Filters

Filters are objects with special fields starting with $ which represent comparison operators. This notation takes a bit of time to get used to, but it is really convenient, since it matches the structure of the documents themselves.

Some examples:

{ name: "pauek" }                             // name = "pauek"
{ age: { $gt: 20 } }                          // age > 20
{ status: { $in: ["A", "B"] } }               // status IN ("A", "B")
{ $and: [{ a: { $gte: 1 } }, { b: "two" }] }  // a >= 1 AND b = 'two'
{ $or:  [{ a: { $lt: 1 } }, { b: "two" }]}    // a < 1 OR b = 'two'
{ $not: { $or: [{ a: 1 }, { b: "two" }] } }   // NOT (a = 1 OR b = 'two')
{ name: /^z/ }                                // name LIKE 'z%'
{ "position.coords.x": { $gt: 10 } }          // position.coords.x > 10
{ tags: "jazz" }                              // tags contains "jazz"
{ tags: ["jazz", "indie"] }                   // tags = ["jazz", "indie"]
{ tags: { $all: ["classical", "popular"] } }  // has all tags mentioned
{
   place: /land$/,
   coords: { x: { $gt: 10 }, y: { $lt: 10 } } // querying subdocs
}

Find

findOne looks for the first document matching the query parameter:

db.users.findOne({ isChild: true });

{
  "_id": ObjectId("5ee9fa446b327281a5213f44"),
  "username": "zipi",
  "age": 9
}

find returns a cursor for the whole sequence of documents matching a certain query:

db.users.find({ name: { $regex: "^z" } });

{
  "_id": ObjectId("5ee9fa446b327281a5213f44"),
  "username": "zipi",
  "age": 9
}
{
  "_id": ObjectId("5ee9fa446b327281a5213f45"),
  "username": "zape",
  "age": 9
}

find and findOne return a cursor, an object which can produce a sequence of the results of a query. Within the Mongo Shell, cursors are iterated automatically and the documents shown on screen. Cursors have methods to alter the result of the search and filter, sort, limit, etc.

Sorting

The sort method can be applied to the result of find, and receives an object specifying what fields to order by.

Some examples:

db.users.find().sort({ name: 1 }); // Ascending

db.users.find().sort({ age: -1 }); // Descending

db.users.find().sort({
  // Two criteria
  username: 1,
  age: -1,
});

Projecting

To choose what fields to retrieve from each document you can use projection. The projection is an object which selects which fields to return by setting them to 1:

db.users.find({}, { name: 1 }); // Only the `name` field

db.users.find({ age: { $gte: 5 } }, { age: 1 }); // Only the `age`

db.users.find({}).projection({ name: 1 }); // project afterwards

Distinct values

distinct returns all the different salues (similar to SELECT DISTINCT in SQL).

db.users.distinct("age"); // distinct ages

db.users.distinct("age", { age: { $gte: 5 } }); // distinct ages > 10

Paging

To limit results to 20 documents:

db.users.find().limit(20); // First page of 20 results

db.users.find().skip(20).limit(20); // Second page of 20 results

db.users.find().skip(40).limit(20); // Third page of 20 results
// ...