Author: Satish

Count Method: MongoDB

Let’s learn to use count() method in MongoDB.

Count method outputs numeric value of the number of documents retrieved.

count-method-mongodb

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> use test
switched to db test
> db.names.find().pretty()
{ "_id" : ObjectId("53be5d4604cc1cb0a7bfc3c0"), "name" : "Alia" }
{ "_id" : ObjectId("53be5d5204cc1cb0a7bfc3c1"), "name" : "Bebo" }
{ "_id" : ObjectId("53be5d5904cc1cb0a7bfc3c2"), "name" : "Chameli" }
{ "_id" : ObjectId("53be5d6104cc1cb0a7bfc3c3"), "name" : "Dev D" }
{ "_id" : ObjectId("53be5d6804cc1cb0a7bfc3c4"), "name" : "Emli" }
{ "_id" : ObjectId("53be5d8604cc1cb0a7bfc3c5"), "name" : "Farhan" }
{ "_id" : ObjectId("53be5d9204cc1cb0a7bfc3c6"), "name" : "Gangs" }
{ "_id" : ObjectId("53be5d9904cc1cb0a7bfc3c7"), "name" : "Hum" }
{ "_id" : ObjectId("53be5e3704cc1cb0a7bfc3c8"), "name" : 25 }
> db.names.count();
9
> db.names.count({"name": {$type: 2}});
8
> db.names.count({"name": {$type: 1}});
1
> db.names.count({"name": {$regex: "e"}});
3

We have 9 documents, out of which 8 documents have string values as name and 1 document has numeric value as it’s name.

Count Method: MongoDB


[youtube https://www.youtube.com/watch?v=1Cx0KT43xlw]

YouTube Link: https://www.youtube.com/watch?v=1Cx0KT43xlw [Watch the Video In Full Screen.]



Related Read: $exists, $type, $regex operators: MongoDB

$type: 2, represents string value in BSON specification. And we have 8 documents with string value for the name field.

$type: 1, represents double value in BSON specification. And we have 1 document with numeric value for the name field.

$regex: “e”, we have 3 documents which has small letter e in the string value for name field.

Note: We can make use of count() method, when we want to show the number of friends or followers each member has. So MongoDB makes it easy by facilitating developers with count() method.

SELECT / LIST Records From Database Table: PHP & MySQL
In sql, we have SELECT count(*) FROM table_name;

Cursor Object: MongoDB

Lets have a deeper look into the MongoDB cursor object.

cursor-object-mongodb

Documents in our collection
test database, names collection.

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> use test
switched to db test
> show collections
names
system.indexes
> db.names.find();
{ "_id" : ObjectId("53be5d4604cc1cb0a7bfc3c0"), "name" : "Alia" }
{ "_id" : ObjectId("53be5d5204cc1cb0a7bfc3c1"), "name" : "Bebo" }
{ "_id" : ObjectId("53be5d5904cc1cb0a7bfc3c2"), "name" : "Chameli" }
{ "_id" : ObjectId("53be5d6104cc1cb0a7bfc3c3"), "name" : "Dev D" }
{ "_id" : ObjectId("53be5d6804cc1cb0a7bfc3c4"), "name" : "Emli" }
{ "_id" : ObjectId("53be5d8604cc1cb0a7bfc3c5"), "name" : "Farhan" }
{ "_id" : ObjectId("53be5d9204cc1cb0a7bfc3c6"), "name" : "Gangs" }
{ "_id" : ObjectId("53be5d9904cc1cb0a7bfc3c7"), "name" : "Hum" }
{ "_id" : ObjectId("53be5e3704cc1cb0a7bfc3c8"), "name" : 25 }

We have 8 documents with name in alphabetical order, and 1 document with name as 25. So in total we have 9 documents in our names collection.

Establishing Cursor

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> var cur = db.names.find();
 
> cur
{ "_id" : ObjectId("53be5d4604cc1cb0a7bfc3c0"), "name" : "Alia" }
{ "_id" : ObjectId("53be5d5204cc1cb0a7bfc3c1"), "name" : "Bebo" }
{ "_id" : ObjectId("53be5d5904cc1cb0a7bfc3c2"), "name" : "Chameli" }
{ "_id" : ObjectId("53be5d6104cc1cb0a7bfc3c3"), "name" : "Dev D" }
{ "_id" : ObjectId("53be5d6804cc1cb0a7bfc3c4"), "name" : "Emli" }
{ "_id" : ObjectId("53be5d8604cc1cb0a7bfc3c5"), "name" : "Farhan" }
{ "_id" : ObjectId("53be5d9204cc1cb0a7bfc3c6"), "name" : "Gangs" }
{ "_id" : ObjectId("53be5d9904cc1cb0a7bfc3c7"), "name" : "Hum" }
{ "_id" : ObjectId("53be5e3704cc1cb0a7bfc3c8"), "name" : 25 }

