To concatenate, use $concat in MongoDB aggregate(). Let us first create a collection with documents −> db.demo377.insertOne({"ListOfIds":[1001, 1002, 1003, 1004, 1005, 1006, 1007]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e5a73462ae06a1609a00b0e") }Display all documents from a collection with the help of find() method −> db.demo377.find().pretty();This will produce the following output −{    "_id" : ObjectId("5e5a73462ae06a1609a00b0e"),    "ListOfIds" : [       1001,       1002,       1003,       1004,       1005,       1006,       1007    ] }Following is the query to concatenate an array of integer ... Read More

To update documents, you cannot use aggregation pipeline. You can use update(). Let us first create a collection with documents −> db.demo376.insertOne( ...    { ... ...       "id" :101, ... ...       "details" : [ ...          { ...             Name:"Chris", ...             Age:21, ...             Score:45 ...          }, ...          { ...             Name:"David", ...             Age:23, ...   ... Read More

JavaScript function can be saved for reuse using a system collection called system.js. To store a function, use the db.collection.save(),Let us first create a function. Following is the query −> db.system.js.save({ ...    _id: "displayMessage", ...    value: function (data) { ...       return 'The Name is: ' + data; ...    } ... })This will produce the following output −WriteResult({    "nMatched" : 0,    "nUpserted" : 1,    "nModified" : 0,    "_id" : "displayMessage" })Following is the query to execute stored function −> db.eval("displayMessage('John')") WARNING: db.eval is deprecatedThis will produce the following output −The Name is: John

There is no difference between show dbs and show databases. Both commands internally call listDatabases command.The show dbs command is as follows −> show dbsThis will produce the following output −admin             0.002GB app                 0.000GB business           0.000GB config             0.000GB local             0.000GB main             0.000GB ... Read More

To find when the keys are unknown, use $addField and $objectToArray. Let us first create a collection with documents −> db.demo375.insertOne( ...    { ...       "details":{ ...          "Name":"John", ...          "Age":23 ...       } ...    } ... ); {    "acknowledged" : true,    "insertedId" : ObjectId("5e5a0ae42ae06a1609a00b06") } > db.demo375.insertOne( ...    { ...       "details":{ ...          "Name":"David", ...          "Age":21 ...       } ...    } ... ); {    "acknowledged" : true,    "insertedId" ... Read More

To group by another field, use $group along with $project. Let us first create a collection with documents −> db.demo374.insertOne( ...    { ... ...       "Name" : "Chris", ...       "HobbyDetails" : [ ...          "Reading Book", ...          "Playing Football" ...       ], ...       "CountryName" : "US" ...    } ... ); {    "acknowledged" : true,    "insertedId" : ObjectId("5e5a04402ae06a1609a00b04") } > db.demo374.insertOne( ...    { ... ...       "Name" : "Chris", ...       "HobbyDetails" : [ ... ... Read More

Let us first create a collection with documents −> db.demo373.createIndex({"Name":1, "CountryName":1}); {    "createdCollectionAutomatically" : true,    "numIndexesBefore" : 1,    "numIndexesAfter" : 2,    "ok" : 1 } > db.demo373.insertOne({"Name":"Chris", "Age":22, "CountryName":"US"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e59ffde2ae06a1609a00aff") } > db.demo373.insertOne({"Name":"David", "Age":21, "CountryName":"AUS"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e59ffe82ae06a1609a00b00") } > db.demo373.insertOne({"Name":"Bob", "Age":23, "CountryName":"UK"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e59fff42ae06a1609a00b01") } > db.demo373.insertOne({"Name":"John", "Age":21, "CountryName":"US"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e59ffff2ae06a1609a00b02") } > db.demo373.insertOne({"Name":"Carol", "Age":23, "CountryName":"AUS"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e5a00082ae06a1609a00b03") }Display all ... Read More

The $ifNull evaluates an expression and returns the value of the expression if the expression evaluates to a non-null value.Let us first create a collection with documents −> db.demo372.insertOne({"FirstName":"Chris"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e591aea2ae06a1609a00af6") } > db.demo372.insertOne({"FirstName":null}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e591aef2ae06a1609a00af7") } > db.demo372.insertOne({"FirstName":"David"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e591af42ae06a1609a00af8") } > db.demo372.insertOne({"FirstName":null}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e591afb2ae06a1609a00af9") }Display all documents from a collection with the help of find() method −> db.demo372.find();This will produce the following output −{ "_id" : ObjectId("5e591aea2ae06a1609a00af6"), "FirstName" : "Chris" ... Read More

To change the primary key, you need to first delete it. Use forEach() along with delete to remove and then get a new primary key. Let us create a collection with documents −> db.demo41.insertOne({"StudentName":"Carol"}); {    "acknowledged" : true,    "insertedId" : ObjectId("5e25ce4acfb11e5c34d898e3") }Display all documents from a collection with the help of find() method −> db.demo41.find();This will produce the following output −{ "_id" : ObjectId("5e25ce4acfb11e5c34d898e3"), "StudentName" : "Carol" }Here is the query to change the primary key on a MongoDB collection −> var next = db.demo41.find() > > next.forEach(function(s) { ...    var prevId=s._id; ...    delete s._id; ... ... Read More

For this, use save() in MongoDB. Following is the syntax −var anyVaribaleName=yourValue db.anyCollectionName.save(yourVariableName); yourVariableName;Let us first create an object for our example −> var studentDetails={"StudentName":"Chris","ListOfMarks":[56,78,89],"ListOfSubject":["MySQL","Java","MongoDB"]};Let us save the above created object “studentDetails” −> db.demo40.save(studentDetails); WriteResult({ "nInserted" : 1 })Let us display the value −> studentDetails;This will produce the following output −{    "StudentName" : "Chris",    "ListOfMarks" : [       56,       78,       89    ],    "ListOfSubject" : [       "MySQL",       "Java",       "MongoDB"    ],    "_id" : ObjectId("5e177757cfb11e5c34d898e2") }