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Tuesday, 1 December 2015

MongoDB Interview Questions and Answer on Database

MongoDB Interview Questions and Answer on Database

Question: What is Relationships in MongoDb?
Relationships in MongoDb represent how various documents are related to each other.
For Example:
There are various documents like user, address, payment details etc.
Connection between these documents know as Relationships.


Question: What is Two type Relationships?
  1. Modeling Embedded Relationships
  2. Modeling Referenced Relationships



Question: What is Modeling Embedded Relationships?
In this Approach, we embed the one document into other.
For Example:
{
   "_id":ObjectId("521234cd85242f436000007"),
   "contact": "987685555555",
   "dob": "01-01-1992",
   "name": "User1",
   "address": [
      {         
         "pincode": 160011,
         "city": "Mohali",
       
      },
      {
         "pincode": 140301,
         "city": "Mohali",
      }
   ]
}

We have added the both address in in user document.


Question: Modeling Referenced Relationships?
In this approach, We add the referenceId instead of whole document, thats why know as Referenced Relationships.
For Eample:
{
   "_id":ObjectId("521234cd85242f436000007"),
   "contact": "987685555555",
   "dob": "01-01-1992",
   "name": "User1",
   "address": [
      ObjectId("521234ce85242f436000001"),
      ObjectId("521234ce85242f436000002")
   ]
}


Above is also know as Manual References because we put the RefereceId static.


Question: What is different between "Manual References" and "DBRefs References"?
Manual References
where you save the _id field of one document in another document as a reference know as Manual References.
Just see the above question for Example.

DBRefs References
References the one document with another using the value of first document's _id field, collection name, and database name.



Question: Explain the DBRefs References in detail?
There are three fields in DBRefs:
$ref: This field specifies the collection of the referenced document.
$id: This field specifies the _id field of the referenced document.
$db: Database name where referenced document lies. This is Optional.
{
   "_id":ObjectId("521234cd85242f436000007"),
   "address": {
   "$ref": "address_home",
   "$id": ObjectId("521234cd85242f436000007"),
   "$db": "webtechnology"},
   "contact": "987685555555",
   "dob": "01-01-1991",
   "name": "web-tech"
} 



Question: What is a Covered Query?
Covered query is a query in which all the "fields of Query" and "fields returned in the query" are same index.
For Example:
{
   "_id":ObjectId("521234cd85242f436000007"),
   "address": {
   "$ref": "address_home",
   "$id": ObjectId("521234cd85242f436000007"),
   "$db": "webtechnology"},
   "contact": "987685555555",
   "dob": "01-01-1991",
   "name": "web-tech"
} 

Add "Index"
db.users.ensureIndex({name:1,usercontact:1})


If we are using below:
db.users.find({name:"web-tech"},{usercontact:"987685555555",_id:0})

This is best example for Covered Query.




Question: How to Analyzing the queries in MongoDB?
$explain: The $explain operator provides information on the query, indexes and other statistics. For analyse the use just add .explain() at the end of query.
db.users.find({name:"web-tech"}).explain()


Question: What is use of $hint operator?
$hint: operator is used to forces the query optimizer to use the specified index to run a query.
It is useful when you want to test performance of a query with different indexes with applying the index in real.
db.users.find({name:"web-tech"}).hint({dob:"01-01-2015"}).explain()



Question: Does MongoDb provides atomic transactions?
Yes, but provides only for single document.
In this way, Either it update all fields or none.
db.myproducts.findAndModify({ 
   query:{_id:2,product_available:{$gt:0}}, 
   update:{ 
      $inc:{product_available:-1}, 
      $push:{product_bought_by:{customer:"webtech",date:"19-Jan-2014"}} 
   }    
})



Question: How to add index on document?
db.users.ensureIndex({"dob":1})




Question: How to add index on sub document?
db.users.ensureIndex({"address.city":1,"address.state":1})




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