Look at the contents of our cursor object cur.

hasNext() and next() methods on Cursor

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> var cur = db.names.find();
> cur.hasNext();
true
> cur.next()
{ "_id" : ObjectId("53be5d4604cc1cb0a7bfc3c0"), "name" : "Alia" }
> cur.next()
{ "_id" : ObjectId("53be5d5204cc1cb0a7bfc3c1"), "name" : "Bebo" }
> cur.next()
{ "_id" : ObjectId("53be5d5904cc1cb0a7bfc3c2"), "name" : "Chameli" }

If there are any documents to iterate inside cursor object, then hasNext() will return true orelse it’ll return false. If hasNext() returns true, then we can iterate through the documents using next() method on the cursor object.

sort() method on Cursor Object

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> var cur = db.names.find();
 
> cur.sort({"name": -1});
{ "_id" : ObjectId("53be5d9904cc1cb0a7bfc3c7"), "name" : "Hum" }
{ "_id" : ObjectId("53be5d9204cc1cb0a7bfc3c6"), "name" : "Gangs" }
{ "_id" : ObjectId("53be5d8604cc1cb0a7bfc3c5"), "name" : "Farhan" }
{ "_id" : ObjectId("53be5d6804cc1cb0a7bfc3c4"), "name" : "Emli" }
{ "_id" : ObjectId("53be5d6104cc1cb0a7bfc3c3"), "name" : "Dev D" }
{ "_id" : ObjectId("53be5d5904cc1cb0a7bfc3c2"), "name" : "Chameli" }
{ "_id" : ObjectId("53be5d5204cc1cb0a7bfc3c1"), "name" : "Bebo" }
{ "_id" : ObjectId("53be5d4604cc1cb0a7bfc3c0"), "name" : "Alia" }
{ "_id" : ObjectId("53be5e3704cc1cb0a7bfc3c8"), "name" : 25 }

We could modify the cursor object using methods like sort(), limit() and skip(). In above example, we are modifying cursor object using sort() method, and we are sorting it in reverse lexicographical order on the name field.

limit() method on Cursor Object

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> var cur = db.names.find();
> cur.limit(3);
{ "_id" : ObjectId("53be5d4604cc1cb0a7bfc3c0"), "name" : "Alia" }
{ "_id" : ObjectId("53be5d5204cc1cb0a7bfc3c1"), "name" : "Bebo" }
{ "_id" : ObjectId("53be5d5904cc1cb0a7bfc3c2"), "name" : "Chameli" }

We could limit the output/result using limit() method.

Chaining method on Cursor Object

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> var cur = db.names.find();
 
> cur.sort({"name": -1}).limit(5).skip(2);
{ "_id" : ObjectId("53be5d8604cc1cb0a7bfc3c5"), "name" : "Farhan" }
{ "_id" : ObjectId("53be5d6804cc1cb0a7bfc3c4"), "name" : "Emli" }
{ "_id" : ObjectId("53be5d6104cc1cb0a7bfc3c3"), "name" : "Dev D" }
{ "_id" : ObjectId("53be5d5904cc1cb0a7bfc3c2"), "name" : "Chameli" }
{ "_id" : ObjectId("53be5d5204cc1cb0a7bfc3c1"), "name" : "Bebo" }

Here we chain the methods sort(), limit() and skip(). We are sorting in reverse lexicographical order on the name field, then skipping the first 2 documents and then limiting the result/output to 5 documents.

The order in which these 3 methods are applied are: First sort, then skip and then limit.
Also note that, these methods modify cursor object at the server side and not on client site.

Cursor Object: MongoDB


[youtube https://www.youtube.com/watch?v=_LexQW0vSlQ]

YouTube Link: https://www.youtube.com/watch?v=_LexQW0vSlQ [Watch the Video In Full Screen.]



explain() method on Cursor Object

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> db.names.find().explain();
{
        "cursor" : "BasicCursor",
        "isMultiKey" : false,
        "n" : 9,
        "nscannedObjects" : 9,
        "nscanned" : 9,
        "nscannedObjectsAllPlans" : 9,
        "nscannedAllPlans" : 9,
        "scanAndOrder" : false,
        "indexOnly" : false,
        "nYields" : 0,
        "nChunkSkips" : 0,
        "millis" : 0,
        "server" : "PC:27017",
        "filterSet" : false
}

explain() method shows that find() returns a basic cursor. More on explain() method in coming videos.

