MongoDB Query Examples

Feb 01, 2017

MongoDB Query Examples

In this blog, I will explain with examples the common queries and operation that can be executed on a MongoDB collection.

Let consider a scenario where we have a messaging APP and all of the users communication are stored in MongoDB

We have a collection named "messages" in MongoDB. Below is a sample documents that are stored in the collection:



{
	"_id": "582a15149bbc2d2f8898628b",
	"senderUserId": 1234,
	"receiverUserId": 7890,
	"message": Hi, How are you?,
	"sentDate": ISODate("2016-11-14T19:48:36.379Z"),
	"read": false
},
{
	"_id": "722a15149bbc4d2f8898628b",
	"senderUserId": 7890,
	"receiverUserId": 1234,
	"message": "I am doing well. How about you?",
	"sentDate": ISODate("2016-11-15T2:10:11.123Z")
	"read": true
}, {
	"_id": "150000149bb42d2f8898628b",
	"senderUserId": 5678,
	"receiverUserId": 2222,
	"message": "Hola !!!",
	"sentDate": ISODate("2016-11-17T01:04:6.079Z"),
	"read" : false
	
}

Query #1 - Create document (User sends a message to another user)


db.getCollection('messages').insertOne({
	"_id": "582a15149bbc2d2f8898628b",
	"senderUserId": "1234",
	"receiverUserId": "7890",
	"message": "Hi, How are you?",
	"sentDate": ISODate("2016-11-14T19:48:36.379Z"),
	"read": false
})

Query #2 - Message history between two users with the most recent message at the bottom


db.getCollection('messages').find({
	$or: [{
		senderUserId: 1234,
		receiverUserId: 7890
	}, {
		senderUserId: 7890,
		receiverUserId: 1234
	}]
}).sort({
	sentDate: 1
})

Query #3 - Retrieve all messages received by an user from other users and which are still unread by the receiving user



db.getCollection('messages').find({
	receiverUserId: 1234,
	read: false
}).sort({
	sentDate: 1
})

Query #4 - Retrieve all messages sent by an user to other users



db.getCollection('messages').find({
	senderUserId: 1234,
}).sort({
	sentDate: 1
})

Query #5 - Retrieve count of all messages sent by the user till date



db.getCollection('messages').find({
	senderUserId: 1234,
}).count()

Query #6 - Retrieve grouped count of unread messages per contact


db.getCollection('messages').aggregate([{
		$match: {
			receiverUserId: 1234
		}
	},
	{
		$group: {
			_id: "$senderUserId",
			count: {
				$sum: 1
			}
		}
	}

])

The only problem with the above query is that the order of the result is not guaranteed. We want the results to contain the results where the contact with the most recent communication comes at the top. In order to achieve this order, we need to sort the result in descending order of 'sendDate' field. The above query can be rewritten as following :



db.getCollection('messages').aggregate([{
		$match: {
			receiverUserId: 1234
		}
	},
	{
		$sort: {
			sentDate: -1
		}
	}, {
		$group: {
			_id: "$senderUserId",
			count: {
				$sum: 1
			},
			sentDate: {
				$first: "$sentDate"
			}
		}
	}
	,
	{
		$sort: {
			sentDate: -1
		}
	}, {
		$project: {
			_id: 1,
			count: 1
		}
	}
])

Query #7: Get a unique list of Ids of all people that the user sent messages to



db.getCollection('messages').aggregate([{
		$match: {
			senderUserId: 1234
		}
	}, {
		$group: {
			"_id": "$receiverUserId"
		}
	}, {
		$project: {
			"receiverUserId": "$_id",
			"_id": 0
		}
	}

]);

Query #7: Count of messages sent by the user to each of the contacts



db.getCollection('messages').aggregate([{
		$match: {
			senderUserId: 1234
		}
	}, {
		$group: {
			"_id": "$receiverUserId",
			count: {
				$sum: 1
			}
		}
	},
	{
		$project: {
			"receiverUserId": "$_id",
			"_id": 0,
			count: 1
		}
	}
]);

Query #8: Get paginated list of messages sent between an user and another user [ Pagination Approach #1: Using pageSize and pageNum ]




db.getCollection('messages').find({
	senderUserId: 1234,
	receiverUserId: 7890
}).sort({sentDate:-1}).skip(0).limit(10)       // Fetch the first page of the paginated messages. Each page will have 10 messages


db.getCollection('messages').find({
	senderUserId: 1234,
	receiverUserId: 7890
}).sort({sentDate:-1}).skip(10).limit(10)       // Fetch the second page of the paginated messages.



General format :

db.getCollection('messages').find({
	senderUserId: 1234,
	receiverUserId: 7890
}).sort({sentDate:-1}).skip(pageNum-1 * pageSize ).limit(pageSize)       // General format for fetching messages

Query #9: Get list of messages sent between an user and another match [Pagination Approach #2 : Range based pagination]



// Fetch 10 messages that were exchanged between the users before the provided time frame.

db.getCollection('messages').find({
	senderUserId: 1234,
	receiverUserId: 7890,
	sentDate: {
		$lt: ISODate("2016-11-14T19:48:36.379Z")
	}
}).sort({
	sentDate: -1
}).limit(10)



// Fetch 10 messages that were exchanged between the users after the provided time frame.

db.getCollection('messages').find({
	senderUserId: 1234,
	receiverUserId: 7890,
	sentDate: {
		$gt: ISODate("2016-11-14T19:48:36.379Z")
	}
}).sort({
	sentDate: 1
}).limit(10)

Query #10: Update messages received by an an user from another user as read.


db.getCollection('messages').update({
	receiverUserId: 1234,
	senderUserId: 7890,
	read: false
}, {
	$set: {
		read: true
	}
}, {
	multi: true
})