MongoDB Queries - 2

MongoDB Update() method

MongoDB's update() method is used to update document into a collection. The update() method update values in the existing document.


> db.democol.find().pretty()

Result:

{
        "_id" : ObjectId("52963c2a6f63f810a98b7a98"),
        "title" : "Learn mongo",
        "by" : "mani",
        "likes" : 100
}
{
        "_id" : ObjectId("52963ce96f63f810a98b7a99"),
        "by" : "subramanian",
        "comments" : [
                {
                        "user" : "Tiara",
                        "message" : "Worth to read"
                }
        ],
        "likes" : 20,
        "title" : "Basic SQL"
}

> db.democol.update({'title':'Basic SQL'},{$set:{'title':'Learn SQL'}})
> db.democol.find().pretty()

Result:

{
        "_id" : ObjectId("52963c2a6f63f810a98b7a98"),
        "title" : "Learn mongo",
        "by" : "mani",
        "likes" : 100
}
{
        "_id" : ObjectId("52963ce96f63f810a98b7a99"),
        "by" : "subramanian",
        "comments" : [
                {
                        "user" : "Tiara",
                        "message" : "Worth to read"
                }
        ],
        "likes" : 20,
        "title" : "Learn SQL"
}

By default mongodb will update only single document, to update multiple you need to set a paramter 'multi' to true.

>db.democol.update({'title':'Basic SQL'},{$set:{'title':'Learn SQL'}},{multi:true})



The remove() Method

MongoDB's remove() method is used to remove document from the collection. remove() method accepts two parameters. One is deletion criteria and second is justOne flag

deletion criteria : (Optional) deletion criteria according to documents will be removed.

justOne : (Optional) if set to true or 1, then remove only one document.

>db.mycol.find()

Result:

{ "_id" : ObjectId(5983548781331adf45ec5), "title":"MongoDB Overview"}
{ "_id" : ObjectId(5983548781331adf45ec6), "title":"NoSQL Overview"}
{ "_id" : ObjectId(5983548781331adf45ec7), "title":"Tutorials Point Overview"}
Following example will remove all the documents whose title is 'MongoDB Overview'

>db.mycol.remove({'title':'MongoDB Overview'})
>db.mycol.find()

Result:

{ "_id" : ObjectId(5983548781331adf45ec6), "title":"NoSQL Overview"}
{ "_id" : ObjectId(5983548781331adf45ec7), "title":"Tutorials Point Overview"}


Remove only one

If there are multiple records and you want to delete only first record, then set justOne parameter in remove() method

>db.COLLECTION_NAME.remove(DELETION_CRITERIA,1)



MongoDB Projection

In mongodb projection meaning is selecting only necessary data rather than selecting whole of the data of a document. If a document has 5 fields and you need to show only 3, then select only 3 fields from them.

The find() Method

MongoDB's find() method, explained in MongoDB Query Document accepts second optional parameter that is list of fields that you want to retrieve. In MongoDB when you execute find() method, then it displays all fields of a document. To limit this you need to set list of fields with value 1 or 0. 1 is used to show the filed while 0 is used to hide the field.

> db.democol.find({},{title:1,_id:0})

Result:

{ "title" : "Learn mongo" }
{ "title" : "Learn SQL" }

MondoDB Queries

The find() Method

To query data from MongoDB collection, you need to use MongoDB's find() method.

> db.democol.find()

Result:

{ "_id" : ObjectId("52963c2a6f63f810a98b7a98"), "title" : "Learn mongo", "by" :
"mani", "likes" : 100 }
{ "_id" : ObjectId("52963ce96f63f810a98b7a99"), "title" : "Learn SQL", "by" : "s
ubramanian", "likes" : 20, "comments" : [  {  "user" : "Tiara",  "message" : "Wo
rth to read" } ] }


The pretty() Method

To display the results in a formatted way, you can use pretty() method.

> db.democol.find().pretty()

Result:

{
        "_id" : ObjectId("52963c2a6f63f810a98b7a98"),
        "title" : "Learn mongo",
        "by" : "mani",
        "likes" : 100
}
{
        "_id" : ObjectId("52963ce96f63f810a98b7a99"),
        "title" : "Learn SQL",
        "by" : "subramanian",
        "likes" : 20,
        "comments" : [
                {
                        "user" : "Tiara",
                        "message" : "Worth to read"
                }
        ]
}


AND in MongoDB

In the find() method if you pass multiple keys by separating them by ',' then MongoDB treats it AND condition. Basic syntax of AND is shown below:

> db.democol.find({by:"mani"}).pretty()

Result:

> db.democol.find({by:"mani"}).pretty()
{
        "_id" : ObjectId("52963c2a6f63f810a98b7a98"),
        "title" : "Learn mongo",
        "by" : "mani",
        "likes" : 100
}

> db.democol.find({by:"mani",title:"Learn mongo"}).pretty()

Result:

{
        "_id" : ObjectId("52963c2a6f63f810a98b7a98"),
        "title" : "Learn mongo",
        "by" : "mani",
        "likes" : 100
}


OR in MongoDB

To query documents based on the OR condition, you need to use $or keyword. Basic syntax of OR is shown below:

> db.democol.find({$or:[{by:"mani"},{title:"Learn SQL"}]}).pretty()

Result:

{
        "_id" : ObjectId("52963c2a6f63f810a98b7a98"),
        "title" : "Learn mongo",
        "by" : "mani",
        "likes" : 100
}
{
        "_id" : ObjectId("52963ce96f63f810a98b7a99"),
        "title" : "Learn SQL",
        "by" : "subramanian",
        "likes" : 20,
        "comments" : [
                {
                        "user" : "Tiara",
                        "message" : "Worth to read"
                }
        ]
}