Programmatic way of printing Cursor Object content

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> var cur = db.names.find();
 
> while(cur.hasNext()) printjson(cur.next());
{ "_id" : ObjectId("53be5d4604cc1cb0a7bfc3c0"), "name" : "Alia" }
{ "_id" : ObjectId("53be5d5204cc1cb0a7bfc3c1"), "name" : "Bebo" }
{ "_id" : ObjectId("53be5d5904cc1cb0a7bfc3c2"), "name" : "Chameli" }
{ "_id" : ObjectId("53be5d6104cc1cb0a7bfc3c3"), "name" : "Dev D" }
{ "_id" : ObjectId("53be5d6804cc1cb0a7bfc3c4"), "name" : "Emli" }
{ "_id" : ObjectId("53be5d8604cc1cb0a7bfc3c5"), "name" : "Farhan" }
{ "_id" : ObjectId("53be5d9204cc1cb0a7bfc3c6"), "name" : "Gangs" }
{ "_id" : ObjectId("53be5d9904cc1cb0a7bfc3c7"), "name" : "Hum" }
{ "_id" : ObjectId("53be5e3704cc1cb0a7bfc3c8"), "name" : 25 }

While loop executes until cur.hasNext() returns true. Until cur.hasNext() is true, cur.next() keeps printing next document in the cursor cur.

Dot Notation To Access Sub Document: MongoDB

Today lets see how we can access sub-document or sub-object using Dot notation, in MongoDB.

Documents in our collection
nesting database, users collection.

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> use nesting
switched to db nesting
 
> db.users.insert({"name": "Satish", 
                   "email": {"work": "[email protected]", 
                   "personal": "[email protected]"}});
WriteResult({ "nInserted" : 1 })
 
> db.users.find().pretty()
{
        "_id" : ObjectId("53bfc9f54ed48cddc53effa8"),
        "name" : "Satish",
        "email" : {
                "work" : "[email protected]",
                "personal" : "[email protected]"
        }
}

Here we have a sub-document called “email” – with work and personal keys.

Dot Notation To Access Sub Object: MongoDB


[youtube https://www.youtube.com/watch?v=mqB_ilUXg1o]

YouTube Link: https://www.youtube.com/watch?v=mqB_ilUXg1o [Watch the Video In Full Screen.]



Finding document using sub-object

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> db.users.find({"email": {"work": "[email protected]", 
                                   "personal": "[email protected]"}});
{ "_id" : ObjectId("53bfc9f54ed48cddc53effa8"), 
  "name" : "Satish", 
  "email" : { "work" : "[email protected]", 
              "personal" : "[email protected]" } 
}

But if the order of fields is changed it doesn’t retrieve the document, as it does not match the BSON bytes stored in the database.

Finding document using sub-object: Does not work

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> db.users.find({"email": {"work": "[email protected]"}});
> db.users.find({"email": {"personal": "[email protected]", 
                           "work": "[email protected]"}});

Finding document with dot notation

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> db.users.find({"email.work": "[email protected]"});
{ "_id" : ObjectId("53bfc9f54ed48cddc53effa8"), 
  "name" : "Satish", 
  "email" : { "work" : "[email protected]", 
              "personal" : "[email protected]" } }

Using dot notation is the best way to access sub-document or sub-object in MongoDB.

$and operator: MongoDB

Lets quickly learn about using $and operator in MongoDB.

Related Read:
$exists, $type, $regex operators: MongoDB
$or (Union) Operator: MongoDB

Documents in our collection
test database, names collection.

