Projection in MongoDB [With Examples]

To make it easier to understand this operation can be interpreted as the following SQL statement: 

SELECT * from upGradCollection WHERE status = "A" 

Though there is more that we can do with projections, let's suppose we want to get only some associated fields from the document, A projection can also explicitly enclose several fields by setting the <field> to 1 in the projection document.

db.inventory.find( { status: "A" }, { item: 1, status: 1, _id: 0 } ) 

In the result set for the above query, only the item and status fields are returned in the matching documents whereas the _id field is Suppressed

Similarly, this operation can be interpreted as the following SQL statement: 

SELECT item, status from upGradCollection WHERE status = "A" 

MongoDB administration certification can help you further understand the Storage of data and other functionality related to MongoDB. 

MongoDB Projection Operators 

MongoDB projection method positively impacts database performance as it reduces the workload of the find query when trying to retrieve specific data from a document, minimizing resource usage. 

To enhance the querying and reduce the workload, multiple operators can be used within a projection query like the ones below: 

• $ 
• $elemMatch 
• $slice 
• $meta 

Let us dive in to understand each one in depth: 

1. $ Operator 

The $ operator is utilized in scenarios where there is a need to limit an array to project only the first element that matches the condition of the query, if no query condition is present, the first element is returned in the specified MongoDB projection array, and a sample syntax of the same can be seen below: 

db.collection.find( { <array>: <condition> ... }, { "<array>.$": 1 } ) 

Example:

Let us check out the "students" Collection this time to understand the functioning of the $ operator. 

db.students.find( { semester: 1, grades: { $gte: 85 } }, { "grades.$": 1 } ) 

Using the $ operator will only return the MongoDB projection first element of array where it is equal to or greater than 85. 

Limitations of the $ operator: 

• In a single MongoDB projection query, only a single $ operator can be used. 
• It will only be applied to a single condition on the array field
• The sort() function in the find() is applied before the $ operator and this function may cause the sort order to get distorted. 
• The $ operator can only be utilized at the end of a field path. This restriction helps to mitigate any formatting issues. 

2. $elemMatch Operator

Similar to the $ operator, the $elemMatch operator also limits the contents of an array to the first element that fits the given constraint. Though, there is a minor difference from the $ operator because the $elemMatch projection operator needs an explicit condition argument. 

Syntax as follows:  

db.collection.find( { <array>: <condition> ... }, { "<array>.$elemMatch": ($elemMatch operator) } ) 

Example:

Below you can check the "schoolsdData" collection that we will use to demonstrate the $elemMatch operator. 

We need to write an operation that returns the following documents that have a zipcode equal to `63109` and projects the students array using $elemMatch: 

db.schools.find( { zipcode: "63109" }, { students: { $elemMatch: { school: 102 } } } ) 

Limitations of $elemMatch operator are as follows: 

• The field to which the $elemMatch projection is applied is returned as the last field of the document, regardless of the order in which the fields are ordered. 
• $elemMatch projection operator is not supported by find() operations on MongoDB views. 
• The $elemMatch operator does not handle $text query expressions. 

3. $slice Projection Operator

The $slice operator bounds the number of elements that should be returned as the output of a MongoDB projection query. 

Syntax: 

db.collection.find( <query>, { <array Field>: { $slice: <number> } }) 

Now we'll use the "postsData" collection to demonstrate the $slice operator. 

We want to get data from the comments array to return the array with its first three elements. If the array contains less than three elements, all elements of the array are returned. 

db.posts.find( {}, { comments: { $slice: 3 } } )

Limitations in $slice operator: 

• In a nested array the $slice operator will only return the sliced element and will not return any other item, especially from MongoDB 4.4. 
• The find() action is not supported by the $slice operator on MongoDB views. 
• Due to a constraint imposed by MongoDB, the $slice operator cannot be combined with the $ projection operator. This is because top-level fields are not permitted to contain the $ symbol as part of the field name. 

4. $meta Operator with Examples 

The $meta operator gives us the capability to extract the metadata associated with a document. Both MongoDB projection and Aggregation support the $meta operator, though for now let us just dive into Projections. 

