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MongoDB Coding Practice
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‎mongodb-code.md

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# MongoDB Coding Practice
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## Q. ***Mention the command to insert a document in a database called school and collection called persons?***
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## Q. ***Mention the command to insert a document in a database called company and collection called employee?***
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```js
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use company;
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db.employee.insert( { name: "John", email: "john.k@gmail.com" } )
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```
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<div align="right">
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<b><a href="#">↥ back to top</a></b>
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</div>
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## Q. ***Mention the command to check whether you are on the master server or not?***
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## Q. ***How can I combine data from multiple collections into one collection?***
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```js
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db.isMaster()
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```
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<div align="right">
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<b><a href="#">↥ back to top</a></b>
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</div>
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## Q. ***How to combine data from multiple collections into one collection?***
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**$lookup**
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Performs a left outer join to an unsharded collection in the same database to filter in documents from the “joined” collection for processing. To each input document, the `$lookup` stage adds a new array field whose elements are the matching documents from the “joined” collection. The `$lookup` stage passes these reshaped documents to the next stage.
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**Syntax**
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```js
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{
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$lookup:
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{
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from: <collection to join>,
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localField: <field from the input documents>,
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foreignField: <field from the documents of the "from" collection>,
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as: <output array field>
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}
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}
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```
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<div align="right">
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<b><a href="#">↥ back to top</a></b>
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</div>
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## Q. ***How do I perform the SQL JOIN equivalent in MongoDB?***
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## Q. ***How to query MongoDB with %like%?***
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```js
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// Sample Records
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comments
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{ uid:12345, pid:444, comment="blah" }
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{ uid:12345, pid:888, comment="asdf" }
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{ uid:99999, pid:444, comment="qwer" }
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users
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{ uid:12345, name:"john" }
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{ uid:99999, name:"mia" }
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```
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**Answers**
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```js
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{
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$lookup:
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{
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from: <collection to join>,
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localField: <field from the input documents>,
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foreignField: <field from the documents of the "from" collection>,
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as: <output array field>
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}
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}
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```
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## Q. ***How to query MongoDB with "like"?***
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```js
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db.users.insert({name: 'paulo'})
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db.users.insert({name: 'patric'})
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db.users.insert({name: 'pedro'})
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db.users.find({name: /a/}) //like '%a%'
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db.users.find({name: /^pa/}) //like 'pa%'
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db.users.find({name: /ro$/}) //like '%ro'
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```
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<div align="right">
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<b><a href="#">↥ back to top</a></b>
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</div>
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## Q. ***Find objects between two dates MongoDB?***
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## Q. ***How to query MongoDB with “like”?***
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## Q. ***Update MongoDB field using value of another field***
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Operator `$gte` and `$lt` is used to find objects between two dates in MongoDB.
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**Example**: Creating a collection
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```js
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>db.order.insert({"OrderId":1,"OrderAddrees":"US","OrderDateTime":ISODate("2020-02-19")};
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WriteResult({ "nInserted" : 1 })
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>db.order.insert({"OrderId":2,"OrderAddrees":"UK","OrderDateTime":ISODate("2020-02-26")};
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WriteResult({ "nInserted" : 1 })
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```
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Display all documents from the collection using `find()` method.
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```js
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> db.order.find().pretty();
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// Output
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{
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"_id" : ObjectId("5c6c072068174aae23f5ef57"),
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"OrderId" : 1,
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"OrderAddrees" : "US",
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"OrderDateTime" : ISODate("2020-02-19T00:00:00Z")
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}
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{
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"_id" : ObjectId("5c6c073568174aae23f5ef58"),
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"OrderId" : 2,
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"OrderAddrees" : "UK",
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"OrderDateTime" : ISODate("2020-02-26T00:00:00Z")
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}
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```
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Here is the query to find objects between two dates:
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```js
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> db.order.find({"OrderDateTime":{ $gte:ISODate("2020-02-10"), $lt:ISODate("2020-02-21") }
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}).pretty();
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// Output
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{
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"_id" : ObjectId("5c6c072068174aae23f5ef57"),
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"OrderId" : 1,
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"OrderAddrees" : "US",
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"OrderDateTime" : ISODate("2020-02-19T00:00:00Z")
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}
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```
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<div align="right">
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<b><a href="#">↥ back to top</a></b>
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</div>
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## Q. ***Is it possible to update MongoDB field using value of another field?***
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The aggregate function can be used to update MongoDB field using the value of another field.
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**Example**
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```js
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db.collection.<update method>(
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{},
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[
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{"$set": {"name": { "$concat": ["$firstName", " ", "$lastName"]}}}
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]
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)
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```
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<div align="right">
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<b><a href="#">↥ back to top</a></b>
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</div>
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## Q. ***How to check if a field contains a substring?***
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The `$regex` operator can be used to check if a field contains a string in MongoDB.
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```js
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db.users.findOne({"username" : {$regex : ".*some_string.*"}});
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```
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<div align="right">
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<b><a href="#">↥ back to top</a></b>
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</div>
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## Q. ***How to get the last N records from find?***
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## Q. ***MongoDB relationships. What to use - embed or reference?***
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```js
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// Syntax
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db.<CollectionName>.find().sort({$natural:-1}).limit(value)
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// Example
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db.employee.find().sort({$natural:-1}).limit(100)
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```
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<div align="right">
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<b><a href="#">↥ back to top</a></b>
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</div>
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## Q. ***How to remove a field completely from a MongoDB document?***
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## Q. ***By default, MongoDB writes and reads data from both primary and secondary replica sets. True or False.***
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How do I remove words completely from all the documents in this collection?
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```js
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{
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name: 'book',
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tags: {
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words: ['abc','123'], // <-- remove it comletely
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lat: 33,
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long: 22
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}
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}
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```
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**Answer**
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```js
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db.example.update({}, {$unset: {words: 1}}, false, true);
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```
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<div align="right">
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<b><a href="#">↥ back to top</a></b>
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</div>
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## Q. ***How to find MongoDB records where array field is not empty?***
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## Q. ***Is it possible to update MongoDB field using value of another field?***
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```js
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db.inventory.find({ pictures: { $exists: true, $ne: [] } })
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```
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<div align="right">
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<b><a href="#">↥ back to top</a></b>
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</div>
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## Q. ***How to find document with array that contains a specific value?***
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Populate the inventory collection
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```js
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db.inventory.insertMany([
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{ item: "journal", qty: 25, tags: ["blank", "red"], dim_cm: [ 14, 21 ] },
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{ item: "notebook", qty: 50, tags: ["red", "blank"], dim_cm: [ 14, 21 ] },
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{ item: "paper", qty: 100, tags: ["red", "blank", "plain"], dim_cm: [ 14, 21 ] },
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{ item: "planner", qty: 75, tags: ["blank", "red"], dim_cm: [ 22.85, 30 ] },
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{ item: "postcard", qty: 45, tags: ["blue"], dim_cm: [ 10, 15.25 ] }
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]);
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```
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To query if the array field contains at least one element with the specified value, use the filter { `<field>`: `<value>` } where `<value>` is the element value.
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```js
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db.inventory.find( { tags: "red" } )
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```
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<div align="right">
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<b><a href="#">↥ back to top</a></b>
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</div>

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