MongoDB Collection Methods (Day 11)

This query returns the distinct value from the “Tags” field of documents where the value of the “Likes” field is greater than 5.

Output

[ "Database", "MongoDB", "NoSQL" ]

Note

If the value of a specific field is an array, then the distinct() method considers each value as a separate value. In other words, the [“Name”] and “Name” values of a field are treated as separate values in the distinct method.

Let's see an example.
Now we will use the distinct() method for the “Tags” array and examine the result.

Query
Output

[ "MongoDB", "NoSQL", [ "NoSQL" ] ]


findOne() Method

The findOne() method is used to return a document that satisfies the specified selection criteria. If more than one document matches the selection criteria, then the first document will return a value that reflects the order of documents on the disk.

Syntax
Parameters

Parameter	Type	Description
Query	Document	Specify selection criteria
Projection	Document	Specify the field that the findOne method will return. I or true for include and 0 or false to exclude the field

Return single document

To return the single document without any selection criteria, use the findOne method without the <query> parameter.

Example

db.Post.findOne()

This query returns the first document from the collection.

Output

findOne() method with selection criteria

To determine the first document that matches a specified selection criteria, use the <query> parameter in the findOne() method.

Example
This query returns the first document in which the value of the “Likes” field is greater than 5.

Output
Return specific field

To return the specific field from the document, use the <projection> parameter in the findOne() method.

Example
This query returns the “Title” and “Tags” fields from first document that match the selection criteria.

Output
Note

We cannot apply cursor methods (hasNext, Next) to the result of findOne() because the findOne() method returns only a single document. If we try to use the cursor method then MongoDB will throw an error.

Example

Parameters	Type	Description
Document	Document	Specify document to save to the collection

Document without _id field

If we use a document without defining an _id field, then the save() method calls the insert() method and a new document will be inserted into the collection.

Example

This query inserts a new document into the collection.

Now we will check the existence of the newly inserted document.
Output
Document with _id field

If a document contains an _id field then the save() method is the equivalent of the update method with the upsert method set to true. In other words, if the document contains an _id field, then there are two possibilities.

Case 1: _id already exists.

If _id already exists, then the field of an existing document will replace the new document's field.

Example:

We use the document whose _id is ObjectId("55c5481bacae58b1a7d7dffa").
Now, we execute the save method and retrieve the document.
Document after replacing:

Query
Output
Case 2: _id doesn't exist

If _id does not already exist in the collection, then a new document will be inserted.

Example

First, we check that _id already exists.

Query
When we execute the preceding query, we did not retrieve a document. In other words this _id does not exist.

Now we insert a new document using the save() method.
Now we check this document.

Query
Output
The above result ensures that a new document is inserted into the “Post” collection.

Today we learned some collection methods. In the next article, I will explain a new topic. Until then keep coding and continue practicing.