Use Populate to Join Queries

Mongoose supports join queries by doing mutliple queries. Note: State of collections in the database can change during the join query.

usersRouter.get('/', async (request, response) => {
  const users = await User.find({}).populate('blogs', { title: 1, author: 1, url: 1 })
  response.json(users)
})

Example output:

[
 {
    "blogs": [
      {
        "title": "Whatthefuck.is",
        "author": "Dan Abramov",
        "url": "https://whatthefuck.is/",
        "id": "5f1927587228728f020a705a"
      },
      {
        "title": "Josh Branchaud",
        "author": "Josh Branchaud",
        "url": "https://dev.to/jbranchaud",
        "id": "5f1928427228728f020a705b"
      }
    ],
    "username": "mburham",
    "name": "Michael Burham",
    "id": "5f18c2a4092dee858862166f"
  } 
]

blogsRouter.get('/', async (request, response) => {
  const blogs = await Blog.find({}).populate('user', { username: 1, name: 1 })
  response.json(blogs)
})

Example output:

[
  {
    "likes": 0,
    "title": "Josh Branchaud",
    "author": "Josh Branchaud",
    "url": "https://dev.to/jbranchaud",
    "user": {
      "username": "mburham",
      "name": "Michael Burham",
      "id": "5f18c2a4092dee858862166f"
    },
    "id": "5f1928427228728f020a705b"
  },
  {
    "likes": 0,
    "title": "Life at the bleeding edge(of web standards)",
    "author": "Lea Verou",
    "url": "https://lea.verou.me/",
    "user": {
      "username": "hobbes",
      "name": "Hobbes the cat",
      "id": "5f188925e811ec705b7525a5"
    },
    "id": "5f192be8837fa0918991877c"
  },
]

Source -> Mongoose: Populate method