The $meta operator uses the following syntax. 

{ $meta: <metaDataKeyword> } 

The "metaDataKeyword", represents these values: 

• "textScore" Keyword. This keyword effectively finds how precisely a search term is related with the given field in a document. The "textScore" yields the score associated with the provided $text.
• "indexKey" Keyword. Introduced in MongoDB 4.4 as a preferred alternative to the cursor.returnKey() function. For a document that uses a non-text index, this function returns the index key. A key thing to note is that this keyword should not be used in production situations; it should only be used for debugging purposes. 

Now, let's look at a few $meta Operator MongoDB projection examples : 

For {$meta: "textScore"}, we will be using the articles collection like below: 

We first add an index to the Document and then we use the textScore meta operator to include the score assigned to each matching document. 

db.articles.createIndex( { title: "text"} ) 
db.articles.find( 
{ $text: { $search: "cake" } }, 
{ score: { $meta: "textScore" } } 
) 

Usage and Limitations of $meta operator in projection: 

• The {$meta: "$textScore"} expression is used in a projection query to obtain the text score. The expression can be set up as an inclusion or exclusion in the projection statement. The limitation that it comes with is that if the expression is set to an existing field the projected metadata value will get overwritten by the current value in the field. 
• The $meta expression when paired with sort() function will also sort the "textScore" metadata in descending order. 
• Both the projection and the sort procedures can use a different field name for the expression when $meta: "$textScore" is used. The query system will ignore the sort's field name (). 

Projection Queries with Examples 

Let us look at some example projection queries in this section.

Not specifying fields in a query is similar to SELECT * in SQL. If you are looking only to return a subset of fields, you'd specify them in the SELECT clause like below: 

SELECT Id, name, is_active FROM customers 

This is similar to returning all fields in matching Documents just like one of the above queries 

db.inventory.find( { status: "A"} ) 

Let us check out some other examples to understand the variations in queries.

1. Return Only Specified Fields 

In case you need to return only the specified fields from a document, several fields can be included in a projection by setting the <field> to 1 in the projection document.

A good MongoDB projection example could be the below Query 

db.inventory.find( { status: "A" }, { item: 1, status: 1 } )

The result set associated with this query will contain only the item, status, and, the _id (by default) fields in the matching documents. 

2. Return all Except Excluded Fields 

Similarly, when you want to remove or suppress the fields returning in the matching document, projections can be used to exclude specific fields. The following example will yield all fields except for the status and the instock fields in the corresponding documents: 

db.inventory.find( { status: "A" }, { status: 0, instock: 0 } ) 

3. Return Specified Fields in an Embedded Document 

In an embedded document, certain fields can be returned as per the need of the result set. To do so set the embedded field in the projection document to 1 and use the dot notation to refer to it. 

The following example returns: 

• The _id field (returned by default), 
• The item field, 
• The uom field in the size document. 

db.inventory.find( 
{ status: "A" }, 
{ item: 1, "size.uom": 1 } 
) 

Starting in MongoDB 4.4, you can also specify embedded fields using the nested form, e.g. { item: 1, status: 1, size: { uom: 1 } }. 

4. Return Specified Fields in an Embedded Document in an Array 

To return specific fields in an array format you can use dot notation to grab the project-specific fields from inside documents embedded in an array. 

Let us take this example that specifies a projection to return: 

• The _id field (default Value), 
• The item field, 
• The qty field, that is let’s say embedded in the instock array. 

The query for the same on inventory collection will be as below: 

db.inventory.find( { status: "A" }, { item: 1, "instock.qty": 1 } ) 

Conclusion 

I see projections in MongoDB as similar to choosing specific columns in SQL. Even though it may seem a bit odd to use them for fetching less data, as the database grows, using projections allows us to control what I retrieve with each read or write. This can enhance efficiency and overall performance.

I hope this article clears up the concept of projections in MongoDB and how to use them to create efficient MongoDB queries. UpGrad's MongoDB tutorial covers MongoDB Projection in depth.

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