Modifying a stream with $project

The find function allowed us to do simple whitelist projection. The aggregate pipeline gives us many more options.

We can use $project to modify all the documents in the pipeline. We can remove elements, allow elements through, rename elements, and even create brand new elements based on expressions.

Say we have a set of voucher codes, like this:

  {
    "firstName" : "Dave",
    "lastName" : "Jones",
    "code" : "74wy zphz",
    "usedAt" : ISODate("2015-06-13T17:47:20.423Z"),
    "email" : "123@gmail.com"
  },
  {
    "firstName" : "Stuart",
    "lastName" : "Hat",
    "code" : "7uwz e3cw",
    "usedAt" : ISODate("2015-06-13T17:47:50.489Z"),
    "email" : "456@gmail.com"
  }
  ...

We can use project to restrict the fields that are passed through. We pick the fields we like and set true to pass them through unchanged.

db.entrycodes.aggregate({
  $project: {
    email: true,
    code: true,
  },
});

Removing the id

Remove the id field bay passing _id: false.

This will yield a set something like the following:

[
  {
    code: "7uwy zphz",
    email: "123@gmail.com",
  },
  {
    code: "7uwz eccw",
    email: "123@gmail.com",
  },
];

Renaming Fields

We can use project to rename fields if we want to. We use an expression: $lastName to pull out the value from the lastName field, and project it forward into the surname field.

db.entrycodes.aggregate({
  $project: {
    surname: "$lastName",
  },
});

This will yield something like the following:

  [
    {
      surname : "Jones"
    },
    {
      surname : "Hat"
    }
    ...
  ]

Chaining $match and$ project

We can chain $match and$ project together. Say we have a list of codes, and some have not yet been used. We want to pull out the names and emails, but only from the codes which have been used.

  [,
    {
      "code" : "7uwz eccw"
    }
    {
      "firstName" : "Dave",
      "lastName" : "Jones",
      "code" : "7uwy zphz",
      "usedAt" : ISODate("2015-06-13T17:47:20.423Z"),
      "email" : "123@gmail.com",
      "winner": true
    },
    {
      "firstName" : "Stuart",
      "lastName" : "Hat",
      "code" : "7uwz eccw",
      "usedAt" : ISODate("2015-06-13T17:47:50.489Z"),
      "email" : "123@gmail.com"
    },
    {
      "code" : "7uwz eccw"
    }
    ...
  ]

We might first $match the codes which have a usedAt field, and then use$ project to pull out the names and emails from the remainder.

db.entrycodes.aggregate(
  {
    $match: {
      usedAt: {
        $exists: true,
      },
    },
  },
  {
    $project: {
      firstName: true,
      lastName: true,
      email: true,
      _id: false,
    },
  }
);

Exercise

Make a list of cat names.
First $match people with cats, or the output will be a bit sparse.
Now use $project to pull out only the cat names. You will need to use the dot syntax: '$ cat.name’.

Exercise - Stocks