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> db.names.find().pretty()
{ "_id" : ObjectId("53be5d4604cc1cb0a7bfc3c0"), "name" : "Alia" }
{ "_id" : ObjectId("53be5d5204cc1cb0a7bfc3c1"), "name" : "Bebo" }
{ "_id" : ObjectId("53be5d5904cc1cb0a7bfc3c2"), "name" : "Chameli" }
{ "_id" : ObjectId("53be5d6104cc1cb0a7bfc3c3"), "name" : "Dev D" }
{ "_id" : ObjectId("53be5d6804cc1cb0a7bfc3c4"), "name" : "Emli" }
{ "_id" : ObjectId("53be5d8604cc1cb0a7bfc3c5"), "name" : "Farhan" }
{ "_id" : ObjectId("53be5d9204cc1cb0a7bfc3c6"), "name" : "Gangs" }
{ "_id" : ObjectId("53be5d9904cc1cb0a7bfc3c7"), "name" : "Hum" }
{ "_id" : ObjectId("53be5e3704cc1cb0a7bfc3c8"), "name" : 25 }
{
        "_id" : ObjectId("53beaa0f6a8a31dc255d4589"),
        "name" : "Satish",
        "age" : 27
}

$and operator: MongoDB


[youtube https://www.youtube.com/watch?v=Ur0Mmj7yEOI]

YouTube Link: https://www.youtube.com/watch?v=Ur0Mmj7yEOI [Watch the Video In Full Screen.]



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> db.names.find({$and: [{"name": {$regex: "e"}}, {"name": {$gt: "C"}}]});
{ "_id" : ObjectId("53be5d5904cc1cb0a7bfc3c2"), "name" : "Chameli" }
{ "_id" : ObjectId("53be5d6104cc1cb0a7bfc3c3"), "name" : "Dev D" }

As you can see, $and works/outputs results only when all the conditions in the array is met. i.e., in above command, the name must have small letter “e” in it and must be lexicographically greater than capital letter “C”.

$or (Union) Operator: MongoDB

Today lets learn about $or operator.

In set theory, the union (denoted by ∪) of a collection of sets is the set of all distinct elements in the collection. It is one of the fundamental operations through which sets can be combined and related to each other.

Union of two sets
The union of two sets A and B is the collection of points which are in A or in B or in both A and B. In symbols,

union-of-two-sets
For example, if A = {1, 3, 5, 7} and B = {1, 2, 4, 6} then A ∪ B = {1, 2, 3, 4, 5, 6, 7}.

$or (Union) Operator: MongoDB


[youtube https://www.youtube.com/watch?v=84GxdweNoCQ]

YouTube Link: https://www.youtube.com/watch?v=84GxdweNoCQ [Watch the Video In Full Screen.]



Documents in our collection
test database, names collection.

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> db.names.find().pretty()
{ "_id" : ObjectId("53be5d4604cc1cb0a7bfc3c0"), "name" : "Alia" }
{ "_id" : ObjectId("53be5d5204cc1cb0a7bfc3c1"), "name" : "Bebo" }
{ "_id" : ObjectId("53be5d5904cc1cb0a7bfc3c2"), "name" : "Chameli" }
{ "_id" : ObjectId("53be5d6104cc1cb0a7bfc3c3"), "name" : "Dev D" }
{ "_id" : ObjectId("53be5d6804cc1cb0a7bfc3c4"), "name" : "Emli" }
{ "_id" : ObjectId("53be5d8604cc1cb0a7bfc3c5"), "name" : "Farhan" }
{ "_id" : ObjectId("53be5d9204cc1cb0a7bfc3c6"), "name" : "Gangs" }
{ "_id" : ObjectId("53be5d9904cc1cb0a7bfc3c7"), "name" : "Hum" }
{ "_id" : ObjectId("53be5e3704cc1cb0a7bfc3c8"), "name" : 25 }
{
        "_id" : ObjectId("53beaa0f6a8a31dc255d4589"),
        "name" : "Satish",
        "age" : 27
}

Related Read: $exists, $type, $regex operators: MongoDB

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> db.names.find({$or: [{"name": {$regex: "^E"}}, {"age": {$exists: true}}]});
{ "_id" : ObjectId("53be5d6804cc1cb0a7bfc3c4"), "name" : "Emli" }
{ "_id" : ObjectId("53beaa0f6a8a31dc255d4589"), "name" : "Satish", "age" : 27 }

$or is a prefix operator. It takes array as it’s value. The array can contain any number of objects for the union. Mongo Shell fetches all the documents which matches any of the documents which the individual objects inside the array points to.

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> db.names.find({$or: [{"name": {$regex: "^E"}}, {"name": {$type: 1}}]});
{ "_id" : ObjectId("53be5d6804cc1cb0a7bfc3c4"), "name" : "Emli" }
{ "_id" : ObjectId("53be5e3704cc1cb0a7bfc3c8"), "name" : 25 }

$type: 1, points to Boolean value.

Note: Next we’ll learn how to make use of $and operator.