id": "2", vehicles: {$elemMatch: {make: "Toyota", model: "Tacoma"}}} ).project({serviceLogs: 1});

More info here: https://docs.mongodb.com/manual/tutorial/project-fields-from-query-results/

Answer 2

Although I did not get the exact answer I initially expected, I have a query that also got the vehicle information along with the service logs. This is better as I am able to retrieve the vehicle make, model, and all of its service logs. This query will return only one vehicle that meets the _id and vehicle._id . Below is an example query for the vehicle with _id:"2" .

db.collection.aggregate([
    { $match: { _id: "2" } },
    {
      $project: {
        vehicles: {
          $filter: {
            input: "$vehicles",
            as: "vehicle",
            cond: { $eq: ["$$vehicle._id", 14] }
          }
        },
        _id: 0
      }
    }
  ]).pretty()

This query will yield the result below:

{
    "vehicles" : [
            {
                    "_id" : 14,
                    "make" : "Toyota",
                    "model" : "Tacoma",
                    "serviceLogs" : [
                            {
                                    "_id" : 111,
                                    "miles" : 2134,
                                    "service" : "Brake pads replacement"
                            },
                            {
                                    "_id" : 112,
                                    "miles" : 5678,
                                    "service" : "VSS replacement"
                            }
                    ]
            }
    ]
}

Using MongoDB's aggregate and $project , I am able to retrieve just the data from the vehicle I am targeting. In my React application, below is the sample code I used to assign the vehicle data to React states.

API.getOneVehicleForUser(this.state.uid, this.state.vehicleId) // API call passing in the user ID and the vehicle id we want to target
  .then(res => { // After the API call returns successfully, we assign the data to React states
      this.setState({
        make: res.data[0].vehicles[0].make,
        model: res.data[0].vehicles[0].model,
        serviceLogs: res.data[0].vehicles[0].serviceLogs
      })
  .catch(err => console.log(err)); // Do anything you want here to handle errors if the API call should fail

this.state.serviceLogs is an array, which stores all of the vehicle service logs. Here I can do anything I want, such as count and display how many service logs the vehicle has using this.state.serviceLogs.length or display each service log by their miles and service type.

I know this may not be the cleanest solution, but for my application, it has been working very well.